John asks: Is it better to use Icynene (open-cell) or closed-cell spray foam in an unvented cathedral ceiling? We are building a new home in Atlanta, GA.
Steve Bliss, of BuildingAdvisor.com, responds: I don’t really like unvented cathedral ceilings, also called “hot roofs”, because they are very unforgiving. If the roof sheathing or rafters ever gets wet from a roof leak, or from indoor moisture, the wood will have a difficult time drying out and wood that remains wet in warm weather will decay.
However, if you are determined to build an unvented roof, then I think that closed-cell foam insulation is a better choice. Icynene and similar open-cell foams have an R-Value of about 3.6/in. and are highly permeable to water vapor. Closed-cell polyurethane foam has an R-value of R-6 to R-7 per inch., and is a very effective vapor retarder.
With an unvented cathedral ceiling, it is critical to keep moist air and water vapor away from the roof sheathing where it can build up and lead to wood decay. Moisture problems in unvented or poorly vented cathedral ceilings are not uncommon, especially in cold climates where water vapor can enter the joist space and condense on the underside of the sheathing in cold weather. Atlanta has enough cold winter weather to be concerned.
While moisture damage and wood decay in unvented cathedral ceilings have primarily been a problem in cold climates, similar problems have been reported in warm, moist climates such as the Southeast.
While it is possible to use a permeable insulation – open-cell foam, fiberglass, or cellulose – in an unvented cathedral ceiling, it is risky and I would not recommend it. This approach relies on a near-perfect air and vapor barrier on the house side of the ceiling cavity to keep moist household air and water vapor out of the roof cavity. In a very simple ceiling without complex framing, skylights, recessed lights, or other penetrations, this may be achievable by an experienced energy-efficient builder with the help of a blower door. But, more often than not, there will be air leaks. I would not trust this detail on my own house.
Unvented cathedral ceilings. Where an unvented cathedral ceiling is required, the preferred (and most expensive) approach is closed-cell spray foam. When properly applied by an experienced installer, it will create the type of air and vapor barrier needed to protect the sheathing from moisture. This is a good approach for ceilings where good ventilation is difficult to achieve due to complicated framing, multiple skylights, or other issues. It’s also useful in renovations where achieving a perfect air barrier is especially challenging. To save money, it’s possible to spray a couple of inches of the expensive closed-cell foam against the roof sheathing and fill the rest of the cavity with a less expensive open-cell foam or fibrous (fiberglass or cellulose) insulation.
Cautions with spray foam. Spray foam in cathedral ceilings can work well, but it’s important to have skilled installers and follow these recommendations:
1) Since unvented roofs are designed to dry mainly to the interior, DO NOT use a vapor barrier on the underside of the cathedral ceiling. Do not use polyurethane sheeting or vapor barrier paint. If a vapor retarder is required by code, use one of the new “smart” (and expensive) vapor retarders such as MemBrain or Intello.
2) Choose a roofing underlayment that is permeable to water vapor, allowing some drying to the exterior in the case of a roof leak or other moisture problem. That usually means traditional roofing felt as most synthetic underlayments have very low permeability. Use ASTM-rated roofing felt as the generic type is thin and often of poor quality. There are a few high-perm synthetic underlayments on the market, including Deck-Armor, Delta-Maxx, Delta-Foxx, and SlopeShield Plus Self-Adhered — the only breathable self-adhered membrane I know of. These tend to be costly and work best with a breathable roofing such as tile, wood shingles, or metal roofing set on battens to create a vent space. With asphalt shingles its doubtful that a breathable underlayment does much good.
3) With spray foam, always use a well-trained, professional applicator as these materials take a lot of expertise to install properly. An unskilled installer can make a mess and may leave a lot of voids – increasing the likelihood of future moisture problems.
Vented cathedral ceilings. A less expensive alternative is the more traditional vented cathedral ceiling insulated with fiberglass, cellulose, or open-cell foam. This is best for simple cathedral ceilings that are easy to ventilate. These should have straightforward framing with no skylights and with direct access to soffit and ridge vents. You will need an airtight air barrier below the ceiling insulation along with an effective vapor retarder, which could be poly, vapor barrier paint, or a smart membrane. Minimize penetrations to the ceiling and especially avoid recessed ceiling lights, which have been the cause of numerous moisture problems in cathedral ceilings — with or without vent channels.
Above the insulation, leave a 1-1/2 in. vent channel separated from the insulation by a foam, cardboard, or plywood baffle, and provide balanced venting at eaves and ridge. The vent space below the sheathing will help promote drying – whether from roof leak above or from moist air or water vapor entering from below. But do not rely on the vent channel to prevent moisture problems.
Research that has shown the roof ventilation is not very effective at preventing moisture problems in a house with a leaky air barrier and high indoor humidity. The key is an airtight air barrier below the insulation that keeps moist air out of the ceiling cavities. All penetrations need sealing with spray foam or another long-lasting sealant. A blower-door test to confirm that the ceiling is free of air leaks is always a good idea with this type of roof construction.
Effective air and vapor sealing is your primary protection. Roof ventilation is a backup that will promote drying and help prevent wood decay if a roof leak allows water to enter the roof structure.
Read more on Cathedral Ceilings:
Combining Spray Foam & Fiber Insulation
Insulating with Foam Board
Preventing Condensation in Cathedral Ceilings
Sealing Unvented Cathedral Ceilings
Is Unvented Roof Our Best Option?
We’re building a new home with a great room with an unvented cathedral roof. Why unvented? Rafters ended at top plate. Exposed 4×6 rafter tails extend beyond plate w 1×6 t& g board nailed over tails. Frieze board is nailed into wall.
Roofing is 18″ Alaskan yellow cedar tapersawn shakes on 1×4 skip sheathing.
Insulation is 2-3’” off closed cell and then open cell to achieve R-49 northeast code . Is this the best option or a viable option? The insulator used AccuVent cathedral ceiling vents
There are countless ways to insulate and ventilate (or not) cathedral ceilings. Seems like I hear about a new one every week. There is no consensus on the “best” way, but there is some agreement on the wrong ways — leaky ceiling, recessed lights, blocked ventilation, high interior moisture levels.
You say your roof is unvented, but has AccuVent channels. So I’m a little confused. Maybe the chutes were needed to provide a backing for spaying under the skip sheathing. AccuVents are one of the better plastic vent chutes.
Without soffit and ridge vents, you will not have a conventional vented roof, but with skip sheathing and shakes, the roof will have some natural ventilation which is a good thing. The main downside to a spray foam hot roof is the inability for the sheathing and framing to dry out in the event of a roof leak. Your roofing system should not have this problem.
