Q: What is the best way to insulate an unvented cathedral ceiling in a detached workshop? I plan to insulate before installing an AC and heating system (thinking mini-split). Is closed-cell spray foam the best, or only, good option in this case? I don’t plan on finishing the space with drywall, although wouldn’t rule it out in the future. .And given that it’s only a workshop with occasional use, I only want the minimum insulation (for cost reasons) to prevent moisture issues. I’ve living in Region 5 in Pennsylvania. Hoping you can advise. – Paul
A: Assuming you have full access to the joist spaces from below, you could certainly spray foam up against the roof sheathing. In general, closed-cell foam is the best choice for unvented cathedral ceilings. It is also one of the most expensive, and most building inspectors will not allow it to be left uncovered (for fire-code reasons).
Spray foam is a very expensive option, however. Foam-board insulation would be a more economical, especially if you are supplying the labor (see photo).
Insulation and Moisture
In most cases, however, insulation will not help prevent moisture problems. In fact, insulation can lead to moisture issues if not done properly. The reason is this: In cold weather, insulation in the roof cavity makes the roof sheathing colder (the sheathing is insulated from the warmth below). The colder roof sheathing is then more likely to form condensation or frost on its underside if household moisture reaches the sheathing. The same thing occurs in walls with cavity insulation.
The moisture could get into the ceiling from warm, moist indoor air leaking into the roof cavity through openings for ceiling lights, plumbing vents, or framing details that create gaps. Gaps in the insulation from installation errors can also expose the sheathing.
However, since this is just a workshop, most likely the indoor air will not have the high indoor moisture levels (from kitchens, bathrooms, houseplants, etc.) that can cause moisture problems. Other sources of high levels of indoor moisture include storage of wet firewood, a slab foundation without a sub-slab vapor barrier, or a very wet building site without proper surface drainage and subsurface moisture control.
Roof Leaks
The other problem with foam-filled roofs is that water from a roof leak can wet the sheathing and framing, causing wood decay, long before you see any evidence of a leak on the interior. Trapping water between closed-cell foam underneath and waterproof roofing above is a recipe for mold and wood decay.
These roofs dry out very slowly, and almost entirely to the interior, which is why you should never use a vapor retarder under the finish ceiling with an unvented roof. If a vapor retarder is required by code, you can use one of the expensive “smart” vapor barriers such as MemBrain or Intello. Whether or not you use vapor retarder, you want an air-tight “air barrier” such as well sealed drywall to keep out moisture from the house.
Vented Roof Options
There are certainly other options. You could insulate the roof the traditional way. That is, create a vent channel above the insulation with low and high vents. Then insulate below with fiberglass or cellulose, add a vapor retarder and your finish ceiling. You’ll want a tight ceiling air barrier, with no air leakage into the cathedral ceiling – a critical detail for all cathedral ceiling designs.
Research has shown that the amount of airflow in the vent channels of low-slope roofs is very limited for roofs with less than a 3/12 pitch. However, wind and vapor diffusion also play a role in reducing moisture in the ventilation channels. Also you must have adequate ventilation along both the lower and upper edge of the roof, and continuous venting channels in every rafter bay.
If you really don’t want a finished ceiling below, but you do want some insulation, I would consider using foil-faced polyisocyanurate foam (iso-board) such as R-Max or Thermax. The easiest approach is to nail the form boards below the rafters (see photo above) and tape all the seams. Or to save space, you can cut strips to fit between the rafters. In either case, seal the foam edges well to the rafters with spray foam. For this size job, you would want a large foam canister and professional spray applicator. Most codes allow foil-faced foam to be left exposed. Plus you get the highest R-value per inch for any type of insulation.
If you provide more details, and maybe a photo, I might be able to provide more detailed information. — Steve Bliss, BuildingAdvisor.com
Vic says
Where To Put Air Barrier In Vented Cathedral Ceiling?
I have a tiny building, 8×24 with a pretty steep pitched vented roof, I am re-insulating as my loose insulation compacted down to the eaves from the roof line.
A family member said to use 1 inch Foamular 250 as my vent channel, then put my paper-backed insulation which would provide the air seal once taped. I would then put up paneling or tongue-and-groove boards. Is this OK?
