Using Spray Foam for Unvented Vaulted Ceiling Assemblies
Recently, I worked on a jobsite that called for spray foam. This project is a small shed, one of those you can buy at Home Depot—like a Tuff Shed. It’s being converted into a detached office, and one of the challenges here is dealing with the rafters.
These rafters, or technically, ceiling joists, form a vaulted ceiling with 2x6s. If the building code requires R30 insulation, there are a couple of issues with a vaulted ceiling like this.
First, you can’t get the full R-value, the full R30, with regular fiberglass batting in a 2×6. An R30 fiberglass batt is about 8-11 inches thick depending on the manufacturer, which is too thick for these joists—that’s problem number one. The second issue is that with a vaulted ceiling like this, it’s considered an unvented ceiling assembly. There are no vents at the top or at the eaves.
Without ventilation, if you’re heating this room, like in this case, which will have a mini-split, things can get tricky, especially on colder days. Let’s say you went with just fiberglass batts and have the mini-split pumping in hot air. The thermostat might be set at 70°F, but the hot air will rise and accumulate at the ridge. It could get up to 80-85°F at the ridge while outside temperatures might dip down to 30 or 40°F at night. This temperature difference can lead to condensation issues. which WILL cause mold without a vapor barrier.
I’ve seen cases like this when I used to work in roofing, where customers thought they had leaks. They’d remodeled without using a vapor barrier and just went with fiberglass batting. They’d call back, thinking the roof was leaking because of black mold, water, and leaks appearing, usually around the peak. But when we took the roof apart, there were no signs of leaks—it was due to being an unvented ceiling assembly without a proper vapor barrier.
With spray foam, you can get a ton of R-value in a smaller space, along with a vapor barrier that’s both air- and water-impermeable, so you get 100% of the R-value. Fiberglass batting, by comparison, only gets around 60-70% of its R-value, depending on install quality. Spray foam has many great applications.
Another benefit is that spray foam adds a tremendous amount of shear strength to the building. With structural engineering, you have three main forces: compression, tension, and shear. Compression is a force pushing down on the wall, tension pulls it apart, and shear moves side to side. This shed already has plywood for strength, but adding spray foam insulation doubles down on that stability.
In this case, the customer chose 4 inches of closed-cell spray foam, giving a full R30. Depending on code and framing, some people opt for 2 inches of closed-cell spray foam, which provides a full vapor barrier, and then add fiberglass batting on top—a system known as “flash-and-batt.” With this setup and fiberglass batts on the walls, this will be a comfortable and efficient space to work in.
For more information, check out the our video on Youtube.
https://www.youtube.com/watch?v=cs-x6qSjzwg