There is no harm to venting above a spray foam roof, but it is usually not done as hot roof advocates claim it is not needed. The closed-cell foam keeps indoor air from reaching the cold roof sheathing and condensing. It’s true that you will avoid significant condensation assuming you follow the R-value ratios in Table 1 here, and the spray foam is applied properly. In using the Table, you can substitute open-cell foam for the fiber insulation in the chart.
Read more at the Cathedral Ceiling links below:
Combining Spray Foam & Fiber Insulation
Insulating with Foam Board
How to Air-seal Unvented Roofs
Should I Use Hot Roof On Unvented Bathroom Addition?
I have a small addition 12×12 at a low slope off the back wall of the main house. The attic space I’m the smaller roof is unvented, it does not connect to the attic space of the main house.
I would like to make this space a vaulted ceilings to get a little more head room and vertical space. The rafters are only 2×4 so I was planning on putting larger rafters next to those so I could insulate properly. I have a few different ideas about insulation and not sure what would work best. Soffit vents, 2″ air gap, foil faced foam, rockwool batts. Or does it make more sense to do a tightly sealed hot roof, with spray foam or all foam and tape. Would soffit vents do it anything without venting at the top of the slope?
Also, this space is above a bathroom. Is this even feasible?
There are already moisture problems in that roof, I assume from being unvented. When I opened it the only insulation was 1″ foil backed eps loosely against the roof decking.
Because this is a high-humidity space (a bathroom), you need to pay extra attention to roof insulation and ventilation. You mention that there are already moisture problems in the roof. Could you be more specific? Also, in what climate zone are you located?
With loosely fitting EPS, moist interior air is probably reaching the underside of the sheathing and condensing in cold weather.
If you can sacrifice some ceiling space, my first choice would be to leave a 1 to 2-inch vent space, then insulate with fiberglass or mineral wool batts. For this to work, you need both soffit and ridge vents. Soffit vents alone will do little.
More important than ventilation is an airtight ceiling air barrier keeping moist household air out of the roof structure. This could be one of the “smart” vapor retarders such as Membrain, or low-perm foam insulation (XPS or foil-faced iso board) taped at the seams and sealed at all edges with canned foam.
If you have access to the wall framing, also make sure that there is no leakage around or through holes in the walls’ top plate as air can leak into the walls and then up into the roof. Also seal around any ceiling penetrations including light fixtures and the exhaust fan housing.
Your goal is to eliminate all airflow from the interior air into the rafter spaces.
If this does not seem feasible, then your best option is an unvented host roof. Hire an experienced spray-foam contractor to seal and insulate with closed-cell foam. Cutting and sealing strips of foam insulation between the rafters, sometimes called “flash and batt,” is another option but very difficult to do well enough for a high-moisture space like you have.
In either case, install a properly sized (and well-sealed) bath fan and use it whenever bathing or showering to lower humidity levels in the room.
Read more at these links.
Spray Foam Vs. Vented Roof for Retrofit?
Love your website, thanks for the tips. Insulation question.
I live in an 1890s farmhouse in Vermont. See pic. I am replacing the vinyl siding with cedar. The attic has vermiculite insulation mixed with cellulose which has to be removed in order to replace the fascia. The slopes are full of vermiculite as well. The 2nd story walls are not knee walls, they are exterior (see pic).
The upstairs ceiling is lathe and plaster and we will be taking it down to remove the insulation and start fresh.
I have a slate roof and two gable vents. I get ice dams in the winter, especially around the chimney.
Should I make it a hot roof and add a ton of closed cell spray foam? Or should I add soffit vents and keep it ventilated?
The more I read about this, the more confused I become. The basement does get some water in the summer. The house also gets very hot in the summer, especially upstairs. I worry about water damage and old houses breathing. The insulation guy says the slopes are too skinny to fit ventilation. Don’t know the rafter sizes yet.
Any suggestions welcome, thanks!
To tackle a retrofit insulation job, or any insulation job, you need to start out with a clear definition of your thermal envelope – those are the wall, ceiling, floor, and roof planes that separate the conditioned from unconditioned part of the house. The thermal envelope should be continuous, forming a big bubble that you try to keep warm in winter and cool in summer.
The thermal envelope is also where you should concentrate your air-sealing efforts – creating a continuous airtight layer following the thermal envelope. In theory the air barrier can go anywhere within the thermal envelope – outside surface (taped sheathing, for example), inside surface (taped, caulked, and gasketed drywall, for example), or within the wall (spray foam or a well-sealed smart vapor barrier).
Once you decide where you want your thermal envelope to be, it’s easier to choose an insulation and ventilation strategy that makes sense. For example if you have sloped roof sections, you may need to use spray foam unless you are willing to make deeper spaces for insulation and lose interior head room. You budget and your desired or code-mandated insulation levels also will point you one way or another.
Each approach as pros and cons – there is no perfect solution. And some contractors have strong preferences – some like hot roofs, some hate them.
The easiest approach for your, and probably the most expensive, would be to “add a ton of spray foam.” This will provide your insulation and air barrier in one step, if done well. It will help reduce ice dams by reducing heat loss, but may not eliminate them completely. As long as there is uneven melting of the snow, you can get ice dams and this may still take place around the chimney. Skylights are another common source of ice dams.
The biggest downside of spray foam, from my perspective is that a leaking roof or flashing could cause a lot of damage before it was detected. The wet areas of sheathing and framing would be very slow to dry out. So it’s important to be vigilant about roof leaks with a hot roof. Also, spray foam is a tricky material to install well. Choose a contractor with a lot of experience and a stellar reputation.
Option 2 is a ventilated roof. If you can create functional soffit and ridge vents, leave a min. 1-in vent space (2 in. is better) and find room for enough insulation, then this is usually my first choice as it is a little more forgiving. It can help a lot with ice dams, keeps your roofing material a little cooler, and provides some opportunity for materials to dry out if they get wet from exterior leaks or from interior air that leaks into the roof cavity and condenses.
Thanks again for your help thinking about my old house’s thermal envelope. So we finished the renovations and I am attaching a before and after pic. What a change!
We added 6″ of spray foam to the attic floor and slopes. The slopes had to be built out to achieve this and we removed lots of vermiculite. I have two gable vents and soffit vents with a thin layer of vent space running up along the slopes to cool the small attic. Attaching some foam pics too.
I am including a before pic of my ice dams and an after pic. It is vastly improved but still not good. The area around the chimney is thick with ice. The one upstairs radiator is right next to the chimney. So all of the heat from the 1st floor, 2nd floor and radiator seeps out through the hole in the roof. There is no spray foam around the chimney area due to fire concerns.