I read something about double vapor barriers. But if there isn’t spray foam sealing off the foam board. would that be OK because if moisture got in it could get out. Or do I need to seal it off? So paneling, kraft paper-backed insulation, foam board, vent space, then roof. Also, I have a metal roof so my place gets pretty hot, no shade.
buildingadvisor says
A vented channel under the sheathing of 1-1/2 to 2 inches will provide good ventilation as long as you have adequate vent strips or all along the low end and high end of the roof. Code calls for a min. 1 in. channel, but 2 in. works much better.
To create the baffles, you can use just about any durable material. Manufactured baffles, osb/plywood, or foam board are the most common. The main advantage of using foam board is that you get more R-value than using a non-insulating material. Also the foam board is relatively easy to cut, very durable, and easy to air-seal with spray foam. Using foil-faced foam board will provide a radiant barrier, which will help with your overheating problem.
If you have air-sealed the foam board to the rafter bays, you do not need any additional air sealing below the insulation. In a very cold climate a Class II or III vapor retarder, such a kraft paper or painted drywall is recommended on the interior side of the insulation. In a hot or temperate climate, its best to have no interior vapor retarder.
The most common place for the air barrier is below the insulation. The least expensive option is usually carefully taped drywall, sometimes with gaskets or sealants where needed. Taping Kraft paper well enough to create an airtight barrier sounds difficult to impossible. Poly sheeting is a poor choice as you don’t want a Class I vapor barrier in any ceiling – it does not allow any drying to the interior.
If you don’t want to use drywall, try this: Use inexpensive plastic or cardboard baffles under the sheathing with unfaced batts or blown-in insulation filling the rafter bays. Then nail your 1-inch foam board to the bottom of the rafters, tape the seams, and seal the perimeter. Then you’re ready to install the paneling (and have eliminated the thermal breaks at the rafters).
I don’t know your climate zone or the building use. If it has a high moisture load in a cold climate, your need to take extra precautions or the moisture may end up in your roof cavities. In a dry building (like a workshop) in a moderate problem, you’re unlikely to have moisture problems. In a hot, moist climate, you need to be concerned about moisture driven inward by solar energy. In a hot, arid climate, moisture is usually not a concern.
Vic says
Thank you for replying I appreciate any advice you can give.
It will be a tiny house with a kitchen and bathroom, that I will live in full-time.
Climate is Mediterranean I believe, not sure of the zone. In the last 8 years we have had summers up to 112 degrees F for a couple of weeks, we have lots of rain…like a lot of rain, winters have seen everything from a lot of rain to 19 inches of snow overnight to freezing rain for a week with -15F temps and loss of power frequently in the winter for minimum of 7 days. Eventually I will have a wood stove.
For the unfaced batts, would fiberglass or roxul mineral wool be better as I don’t want it to compress creating voids again.
buildingadvisor says
That’s a pretty extreme climate. Everything I wrote earlier still holds. Just a couple of other reminders:
1) The quality of the ceiling air barrier is much more important than the roof ventilation, which doesn’t always work as intended. If no moist air leaks into the roof structure, you won’t have problems with condensation under the roof. Of course, roofing and flashing leaks are always a concern and are much more likely to cause roof damage than air leaks from the interior.
2) In such a small house, keeping indoor humidity levels under control will be important. If you have water condensing and running down the interior side of insulated glass, then your indoor humidity levels are too high and could lead, over time, to damage to the structure. You can address this in two ways:
• limit the sources of moisture, such as house plants, drying firewood, humidifiers, wet basement, etc.
• Increase the level of indoor ventilation. Start with point-source ventilation such as bath fans and kitchen range hood. Then add whole-house ventilation if needed. This could be a heat-recovery ventilator (air-to-air heat exchanger) or a simple method of running one of your bath fans on a regular schedule using a timer.
Regarding the type of batts, either fiberglass or mineral wool would be fine. Settling is common with loose-fill insulation (fiberglass or cellulose), but is generally not a problem with batts. Maybe the voids you discovered were due to sloppy installation more than compression.
