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 provides a high R-value per inch as well as air and vapor control in one application, assuming it is done correctly. However, most building inspectors will not allow spray foam 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). The foam board can be nailed to the bottom of the rafters, which is pretty straightforward, or cut into strips to fit in between the rafters — the s0-called “cut-and-cobble” approach, which can be very labor intensive and difficult to do well.
Insulation and Moisture
In most cases, 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. Moisture problems are more common in roof spaces than walls due to the “stack effect” — warm, moist household air moving upward toward to roof in cold weather.
Moist indoor air could leak into the roof cavity through openings for ceiling lights, plumbing vents, wiring, or framing details that create gaps. With foam board, gaps in the insulation at joints and around the perimeter can also allow moist indoor air to reach the roof 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 indoor moisture levels include storage of wet firewood, slabs without a sub-slab vapor barrier, or a very wet building site without proper surface drainage and subsurface moisture control.
The other problem with unvented, 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.
Unvented spray-foam roofs dry out very slowly, and almost entirely to the interior, which is why you should never use an impermeable vapor barrier, such a poly sheeting, under the finish ceiling. If a vapor retarder is required by code, it is best to use one of the expensive “smart” vapor barriers such as MemBrain or Intello. With “cut-and-cobble” insulation, you will also want an air-tight “air barrier” such as well sealed drywall to keep household air out of the roof cavities. With properly installed spray foam, an additional air barrier is usually not needed.
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 an airtight ceiling air barrier, with no air leakage into the cathedral ceiling – a critical detail for all cathedral ceiling designs. You can add a layer of rigid insulation below the fiberglass and seal it with tape and canned foam to form an airtight air barrier (illustration below).
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. Also you must have adequate ventilation along both the lower and upper edge of the roof, and continuous venting channels in every rafter bay for effective ventilati0n. Even with limited airflow in the channels, wind and vapor diffusion also play a role in reducing moisture in the roof cavities.
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 preserve headroom, you can cut strips of foamboard to fit between the rafters. In either case, carefully seal any gaps around the perimeter of the foam board with spray foam. Any gaps in the foamboard at seams or around the perimeter can result in condensation on the underside of the sheathing.
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 iso-board provides the highest R-value per inch for any type of insulation.
— Steve Bliss, BuildingAdvisor.com