I have been talking to Brickliners about how to help with this issue. They say it is very unusual. If there is enough clearance, they can add an attic insulation shield around the chimney. But because my contractor boxed it in, all that would have to be redone. And it may or may not help.
You can also see how years of heat loss in this chimney area (it was installed in 2012) have affected the wood and caused it to bend and curve. The eaves are deep, perhaps 12”, but this can’t be good for the new wood.
Obviously, the easiest solution would be to stop using my wood stove and just pack that area with insulation. But my wood stove is great and a good furnace back up. So do you think this is a possible solution? Any other ideas?
Thanks for the follow-up info. Nice to see how a project develops over time. I’m posting your photos here. First is spray foam. Second and third pics show ice dams before and after. They now occur only below the hot flue pipe.
I’m not sure what the Brickliners said was “very unusual”. Even with a well-insulated attic, it’s not unusual to get some ice dams below point sources of heat loss such as chimneys and skylight.
The required clearance around insulated metal chimneys for fireplaces and stoves is usually 18 inches or more for single-wall stovepipe and 6 to 8 inches for double-wall pipe. In general, you cannot fill this space with insulation, even if it is a fireproof material such as mineral wool.
You can block the airflow out of the chimney chase (the opening around the flue) with noncombustible materials such as metal or some types of cement board. You can seal to the pipe with an approved firestop sealant.
Read more about sealing around chimneys
ANdrea Semenuk says
Building a new house in CT. attic will have a cathedral ceiling and simple design. If CI on top of roof deck is too hard for my builder, what is the recommended (cost effective) under the deck approach that will get me to R60? Thicker joists? Tji joists? (Walls are R30+). Thanks
Can I Sandwich Fiberglass between Spray Foam and Foam Board?
HI! i have read a comment/question below and someone were talking about 2 inches of polyurethane + the rest with cellulose. i have a similar situation but i have 2×6 rafters making an unvented cathedral ceiling.
Would this work ?… 2 inch of polyurethane (closed-cell) and filling the rest with fiberglass about 4 inch (R-3.5/inch) and maybe adding a inch layers of rigid board foam (open cell) right under the rafter to stop them from being a “thermal bridge”. Then add 1x3s for an air space above the drywall.
I see two potential problems with your approach:
1) You might have too much additional R-value below the foam, lowering its temperature and leading to condensation.
2) You could potentially trap moisture between two layers of foam, with little ability to dry out the roof structure.
When combining closed-cell spray foam with fibrous insulation, it’s important that the interior surface of the spray foam stay warm enough to prevent condensation. The more insulation you put below the spray foam, the colder the spray foam becomes.
Condensation can occur on the interior foam surface, or within the fiberglass insulation, if indoor air leaks into the rafter cavities or by diffusion of water vapor through the ceiling. Air leakage is usually the more important mechanism for moving moisture into the roof structure.
The optimal ratio of spray foam to fibrous insulation varies with the climate zone. A well-established rule of thumb says that a 2×4 wall or ceiling (what’s left of your 2×6 rafter after spraying the foam) requires R-5 foam for DOE Climate Zone 5, R-7 foam for Zone 6, and R-10 foam for Zones 7 and 8. Two inches of closed-cell urethane provides an aged R-value of about R-12. It’s the ratio of foam R-value to fiberglass R-value that is critical.
So depending on your climate zone, you could add some additional fibrous insulation below the foam. Adding a second layer of foam below the rafters, however, violates another rule: that walls or ceiling with closed-cell foam on the exterior should be able to dry to the interior – that mean no poly vapor barrier or low-perm foam sheathing on the interior.
One inch of EPS (expanded polystyrene) has a perm rating of about 1 or about 0.5 for 2 in. of foam. This makes it a Class II vapor retarder along the lines of Kraft paper. Extruded polystyrene (XPS) is much less permeable. Foil-faced foam is a complete vapor barrier.
Bottom line: sandwiching fibrous insulation between two layers of foam is a risky detail. First, you’d have to do the math to make sure you don’t place too much R-value below the spray foam. If you tape and seal the bottom layer of foam to make a “perfect” air barrier, keeping household air out of the roof structure, you might be fine. But perfection is hard to achieve on a real-world building site.
If you do choose to use foam, I’d stick with no more than an inch of EPS and don’t seal the seams and edges. This would allow some minimal drying to take place. If you want more insulation than you can fit below the spray foam, a better solution might be to build down the rafters to add more fibrous insulation. For example, if you run 2x4s across the rafters, you can add a 3 ½ inches of insulation and reduce thermal bridging at the rafters.
Then create a tight air barrier at the ceiling level with well-sealed drywall or a smart vapor retarder, such as MemBrain, to allow some drying potential to the interior. Another option for higher R-value is to fill the full 5 ½ inches with closed-cell foam.
Cary Brief says
Is Icynene a Good Choice For Unvented Cathedral Ceiling?
First I want to thank you for this valuable information. I am in the middle of a complete rebuild of a 1950’s bungalow in raleigh NC. we opted for a Cathedral ceiling as ceilings were 7′ tall. we rebuilt 90% of the exterior walls, the entire floor system and the front roof (2/10 16 0c. the house started out at about 600sqft and was added on to make it eventually 1330. they doubled the rear roof by sistering 2/6 rafters and it was a mess. Our engineer asked for a 20″ LVL and sistering the 2/6 rafters with 2/10. (the front we tore out the back we did this). new sheathing front and second layer of sheathing over rear roof. Due to the mess of an old ridge a ridge vent was not possible. we are planning spray foam and every single contractor says closed is too expensive and open is just fine. two questions. Is Icynene if 5-6″ enough to deal with humidity? we are putting a Daikin mini split and either with a built in ERV or a stand alone one. wood stove for the winters, I think humidity will be under control. since everything is open (roof is on) we could still opt for some sort of vented roof system – any suggestions?
Wish I had a simple answer for you, but there is still no industry consensus on the best way to build a cathedral ceiling.
Where possible, I still believe it is best to ventilate cathedral ceilings with a 2 in. vent space with soffit and ridge vents. With good ventilation and a tight ceiling air barrier, any type of insulation can be used. The best choice would be an insulation that blocks air movement such as open-cell foam or spray cellulose.
A vented roof is the most forgiving of problems due to moisture from above (roof leaks) or below due to air leakage from the interior. Roof leaks are of particular concern with unvented roofs as the roof decay could occur before the roof has a chance to dry out. Also, there may be no evidence of the roof leak inside the home to alert the homeowner.