Keeping a full depth of roof insulation over the eaves requires special framing details unless the rafters are sufficiently deep. Some contractors raise the rafters where they attach to the top plate to make room for extra insulation over the plate. The traditional connection with a birdsmouth cut in the rafter at the top plate requires that the insulation be trimmed there to allow airflow from the soffits into the roof channels.
Using a layer of foam insulation on the underside of the rafters is a good way to provide adequate insulation at the eaves.
Tyler says
How Much Ventilation Needed for Cathedral Ceiling?
Hi,
Got a 16×20 two story shop that needs insulating. Live in zone 6 semi arid yet potentially prolong snow loads in winter. Concerned about how to insulate and air seal upstairs ceiling. I believe it’s a vented cathedral. The top has ridge center and at the moment there are no soffits just a solid board with gaps along the edges. One question is how many holes in each side to drill for soffit vents and the other is would the rigid foam board with foil on one side cut and spray foamed into the rafter spaces be okay with let’s say drywall on the rafters? The inside roof decking already had a radiant barrier type coating on it. Pitch is I believe 6/12 . There isn’t and won’t be plumbing just electrical. Thanks for your input!!
buildingadvisor says
It is difficult to get adequate ventilation for a cathedral ceiling with vent holes. Continuous vent strips are a better bet. It’s important to get ventilation into every rafter channel. The code requirement of a net free vent area (NFVA) equal to 1/300 of the attic floor does not really apply to cathedral ceilings. Researchers who have studied cathedral ceiling ventilation recommend 10-20 sq. in. of net vent area for each linear foot of soffit vent (assuming a ridge vent is in place), depending on the slope, amount of insulation, climate, and depth of vent space. They recommend a min. 1 1/2 in. air space under the sheathing for ventilation, 2 in. if possible.
FOr the radiant barrier to work, you need to make sure it faces a vent space. YOu can use any type of foam you want between or under the rafters. A layer under the rafters is the easiest to air seal (with tape and spray foam) and reduces thermal bridging through the rafters.
Experts debate the effectiveness of ventilation in cathedral ceilings, but it does help to some extent with ice dams, cooling loads, shingle longevity, and drying potential if the roof structure gets wet. But it has to be part of a system that also includes an airtight air barrier that keeps household air (with its moisture and heat) out of the rafter bays.
Jeff says
This is what I did in my family room with rafter baffles from the soffits all the way to the roof vents, then I faced baffle insulation, then foil faced 1” foam board. Foil Taped all my seems. Before we ever got to putting up the tongue and groove ceiling we have moisture dripping all over from under the tape. We live in central Minnesota. Thoughts?
buildingadvisor says
Is this new construction or are you retrofitting the insulation to an existing roof structure? If it’s new construction, the moisture source could be the new roof/ceiling joists that are drying out in their first heating season. Moisture problems related to new construction often resolve themselves after the first year.
Otherwise, it sounds like there is a significant air leak into the roof cavities and maybe high moisture levels in the air inside your home. Do you know what the relative humidity is inside the house? Do you use bath and kitchen exhaust fans? Have a wet basement, lots of house plants, or firewood drying inside the home? These are common sources of excess indoor humidity.
As for air leakage, do you have any holes in the foam board for ceiling lights or other plumbing or electrical penetrations? Did you get a positive seal around the perimeter of ceiling.
Finally, the source of the moisture could be windblown snow or rain that entered through the ridge vent. Most ridge vents have either wind baffles or filter materials to block entry of windblown moisture, but these systems are not 100% effective.
I’m also wondering about what type of tape you are using to seal the foam board. The tape used to seal seams in insulation is generically called “contractor’s sheathing/seaming tape” or vapor barrier tape. It’s often red like the 3M Construction Seaming Tape. These tapes have very aggressive adhesive that sticks to just about anything and is highly water-resistant.
If water is leaking down through the tape, then maybe air is leaking up through the seams and carrying moist air into the roof cavity.
Do any of these explanations fit for you?
Jeff says
Thanks for the quick input. It is a retrofit because the previous installation had no air baffles. We had to take down the old ceiling and insulation and cut in air passages from the soffits. The ridge vent does have a mesh type filter. I don’t have a real scientific answer on humidity, but we have a indoor/outdoor sensor in the room and it’s about 38%.
