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 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 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 using foil-faced polyisocyanurate foam (iso-board) such as R-Max or Thermax. The easiest approach is to nail the boards below the rafters (see photo) 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
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!