With complex roof designs, however, ventilation is usually not feasible. Or, in your case, the problem is the messy framing of a remodeled roof structure. So an unvented roof may be your only option.
In that situation, the IRC calls for a minimum layer of “air-impermeable insulation in direct contact with the underside of the roof deck”. For your area (DOE Climate Zone 4), the code requires a minimum of R-15 impermeable insulation and R-38 total (or R-30 for up to 500 sq. ft. where R-38 is not feasible). You should keep the same ratio if you increase the overall insulation to insure that the temperature on the underside of the closed-cell foam is safely above the average dew point of the air inside the home.
So one option would be 2-1/2 in. of closed-cell foam (R-15) sprayed against the roof sheathing and 6-1/2 inches of fiberglass, cellulose, or open-cell foam (R-23) below that impermeable layer. You can find a table showing the foam thickness at this link.
NOTE: The most important detail with any type cathedral ceiling is an airtight air-barrier below the insulation – with no recessed lights or other openings cut into the ceiling. If no air leaks into the roof structure, there should be no moisture problem in the roof regardless of the insulation method.
You can create the airtight barrier by carefully caulking and gasketing the drywall – the “airtight-drywall approach” or use one of the smart vapor retarders, which become permeable with high humidity. It’s best to test the assembly with a blower door or you will never know if there are air leaks. Visual inspection of the work is not up to the task.
Some people have successfully used open-cell foam in your climate, which others have experienced problems. Where there are problems, they can usually be traced to high moisture levels in the home combined with a leaky air barrier at the ceiling. Recessed lights are a common culprit.
Read more on the required Thickness of Foam Insulation
Matt B says
Should I Use Smart Vapor Barrier On Unvented Cathedral Ceiling?
I’ve been following this thread and have gained lots of good info! I’m remodeling an older home and we vaulted two ceilings. (Cathedral). I used a lot of information from your threads when deciding on my insulation. I have 2×12″ rafters and we had a very reputable insulation company come in and spray 2″ of closed-cell foam directly on the roof deck followed by 9.5″ of netted and densely packed cellulose to create an unvented ceiling.
I live in central Missouri where we do have very humid summers and some quite cold winters with occasional snow of several inches. That said, my interior air quality will be quite good as we installed a new HVAC system as well as new high-efficiency windows throughout the entire house. The house is also very large (5000 sq ft)
I plan to install a T&G shiplap ceiling. I had planned on drywalling, taping and mudding the ceiling before installing T&G. But then I just read your response about the “smart” membranes- Membrain and Intello. My question is, which option would you recommend – drywall or membrane? And do you have a preference between those brands for my situation? It seem that if the membranes are a good choice thermally, it would be a lot less work than drywall and would leave a much easier surface for installing T&G. Thanks for your help!
Sounds like you should be fine with either solution as long as you keep household moisture out of the rafter bays (and keep the roof in good repair).
An effective vapor retarder and airtight air barrier at the ceiling level will be required to keep the rafter bays dry. The dense-pack cellulose will also help as it resists airflow pretty well. Both will work together to keep moist air and water vapor out of the ceiling cavities.
It’s critical that you keep the roof structure dry as the underside of the spray foam, the most likely place for condensation in the rafter bays, will get pretty cold in winter. The foam is now the condensing plane rather than the underside of the decking. The foam will be warmer than unisulated decking, but will still be cold enough to cause condensation.
In round numbers, you have R-12 for the spray foam layer and R-33 for the cellulose. So the temperature on the bottom surface of the foam will be 25°F when it is 0°F degrees outside and 70°F inside. The calculation is: 12/33 x 70 = 25. When its 20°F outside, the bottom of the foam will be about 38°F. Basically, the more insulation you have below the foam, the colder the bottom surface of the foam.
The dew point of the interior air (70°F at 40%RH) is about 45°F. So much of the winter, the bottom surface of the foam will be below the dew point of the interior air. — meaning you don’t want moist household air getting into the ceiling cavities. These are theoretical calculations, but give you some idea of what’s going on behind the drywall.
The safest approach would be one of the smart membranes. If moisture gets into the rafter bays — despite your best efforts — this will provide the best drying potential as these membranes become highly permeable under high humidity. Last time I checked there were three options: Certainteed Membrain, Pro Clima Intello Plus, and Pro Clima DB+ (lower perm rate and a little cheaper). The Certainteed product is more widely available.
You have the choice of sealing the seams in the vapor barrier with an approved tape, or installing the drywall in an airtight manner, the so-called airtight-drywall approach. If you’re going to the trouble and expense of using a smart membrane, I would tape the seams and use that as the air barrier. Making the drywall airtight is easier said than done.
Kevin Quinlan says
Should I Add Vent Space Above Spray Foam In Roof?
I am looking to insulate an “unvented cathedral ceiling” using spray foam. The ceiling is on 24″ 2×6 centers. I was wondering if I could use 22″ baffles, allowing for a 1-1/2 in. vent channel separated from the spray insulation by a baffle, and provide balanced venting at eaves and ridge. The vent space below the sheathing will help to prevent a “hot roof” AND help promote drying – whether from roof leak above or from moist air or water vapor entering from below.
Most people who use spray foam in roofs spray directly to the underside of the sheathing, creating a hot roof. As you point out, this does not allow for effective drying if the roof structure gets wet from a roof or flashing leak. If the foam is installed correctly, it will block moist air from entering the roof cavity from inside the house, so that should not be a problem. Air-tight sealing is a selling point of closed-cell foam.
Creating a vent space above the foam, as you suggest, creates a more durable system that is less vulnerable to water damage. It is a belt-and-suspenders approach, always a good idea in home construction, since in the real world, few buildings have perfect workmanship. However, you have reduced your 5 ½ in. roof cavity to 4 inches or less, leaving less room for insulation.
Four inches of closed-cell foam will provide a nominal R-value of R-24 to R-26. The actual R-value of the roof is lower due to the thermal bridging of the rafters. Also closed-cell foam is usually the most expensive insulation option.
If you are looking for better thermal performance at a comparable or lower cost, you might consider using just 1.5 inches of spray foam against the underside of the baffles for air sealing. Then place 2 or more inches of rigid foam insulation below the rafters, providing added insulation and preventing thermal bridging. You can fill the remaining cavities with cellulose or fibrous insulation for additional R-value with little added expense. Depending on the type of foam and fibrous insulation, you can get up to about R-30 with little reduction from thermal bridging.