I didn’t know that there was a difference in tapes. We used foil tape that is used on ducting. Maybe I should pull this off and reapply the proper tape? Any recommendations on tape? The foil tape is peeling up all around the perimeter and across seams and this is where the drips are coming out.
The warmer the day, the more moisture. There are 3 modern clip-in recessed lights. Not cans. They are only about 1/2” thick. I’m not sure the best way to seal around these. Now that there is moisture there again, do you recommend pulling down the foam and allowing it to dry before re-sealing, or should I just retape and allow it to dry naturally? It’s not soaking wet by any means, but there is enough that it drips. I’m wondering though, the baffles are only 14 1/2” wide while the rafters are 16” oc, does the insulation touching the roof deck allow for moisture to form? Or is this because the air is leaking through the wrong tape used?
buildingadvisor says
Moisture problems often take some detective work to identify the source. It sounds like your indoor humidity levels are OK, so my guess is that a lot of household air is leaking into the roof cavities. In cold weather, the warm air in your house wants to escape through the roof due to the stack effect. The air will find its way through any small seams and gaps in the ceiling. There is probably frost from condensed moisture on the underside of the roof sheathing that melts on warm days and drips through gaps in the foam.
As mentioned in the original posting, a 2 in. vent space performs much better than a 1-in. space in tests. But the 14½-in. wide baffles should be fine for rafters 16 in. o.c.
Foil-faced tape is a good choice as long as it is UL-listed – that should be printed right on the tape. However if it is peeling, that’s a sign that the adhesive is not providing a good seal. Cleaning the foil facing with a swipe of isopropyl alcohol before applying the tape can help insure a good seal. Also, using a rubber hand roller on the tape can helps make a good bond.
I have mostly used acrylic tapes designed for construction and vapor-barrier joints, such as 3M Construction Seaming tape. But the foil tape designed for ductboard is also a good choice.
As for the recessed lights, sealing them from below is difficult as there are usually a lot of cutouts and holes in the housing unless you purchased special airtight fixtures. With access from above, contractors can build or install airtight covers on IC(insulation contact) -rated units, but this is not an option in a cathedral ceiling.
From below, you can seal around the perimeter of the light housing with canned foam. But it is difficult to seal the rest of the fixture without voiding the light-fixture warranty and possibly creating a fire hazard. Most modern fixtures have built-in overload protection, and modern CLF and LED bulbs create a lot less heat. Still if you want to avoid any risk of overheating, you may be best to use an airtight fixture such as the LED Retrofit Module from Cooper Lighting.
If you can remove the tape and clean off any tape residue with isopropyl alcohol, you may be able to reseal the joints after everything dries out. But that probably means waiting until much warmer weather arrives. Hopefully, you won’t have to remove and reinstall the foam board.
Reid says
Insulating an Unvented Cathedral Ceiling
Our summers in northern California can be pretty hot & I am looking to add more insulation to my attic.
-the attic floor is insulated already
-there is a ridge vent
-there are 2 gable vents, 1 has a fan
-there are 2 between rafter vents
-there are no soffits or eaves, so no vents there
So I wanted add insulation between the roof rafters. I can’t seem to find any information online about my particular design because I have no soffits or soffit vents. If I insulate do I still need to add or make soffit insulation baffles?
Thanks!
buildingadvisor says
First you need to decide whether you want the attic to be conditioned (insulated, heated, and cooled) or unconditioned space.
If I understand you correctly, you have treated the attic as unconditioned space by insulating the attic floor and venting the attic with gable-end, roof, and ridge vents.
If you don’t plan to use the attic for living space, you could add more insulation above the floor. This could be cellulose, unfaced batts, or rigid foam board. You could add a new floor on top of the insulation if you want to use the space for storage or need it for access.
If you want the attic to be conditioned space that is heated and cooled, then your best bet would be an unvented cathedral ceiling — sometimes referred to as a “hot roof”. You could leave the ceiling insulation in place, providing some additional insulation for the living space below.
Experts have debated the pros and cons of hot roofs for decades. I’m not a big fan, but they can work if done carefully and the roof is well-maintained and free of leaks. In a mild and dry climate like California, you should probably be fine If you follow these recommendations:
• Place the insulation between the rafters or below them.
