Q: I am renovating and old home with an uninsulated, non-vented walk up attic built in 1848. The shingle roof is less than 10 years old. The simple gable roof has 2×6 rafters. I’d like to apply 2 in. of closed-cell foam and then install 3 ½ in. of R-15 rockwool and add an Intello “smart” vapor retarder. The finished ceiling will be1/2” nickel gap shiplap.
Will this approach prevent condensation on the underside of the spray foam? I’m also considering using strips of foam board with taped joints and foamed edges – as an alternative to spray foam. I live in Westchester, NY in Climate Zone 5. — Andy
A: Combining spray foam with fiber insulation in an unvented cathedral ceiling (or wall) is often referred to as “flash and batt.” This is a pretty easy way to boost your R-value and get an airtight air barrier without a lot of fussy detailing or extra-deep framing. It provides many of the benefits of closed-cell foam (higher R-value, tight air barrier) without the cost of filling the whole cavity with foam.
The approach also has the downsides of spray foam – high cost and fact that most spray foams use a blowing agent that contributes to global warming. Some of the newer foams use less harmful blowing agents. Open-cell foam is more eco-friendly, but not recommended for this application because of its permeability to moisture.
Dew Points & Condensation
With flash-and-batt insulation, you need to use enough foam to reduce the potential for condensation on the bottom surface of the foam. This is the first cold condensing surface where water vapor in household air may condense into liquid water. The goal is to keep the interior surface of the foam above the dew point of the interior air for most of the winter. So even if indoor air leaks into the roof cavity, no significant condensation will form.
The colder the climate, the more foam you will need. It works like this: if 50% of the total R-value is in the foam and 50% is in the rockwool, then the temperature on the interior face of the foam will be 50% of the temperature difference between inside and outside. So if it’s 0° F outside and 70° F inside, the temperature of the foam surface will be 35 degrees F.
If 40% of the total R-value is foam, then the temperature on the interior surface of the foam will be 28° F (40% X 70).
Code Requirements
The IRC building code requires that the average monthly temperature on the interior face of the foam during the three coldest months be above 45° F (about 7° C) . In Climate Zone 5, that works out to R-20 foam with a total R-value of R-49, based on the 2021 IRC. The key issue is the ratio of foam to fibrous insulation. In this climate zone, the foam should account for at least 41% of the total R-value (see Table 1 below).
In your case, you have a total roof R-value of about R-25 and a foam R-value of R-12 to R-13 (for 2 in. of closed-cell foam). That puts your foam R-value well above the recommended 41% ratio for DOE Climate Zone 5.
Air Barriers and Vapor Retarders
There is no real consensus on the need for an air barrier and vapor retarder below the insulation.
Some contractors and energy experts say that you don’t need an interior air barrier or vapor retarder because the foam provides an excellent air barrier and is warm enough that you don’t need to worry about condensation.
With properly applied spray foam, and the right ratio of foam to fluff, this may be true. If you use “cut-and-cobble” foam board, however, it is harder to achieve a perfect air barrier. Also the wood rafters act as thermal bridges, and will tend to be colder than the interior foam surface.
In addition, workmanship on most job sites is less than perfect (and wood and buildings move over time). So I always prefer a belt-and-suspenders approach. For those reasons, I still recommend a tight interior air barrier and a moderately permeable vapor retarder (1-10 perms) with flash-and batt-systems. Ordinary latex or enamel paint on drywall works fine as the vapor retarder.
A smart vapor retarder like Membrain or Intello is also a good choice as it can provide a good air barrier, if installed carefully, in addition to a vapor retarder that becomes highly permeable when wet. This is an ideal product for this approach and would be a good solution with your shiplap ceiling.
It’s important to retain some drying potential to the interior in case the cathedral ceiling cavity gets wet despite your best efforts. The source of the moisture may be a roof leak rather than condensation from indoor air.
So a low-permeable vapor barrier like poly sheeting should never be used below foam insulation, whether it is foam alone or foam plus fiber. Any moisture that gets trapped between two impermeable materials will linger, leading to mold growth, wood decay, and possible insect damage.
Cut & Cobble
You also mentioned that you might cut strips of foam board to fit between the rafters as an alternative to spray foam. This is definitely cheaper if you provide free labor. However, this approach, sometimes called “cut and cobble” is very labor intensive and getting the required air seal takes effort.
You will want to tape all foam-to-foam joints with a high-quality construction tape that is suitable for the type of foam used. Also, you’ll need to foam the outside edges where the foam board meets the wood framing. Standard home-center canned foam is not up the task. A larger professional-style foam applicator is needed for the job. And wear gloves, mask, and throwaway clothing as spray foam is nasty stuff and impossible to remove from clothing.
So, yes, it’s doable, but definitely get a price from a spray foam contractor and you might think twice about doing this yourself. — Steve Bliss, Editor, BuildingAdvisor.com
Read more about Foam Insulation Thickness on Walls.
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