To save money, you can get rid of the spray foam altogether and fill the entire cavity, up to the baffle, with cellulose, and then insulate below with foam. Tape the foam seams and seal the perimeter with canned foam. This will provide an effective air seal. And you have the vent space under the roof sheathing just in case.
There are lots of options. If space is tight, closed-cell foam might be your best bet. If you can afford to lose a little headroom, foam board under the rafters offers a good alternative.
I have a similar situation. I am building a front entrance 10’x12’ with exposed beams and T&G pine on top of the beams. The insulation contractor is telling to place tyvek on the T&G and then 2×6 rafters. He will then place 5” of closed cell foam between the rafters. I will then place the roof sheeting and roof it. My question is… is this a proper installation and will it work?
Builders have devised countless details for insulating cathedral ceilings – some more successful than others. With exposed boards and beams, the insulation has to go above the t&g decking.
Some buildings lay foam board on top of the decking and then nail the roof sheathing through to the decking with long nails, a problematic detail. Others build a standard vented roof on top of the t&g, which is fine if you don’t mind the extra cost and thickness of the roof structure.
Your builder’s solution seems fine as long as the Tyvek is taped and seams and sealed around the perimeter to keep moist household air out of the roof structure. The contractor can use either spray foam or “cut-and-cobbled” pieces of foam board, as long as the foam pieces are taped at seams and sealed around the perimeter with canned foam – a tedious procedure.
The Tyvek functions as the air barrier here, keeping moist air out of rafter cavities, but allowing some drying to the interior in the event of moisture buildup in the roof. Another, more expensive, option is to use one of the “smart” vapor retarders such as MemBrain. With foam insulation, however, I think the Tyvek will be fine as long as it is installed with care.
After Repair, Unvented Cathedral Ceiling Still Has Moisture Problems
I have a question regarding the result of roofing on my 30 yr. old townhome. The townhome has cathedral unvented roof in cold Colorado climate. They replaced the unvented roof during the winter from the shingles down Meaning roof came off replaced rotted joists/rafters by “mothering” bad with new applied spray foam from roof to ceiling drywall then applied sheathing and asphalt on top.
The spray insulation was applied downwards onto the ceiling drywall leaving a gap between the foam and roof. I always understood foam should be applied from inside to the roof but they did not want to remove my ceiling drywall. This is a unvented cathedral roof with dormer.
Since the first spring the and every spring since, water has run down from the same spot where a dormer joins the ceiling. The HOA says it is my fault but I protest as the hired the contractor and paid for the roof. Now they say I am liable for result of moisture condensation damage to the entire building.
What can I do?
I’m not a big fan of unvented cathedral ceilings as they are not very forgiving if moisture gets into the roof cavity. Unvented roofs usually work fine, but when they go bad, the damage can be extensive. Once discovered, it’s often difficult to determine why one roof did fine and another failed.
The moisture in the roof system could come from indoor air that leaks into the roof cavity, diffusion of indoor water vapor through the ceiling, or a roof leak from above. Once moisture gets into the space, it is slow to dry out. If the rafters and roof sheathing stay wet once the weather gets warm, mold and eventually wood decay will occur. Extensive damage can be done to the rafters and sheathing before water stains show up on the interior.
In your situation, the most important question is what is the source of the moisture in your roof that caused the initial problems, and what is the source of moisture now. In my experience, most moisture problems in roofs are caused by exterior leaks at flashings and other penetrations, but interior moisture is sometimes involved.
In your case, I would start by inspecting the connection where the dormer joins the main roof. A poorly flashed joint is the most likely source of your leak in both the original roof and the new one. Flashing membranes are often the best safeguard against leaks at tricky roof connections and penetrations. I would also consider applying peel-and-stick membrane to the entire roof (if you reroof), assuming you are in an area of high snowfall. You don’t want to risk roof leaks with an unvented cathedral ceiling.
Regarding the roof insulation, closed-cell foam is the best product to use for an unvented cathedral ceiling. If installed properly it will create an effective air and vapor barrier to keep interior moisture away from the roof sheathing, where it can condense and cause problems. Open-cell foam is more prone to problems in this application.
You are correct that the foam is usually sprayed against the roof sheathing in the normal sequence of construction. But it shouldn’t matter that it was sprayed downward against the drywall instead. The gap between the foam and roof sheathing is not a problem. And since there is a gap of at least an inch, it would not hurt to ventilate that space at top and bottom to provide some drying potential.
Unvented cathedral ceilings are designed to dry out mainly to the interior. So you should never use a polyethylene or similar vapor barrier on the ceiling below. Vapor-barrier paint is even discouraged. If you are required by code to install a vapor barrier on the interior, you would want to use one the “intelligent” vapor retarders such as Membrain or Intello, which are highly permeable to moisture when wet.
While you don’t want the ceiling to include a vapor barrier, it’s a good idea to make the ceiling finish as airtight as possible, creating an effective air barrier. This would help keep moisture-laden air from leaking into the ceiling assembly. Even though the closed-cell foam creates an airtight barrier, there may still be air leaks that bypass the foam and allow moist air to reach the sheathing.
For example, you mention that new joists were “mothered” in place (around here we say “sistered”). So gaps could occur between the sistered joists. Gaps can also occur around any interruptions in the roof framing such as skylights, partitions that reach the ceiling, dormers, plumbing vents, chimney chases, and recessed lights – something to definitely avoid in any cathedral ceiling. All these bypass routes, allow heat loss as well as moisture migration.
Another potential bypass for moist air to flow into the cathedral ceiling would be the party wall that separates your townhome from the next. With all these building details, the devil is in the details. A conscientious energy-efficient builder would have the knowhow and commitment to seal off all possible entry points for airflow into the roof, probably with the help of a blower door to verify their work. But many builders do not have this expertise.
Assuming that all interior leakage paths are sealed, then the primary source of the moisture, in your case, is a roof leak, as discussed above.
If moisture is coming from the interior, it may remain frozen in the roof cavity until spring. If the source is a roof leak, it may not occur until the weather warms up bringing rain or melting the snow on your roof. Do you ever see evidence of leaks on the occasional warm winter day?
So job #1 is finding the source of the water. This often takes some detective work and the help of a home inspector or building consultant with the tools and knowledge to diagnose the problem. A blower door, infrared scanner, and sometimes a hose on the roof are needed to pinpoint a leak. Observing the location and patterns of the leakage is an important clue. I would suggest finding a professional who specializes in building moisture damage.
Once you know where the water is coming in, you should be able to stop the leak, let the roof structure dry out, and be good to go. I’m not sure how recently this work was done, but it is unlikely that you have caused substantial damage to the new roof structure in a couple of years. Worst case, you may need to cut (and repair) a small opening in the roof structure to examine the roof structure for damage.