• Use closed-cell spray foam, closed-cell foam board (extruded polystyrene, such as Dow Styrofoam), or foil-faced iso board.
• Seal the insulation or the ceiling surface below against air leakage. You don’t want moist air leaking into the roof cavity.
• Do no use a vapor barrier under the insulation. If required by code, use one of the “smart” vapor retarders such as Membrain.
If you chose spray foam, you can save money by spraying a couple of inches of foam against the underside of the sheathing and filling the rest of the cavity with cellulose or fiberglass. The install drywall and seal it well at the edges to prevent air leakage.
You can read more at these links:
Cathedral Ceilings, Insulating with Spray Foam
Cathedral Ceilings, Unvented, How to Seal
Paul Loughnane says
Hi Steve
I very much appreciate the quick and detailed reply. Yes – it’s completely open from below with the rafters and roof sheathing all exposed (see photo). My goal is simply to make it a reasonably comfortable workshop that I can heat and cool without needing to completely overkill on the HVAC due to zero insulation (although my usage isn’t going to be that high, so I’m not too worried about utility bills).
I didn’t plan on finishing the space so do like the sound of the iso-board solution – in that case can I go without venting or would that cause a problem if I wanted to finish it with drywall on top at a later date?
When you talk about venting, there are no vents to the outside currently, so are you talking about internal vents top and bottom to allow interior air to circulate? Would this need to be provided between all rafters?
My initial research led me down the path of closed cell foam but I didn’t realize how expensive it was (also needed to add intumescent coating if leaving it unfinished).
The second photo is of the adjacent garage space that’s part of the same building. While I’m only considering the workshop space for now, depending how that works out, I may do something similar in there at a later date.
Thanks,
Paul
buildingadvisor says
Hi Paul,
Thanks for the pics. Because of the roof framing layout, you would have a difficult time venting from the lower to upper edge of the roof. You would need to leave a 1-1/2 to 2-in. vent space under the sheathing and drill holes or cut notches in the cross pieces for continuous venting. And you would probably end up with minimal ventilation for your efforts.
Some people nail flat 2x4s on top of their existing roof and build a new roof to create a vent space, but it would be hard to justify the cost for a workshop.
So an unvented roof is probably your best option. As for iso-board vs. spray foam, the iso-board would be a lot cheaper – especially if this is a DIY project. To simplify things, and save a lot of time cutting and fitting, you could just nail the iso-board to the bottom of the rafters. Seal the seams with contractor’s tape and seal the foam board edges to the wall with spray foam.
If everything is well-sealed and the workshop space is pretty dry in the winter (no hot tub or drying firewood), then I wouldn’t worry too much about roof problems caused by interior moisture. I’d be more concerned about roof leaks – make sure you don’t have any currently – and maintain the roof well. When it’s time to reroof, cover the whole thing with peel-and-stick membrane as an underlayment or consider a singly-ply membrane such as EPDM, which can last 30 to 50 years.
To be extra careful, you might consider adding vents at the top and bottom of the roof – soffit vents at the bottom and a soffit vent or shed-roof vent at the top, depending on the detail. Then drill a few one-inch holes in the cross pieces along the middle of each 2x, but not too close to the ends. See our Notching and Boring Guide for where to drill or notch.
With closed-cell spray foam, in theory, you get some drying toward the interior – but not much. With foil-faced insulation, you get none, so the ventilation space is a good idea. Most likely you will never have a problem, but better safe than sorry. The air space above the insulation will provide some relief in the event of a roof leak, even without ventilation. It’s better than having the water sandwiched between the sheathing and spray foam.
As for how much insulation to use, the minimum would be a 2-inch board, which will get you about R-12, plus a little extra from the foil facing. R-12 may not sound like much, but you’ll have continuous insulation with no thermal bridging through the rafters. Compared to what you have now – about R-2 maybe – you’ll be reducing heat loss through the roof by about 80%. If you double the insulation from R-12 to R-24, you will only reduce your heat loss by another 10% for a 90% reduction. That’s how the numbers work for insulation – the biggest bang comes with the first increase in R-value – the so-called Law of Diminishing Returns.
