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.
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