How to Retrofit Insulation in Cathedral Ceiling?
We are about to replace the shingle roof on our house located in southern Wisconsin, built in 1977. I’m interested in retrofitting insulation to improve the R-value, but am getting the sense that our ceiling is a little unusual. Our entire first floor (it’s a ranch) has a cathedral ceiling. The rafters about 2×6. The insulation is foam boards, installed at the time the house was constructed. They are blue and a bit over one inch thick, each. There are several stacked to fill the cavity, but also allow for venting. A soffit vent leads to a ridge vent.
My theory is that the original foam boards may be deteriorating or at least not filling the whole cavity. My “great idea” is to convert our roof to a hot roof and just fill the remaining space in the cavity. The shingles we are looking to use have warranty coverage for such an application. Accessing the cavities cannot likely be done from the inside. We have a tongue in groove ceiling that has a layer of asphalt felt paper immediately over it. So it would need to be done from the ridge or soffit side, on the outside.
Is my “great idea” a bad idea? I’d really like to maximize the insulation of the roof. But I also know we may be opening a can of worms. For what it’s worth, our house is quite dry in the winter, so I suspect little risk of moisture damage from the inside.
It’s always difficult to offer specific insulation details without a complete picture of the jobsite. There are many variables at play and, although a general plan may sound good, the devil is in the details.
If your roof rafters are 2×6 with a vent space, then it sounds like you have no more than 4-5 inches of insulation. If the insulation is crumbly like a foam plastic cup then it is probably expanded polystyrene with an R-value of about 4 per inch. If it higher-density extruded polystyrene, then the R-value is about 5 per inch.
If the boards are not tightly fitted and well-sealed, or they are deteriorating, then the R-value could be considerably less. So improving the insulation would be a good idea. The best way to accomplish this is not so clear.
There’s no easy way to fill the ventilation space without removing the ceiling below or the roof sheathing above. If you do that, then you would want to remove the old insulation and start from scratch. The most effective material would be closed-cell foam, applied from below, which would provide excellent air sealing and the highest R-value per inch. The downside is the cost of removing/replacing the finish ceiling and high cost of spray foam.
Another high-R option would be to use foil-faced polyiso insulation, which could be installed from above or below. The foam boards would need to be cut and fitted in place and well-sealed at all joints with spray foam. The advantage is that you could install this from above by removing the sheathing – a simpler and cheaper option than removing the ceiling below.
There are a variety of other options, such as completely sealing the existing vent space and adding new insulation and sheathing above the existing roof deck. However, this requires a lot of exterior trim work and new flashing, so it can get costly. Also leaving an air space under a hot roof, even if it is well sealed, is a risky proposition. I wouldn’t do it in my house.
You would need to get bids to evaluate the real costs of these options.
If you do go with a hot roof, air sealing is very important, which is why spray foam is the material of choice.
Related Link: Air Sealing Unvented Cathedral Ceilings
Can I Sandwich Roof Deck Between Layers of Foam?
I’m building a new home with 2×12 rafters in Climate Zone 5.
How will this work: R-15 polyiso nail-base above the roof deck, WRB (water-resistant barrier) on the roof deck, and 3″ closed-cell foam sprayed underneath the roof deck. We’ll fill the balance of the cavity with open-cell foam for a total of R-60. I’m concerned with sandwiching the roof deck between two vapor-impermeable materials.
I also plan to install asphalt shingles on top of the polyiso nail base, so I will have a hot roof. Your thoughts please? I am being advised that the ‘sandwich’ is ok but my gut tells me it is not. Thanks
If I understand you correctly, you have a structural roof deck covered with a WRB, with 3 inches of closed-cell spray foam underneath the roof deck and R-15 nail-base polyiso above the roof deck.
Then you are filling the rest of the cavity – approximately 8 inches – with open-cell foam. Do I have this correct?
Builders and designers have tried a wide range of details for unvented cathedral ceilings. There is an almost endless variety and the building codes have been playing catch-up. A lot of these details have not been tested in the field, so you are relying on building-science theory, which only gets you so far. It’s one of the most confusing and rapidly changing topics in the industry.
Also, the devil is in the details. It’s easier to say “airtight ceiling assembly” than to build one.
The goals with any design are:
• Keep moisture out of the roof cavity — water vapor from the house or roof leaks from above.
• Keep the bottom of the roof sheathing, or the bottom of the impermeable foam (if under the roof sheathing), above about 45°F on average in the winter to avoid condensation.
• Allow some potential drying either to the indoors or outdoors in case the sheathing or other framing components get wet.
With those caveats, the detail you are describing sounds OK, but not great in my opinion. It’s a variation on an established approach, but not a field-tested system, which adds some risk. Typically, when impermeable and permeable insulation are used together in an unvented roof, they are either
a) both under the roof deck, or
b) the foam goes above and the permeable insulation goes below the roof deck
These two hybrid details are covered under IRC Section R806.5.
R806.5.1.2. Where air-permeable insulation is provided inside the building thermal envelope, it shall be installed in accordance with Section 5.1.1. In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing in accordance with the R-values in Table R806.5 for condensation control.
R806.5.1.3. Where both air-impermeable and air-permeable insulation are provided, the air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing in accordance with Item 5.1.1 and shall be in accordance with the R-values in Table R806.5 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.
The goal in the code is to keep the average temperature at the underside of the sheathing (or the underside of the closed-cell foam if under the sheathing) above 45 degrees F. This minimizes the risk of condensation if interior moisture reaches the sheathing or spray foam. Table R806.5 recommends minimum R-20 for the impermeable (closed-cell) foam in Climate Zone 5, if used above permeable insulation.
The code writers are assuming that you are adding enough air-permeable insulation to meet code, which is now R-49 in a Zone 5. However, you can calculate the temperature at the bottom of the sheathing by taking the average outdoor temperature and the average indoor temperature (assumed to be 68°F) and multiplying by the ratio of the foam insulation to the permeable insulation. This will show you that the top layer of closed-cell foam should account for at least 40% of the total R-value.
The code requires that these roofs do not have Class I or II vapor retarders below the insulation, allowing some level of drying to occur to the interior. The code also assumes that the spray foam is applied well enough (when under the roof deck) that no moist air can reach the underside of the roof deck. With foam applied to the underside of the deck, some builders create a vent space over the roof deck, with and air space or breathable underlayment.