You can always add a finished ceiling later, either nailing/screwing through the foam or adding a layer of strapping first for easier nailing and another insulting air-space. With a shiny foil surface and a ¾-inch air space, you get a boost of about R-2 for heat flow upward (winter) and double that for heat flow downward in summer.
That’s what I’d do if it was my shop. Being in northern Vermont, I might price out 3-in foam if you can get it, or figure out a way to install R-15 3-1/2-inch fiberglass batts above the foam (you’d have to add some 2×4 joists between the structural one) but the additional savings would be modest.
Paul Loughnane says
I think the iso-board attached directly to the bottom of the rafters is the way to go for all the reasons you’ve outlined (simplicity, cost). I’ll look around to see what thicknesses are available but 2″ seems to be the standard in-stock item at the big-box stores. Lowes has a 2″ Johns Manville product that is R13.
Do I need any kind of vapor barrier or such between the board and the rafters – I don’t see why I would if the board is impervious to moisture but wanted to ask as I saw pictures of this being applied in some of my internet searches!
I understand the need to seal to the interior well, but what about the air pockets between the board and exterior – there may be some minor air leakage to the exterior down around where the rafters meet the tops of the walls – should I seal all that up so the air pockets are totally sealed or does it matter?
And final question (I think) – am I right in thinking that the roof is by far the bigger source of heat gain and loss, but if I wanted, could I use the same method on both the wood stud and cinder block exterior walls , simply attaching the insulation directly on top and sealing as before?
I can tackle most projects and am very careful and thorough when I know what to do, but am simply lacking the know-how/experience in this case so do very much appreciate your time and advice.
Thanks, Paul
buildingadvisor says
In answer to your questions:
1) No, you do not need an additional vapor barrier with foil-faced insulation. The foil facing is impervious to moisture and air as long as you seal the joints (with tape) and seal the edges (with spray foam).
2) Regarding air leakage from the block cores into the ceiling: Unless you are on a very wet site or you have wet snow piled against the unfinished block on the exterior, I wouldn’t be too concerned. Energy experts love to say things like “Make sure the ceiling is 100% airtight.” But in the real world, this is easier said than done. If you can block the tops cores of the block wall with spray foam, that would be good. It’s hard to give a definitive answer. If concerned, you can get a humidity gauge to make sure the indoor RH is below 40% in winter. If necessary (unlikely) you can run a dehumidifier as needed.
3) Regarding roof insulation vs. wall or floor: It is a common misunderstanding that roof insulation is more important than wall insulation because “heat rises.” Hot air does rise, but the temperature at your ceiling is, at most, a couple of degrees warmer than at the floor or walls. So the heat loss from conduction is about the same. Square foot for square foot, walls, ceilings, and floors lose heat at about the same rate. The reason that codes require a lot more insulation in ceilings is because attic insulation is generally cheaper than wall or floor insulation (because you can just blow in gobs of the stuff in an attic). So the cost-effectiveness equation points to more attic insulation than walls. In your case, adding insulation to your open stud cavities would be very cost effective.
In the summer, roof insulation is more important than walls and floor insulation for summertime comfort and air-conditioning costs. If that’s the case, a radiant barrier (or white, reflective roofing) is usually more cost effective than adding lots of high-cost foam insulation.
The economics of insulating cathedral ceilings is more like walls (expensive to insulate) than attics (cheap to insulate). But codes rarely make the distinction between attics and cathedral ceilings in the minimum insulation requirements. So some people go to crazy lengths trying to reach R-40 or R-50 in their cathedral ceiling while their money would have been better spent on better air sealing, better windows, more efficient heating system, or increased wall insulation. It’s a matter of optimizing your dollars spent on energy efficiency to get the best bang for the buck. Putting all your money into ceiling insulation while ignoring the walls, windows, and air-sealing is not going to produce the greatest possible savings.
Energy and moisture advice can be very confusing and sometimes contradictory. Also, there are site-specific issues (like moisture) so that what works in one building can lead to disaster in another – especially if there is an unusual source of moisture. So it’s definitely worth sorting out all the issues ahead of time. Best of luck with your project!