In your case, you have impermeable foam both below and above the roof deck, creating a sandwich. If the roof deck gets wet – most likely from a roof flashing leak – it will probably stay wet and would be subject to rotting. The same problem can occur with detail 5.1.2, but with the multiple layers, it would be a lot more difficult to inspect or repair the damage.
If using your detail or any without the ability to dry in either direction, I would make every effort to prevent roof leakage. For example, I would be inclined to cover the entire roof with a peel-and-stick self-sealing membrane. I’m not sure why you would put the WRB (the roofing underlayment) on the roof deck rather than the nail-base insulation, as you mention. The goal of the underlayment is to keep water out of the roofing structure so it should go directly under the roofing material.
Hi Steve, Thanks!
The membrane on the roof deck is my air barrier.
I am not thrilled with this plan either. Perhaps I should just push for R20 above the deck and all permeable below the deck. My target total r is 60 so I think that is doable. I just really wish all my very well paid experts would design best practices and not random new practices. Ugh stressful!
I am very appreciative of your time and effort. Plan to use Henry vp 100 on nail base with ice shield on edges.
One thing to keep in mind is that the ratio of insulation over and under the roof deck will determine its temperature. So, for example, if you have R-20 insulation above the deck and R-40 below the deck, the temperature of the roof deck will be about 46°F when it is 35°F outside and 68 indoors. The average outside winter temperature in Climate Zone 5 ranges from 30-40°F.
The math is a proportion and works like this. The total R-value is 60. R-20, which is one-third of the total, is above the roof deck. Therefore 1/3 of the total temperature variation occurs above the roof deck. 1/3 x (68°F. -35°F. ) = 11°F. 35°F + 11°F = 46 °F. That’s just above the code target of 45°F.
With an unvented roof, you should never use poly or any other low-perm vapor retarder (Class 1 or 2) at the ceiling, so some drying to the interior is possible. An airtight air barrier at the finished ceiling is not needed, assuming that there is enough foam over the decking to keep the sheathing warm.
On the other hand, a tight air barrier using careful installation of the drywall or one of the intelligent (and expensive) new vapor retarders would provide extra protection against moist air getting into the ceiling cavity. Definitely avoid unsealed recessed lights, unsealed electrical boxes, and other unsealed penetrations in the ceiling.
Best of luck with your project.
I forgot about the ratio! For zone 5 I think I am supposed to target 40% above the deck and 60% below. I appreciate that you are trying to teach me the reason and the math. So if my target total is r6O then I should have r24 above and r36 below. Correct?
My people are not familiar with exterior insulation and I was trying to keep it super simple by using a foam with applied nailbase. Unfortunately the highest r value is 21 with these products. To achieve a higher r value I would have to add a layer of foam:( I am concerned about the success of the install. I honestly don’t know what to do. I am down to the wire. About to give up and just spray some ccsf u see the deck to achieve an overall impermeable r24 and hope to heck water doesn’t get into my sandwich and rot my deck sheathing.
Thanks again! – Andy
If you assume an average outdoor temperature of 30°F, then you’re right – 40% insulation above and 60% below the roof deck.
If you’re after simplicity, how about just spraying 7 ½ inches of closed-cell foam between your 2x12s for a total of about R-49, as required by the IRC for Zone 5. Or if you want to save a bit, use 4.5 inches of foam for R-29 and 6 inches of fiberglass/cellulose for an added R-21.
To go from R-49 to R-60 is not going to save you much due to the Law of Diminishing Returns, which governs the savings from insulation. The principle is that every time you double your R-value, you cut your heat loss in half. After a while, there’s not much left to save, so it is no longer economical to add more insulation. This is the basis of the “optimal” level of insulation in all energy codes.
Let’s say your total heat loss through your roof would be $1,000/year with no insulation (R-1 for the roof sheathing). Increasing the R-value to R-49 would cut the heating cost to $20 (1/49 X $1,000). Increasing the R-value to R-60 would save you an extra $4 – obviously not worth the extra money. The reason building codes require more insulation in roofs/attics than walls is not because “heat rises.” It’s because it’s generally cheaper to insulate attics than walls. With cathedral ceiling using closed-cell foam, these assumptions are out the window, but the code is the code.
Anyway, that’s my 2.5 cents. Best of luck!
Well.. for some reason I seem to be stuck on thermal bridging issue and considered ci (continuous insulation) a good thing. But seems I cannot make it work!
How about 1-2 inches of Styrofoam or beadboard (more permeable) under the drywall – a lot easier and cheaper than adding layers of stuff over the decking. Thermal bridging is real, but assuming the cavity is filled with insulation, you’ve got 12 inches of wood from the indoors to the top of roof deck. At R-1.4/inch, that’s R-17. The framing represents about 10% of your roof surface area. Back to the math, that’s a heat loss through the studs of 1/17 x $1,000 x 10% = $5.80 per year (based on $1,000 heat loss through the roof). Maybe it’s really 2-3 times that, but you get the point. I wouldn’t worry to much about it.
Preventing Moisture Build-Up in Cathedral Ceiling
We live in Kingston, NY and built a cathedral ceiling in the main living space of our 1925 raised ranch. The rafters are 2×6’s. we used R21 fiberglass batts (5 1/2 ” thick) and 1-inch tongue-and-groove pine boards. on the interior. The roof is an unvented shingle roof. Is there an affordable solution to prevent humidity build up?
In cold climates, unvented roof cavities can lead to moisture problems. If moist air gets into the roof cavity, the moisture can condense on the underside of the cold roof sheathing and damage the sheathing or roof framing. Homes with high interior moisture levels are at risk, especially if there is are easy route into the roof cavity – for example, through recessed lighting fixtures in the ceiling.
Many contactors use closed-cell foam for unvented ceilings, because the foam, if properly installed, blocks moist air from entering the roof structure. Fiberglass batts, however, will allow moist air to easily pass through and reach the sheathing.
The best solution is to create an effective air barrier on the interior face of the roof structure. However, this is nearly impossible to do with tongue-and-groove boards.
Ideally, you would have installed an air-barrier material prior to installed the wood finish. One of the new “smart” vapor retarders, such as MemBrain or Intello would be best as these one-way membranes prevent air and moisture from entering the roof cavity, but allow the roof cavity to dry to the interior of the building if moisture builds up. That is, they allow moisture to pass one-way into the living space.
Another option would be standard drywall, carefully sealed at seams and edges to block airflow. You could leave it unpainted or use a standard latex paint and install the wood over this. As long as you don’t use a vapor-barrier paint, drywall is highly permeable to water vapor. You never want to use foil, plastic, or any impermeable material on the underside of an unvented roof as the structure cannot dry out if it gets wet from interior moisture or a roof leak.
Also, keep household moisture levels under control in the winter – no more than 50% RH. Sources of excess moisture may include wet basements, uncovered dirt in a crawlspace, firewood drying in the basement, and kitchens and baths without exhaust vents. Small, tight houses with many occupants will tend to have much higher moisture levels than large, leaky houses with fewer occupants. If you’re seeing moisture condensing on the interior of insulated-glass windows, your moisture levels are too high.
Thank you so much! Your response is incredibly helpful. Unfortunately, we already did 3/4 of the ceiling and are not in the financial position to rip it out and start over. So I guess vigilant monitoring is what we’ll be doing. One last question: would a pellet stove in the main area (15×25′) help with moisture?
Again, thank you for such a quick and thorough response.
Vigilant monitoring sounds like a good option at this point. Look for water stains on the ceiling or other evidence of dripping water.
And, yes, a pellet stove that is drawing its combustion air from inside the home will definitely keep the air dry (maybe even too dry for comfort). As household air is used by the stove, it is replaced by cold outdoor (makeup) air leaking into the house. Cold air holds very little moisture, so its relative humidity drops as it heats up. That’s the reason indoor air is so dry in leaky houses in cold climates.
Just make sure the stove has an adequate air supply — a potential issue in tightly built houses. Check with an HVAC contractor or energy specialist if you are unsure. Best of luck!
Is It OK To Have Air Space Below The Foam?
Hello. We have a new construction roof with 2×10 rafters. We plan to spray 4 inches of closed-cell foam on the underside of the metal standing seam roof–which should be an excellent vapor & thermal barrier. Is it ok to leave the remaining 6 inches between the rafters empty? Or should we fill w/batts or cellulose down to the ceiling? I know the 4″ of foam is enough to do it’s job as a vapor & thermal barrier, so the extra insulation seems pointless (this is NOT a “flash & fill” situation to save money), but the energy auditor is emphatic that the airspace between the ceiling & spray foam must be insulated. Why is this, and please share your thoughts on the best course of action.
Not sure why your energy auditor is concerned about an air space below the foam. Too much air-permeable (fibrous) insulation below the foam can potentially cause problems, but not too little. On the other hand, adding fiberglass or cellulose here is a cheap way to increase your R-value and energy savings, and may be required to meet your local energy code.
Four inches of closed-cell foam provides about R-28, which will not meet the energy cold in many parts of the US. Typically codes require R-30 to R-49 in cathedral ceilings, depending on the climate and local energy code.
The IRC (International Residential Code) requirements for foam in cathedral ceilings are pretty confusing. The code require a minimum thickness of closed-cell foam whether it is above or below the roof sheathing. The required levels appear in Table 806.5 of the 2012 and 2015 IRC, and range from R-5 in the deep south to R-30 along the Canadian border.
Where the foam is above the sheathing, the goal is to keep the sheathing at a minimum of 45° F to prevent condensation on the underside of the sheathing. Basically, the more fibrous insulation you use below, the more foam you need above to keep the sheathing above 45° F.
The foam thickness is less of an issue when the foam is below the sheathing, as in your case. The more insulation, the colder the sheathing will be, so the critical issue becomes the air sealing that keeps moisture away from the sheathing.
With respect to condensation, the concern is having too much fibrous insulation below the foam (which cools the underside surface of the foam or sheathing). Having too little, or no insulation, below the foam will not lead to any moisture problems.
If you do add fibrous insulation, it should be installed tightly to the foam for best performance. Also, adding fiberglass or cellulose here provides good bang for the buck – the reason some people like the “flash and fill” or “flash and batt” approach.
You can view a copy of the DOE Energy Zone Map here.
Also see additional details on IRC Table 806.5.
Best Way To Insulate Cathedral Ceiling?
I’m building a new house in CT. The roof will have a cathedral ceiling with a simple design. If foam on top of the roof deck is too hard for my builder, what is the recommended (cost-effective) under- the-deck approach that will get me to R60? Thicker joists? TJI rafters? (The walls are R30+).
One concern is with thermal bridging with standard insulation between the rafters. What is your advice to reduce thermal bridging?
Short answer is that there is no easy way to achieve R-60 in a cathedral ceiling without using foam somewhere in the system. There are many ways to do this and no industry standard. Every builder seems to have his own preference after some trial and error.
At R-3.5 per inch for most fibrous insulations, you would need 17 inches on insulation, plus a 1-2 inch vent space (2 inches performs much better). Your options are very deep I-Joists or building down with to thicken the rafter space. TJI’s can work well, but are expensive and require special detailing that the contractor may not be familiar with.
Builders have used different approaches to building down the framing depending on the thickness needed. The most common is probably 2x4s or 2x3s nailed across the rafters to eliminate thermal bridging. For even thicker structures, some have built gussets out of plywood to hold a 2×4 (parallel with the rafter) at the desired depth. Typically, you would want a gusset every 16 to 24 inches. Thermal bridging is largely eliminated due to the gussets.
Another approach, which uses less space, is to build a conventional vented roof with fiberglass or cellulose and then add a layer of foam board — as thick as needed for the R-value desired. Foil-faced polyiso provides the most R-value per inch.
The simplest for the builder, and most expensive, is an unvented cathedral ceiling with open-cell or closed-cell spray foam. At about R-6/in. for closed cell foam, you would need about 6 inches. To save money, some builders spray a couple of inches of foam and fill in the rest with fiberglass or cellulose. If you go with foam, use a reputable, experienced installer as this is a tricky material to work with.
Debates about the performance and problems of unvented vs. vented cathedral ceilings will probably go on for many years. I try to avoid unvented “hot” roofs unless it’s the only feasible option. This is sometimes the case in remodels or in very complex roofs that are impossible to vent properly. The thermal performance of foam is very good, but thermal bridging through the rafters degrades the performance somewhat.
With any cathedral ceiling, an airtight air barrier is vital to keep household moisture out of the roof cavity. Spray foam, done correctly, takes care of this problem. With foam board as your bottom layer, taping seams and foaming the perimeter can be very effective.
Avoid recessed lights and carefully seal around any penetrations for plumbing, wiring, or chimneys, etc. Most moisture problems in cathedral ceilings can be traced to a leaky air barrier.
My first choice in your situation would be a vented cathedral ceiling with foil-faced insulation below, carefully taped and foamed to create a near perfect air barrier. This is familiar territory to most builders, eliminates thermal bridging, and is fairly foolproof. Although as an engineer friend once said, “No system is really foolproof; the fools are just too clever.”