Mixed-humid insulation recommendations
I’ve read a great deal about building science and energy-efficient building practices. This site is a great resource. I think I understand the theory, but I have a few questions that are perhaps best answered by experience.
I live in southeast Pennsylvania, in climate zone 4A, and very near climate zone 5A. Because this is a mixed-humid climate, it is recommended that wall assemblies be able dry in both directions. Building Science Corp. recommends XPS exterior insulation to keep the sheathing warm and allow for some limited drying capacity to the outside (1.1 perms for 1″ recommended XPS). BSC also recommends keeping indoor humidity levels below 30% during the peak heating season in the northern parts of zone 4.
I’m contemplating renovations on our house, and would like to take advantage of needed repairs (like new siding, windows, and roof) to improve the house’s energy efficiency. It’s a 1500 sqft Cape (built 1950) with dormers in the second floor bedrooms. I’d like to hit R25 wall and R60 ceiling if possible, based on BSC recommendations. I think with proper insulation and air-tightness it could be conditioned with mini-split(s) and supplemental gas furnaces (room, not central).
We will be renovating parts of the interior, so a gut retrofit of double-wall insulation isn’t out of the question. While that would allow for a bidirectionally vapor-open wall, it leaves the sheathing cold and takes away floor space. Alternatively, exterior foam keeps the sheathing warm, but limits drying potential. Exterior mineral wool seems ideal, but might be more expensive that foam, and perhaps unnecessary.
Is 1 perm sufficiently vapor-open in a mixed-humid climate, or are my drying-potential fears misplaced?
Is cold sheathing a problem in Zone 4A? I’ve read the blog about exterior foam, but I’m unclear if that includes double stud walls. With 3-month average temperatures of 35F, I think there is condensation potential on the sheathing.
What’s the best way to address the roof? Exterior foam is almost definitely needed to reach the desired R-value, but would definitely eliminate exterior drying at the necessary thickness.
Thanks in advance.
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Replies
Jeffrey,
My advice differs from the Building Science Corporation advice (at least as you have summarized it).
Exterior rigid foam works very well, as long as the proper thickness is specified, and as long as proper water-management details are included.
My own philosophy for this type of retrofit is that you shouldn't try to choose a high-permeance foam under the assumption that you will get significant drying through the foam. Instead, you should specify foam that is thick enough to keep the exterior sheathing above the dew point during the winter. Once you do this, the wall should still be designed to dry in two directions, with the rigid foam considered as the center of the wall assembly. Anything on the interior side of the rigid foam should be able to dry to the interior; and anything on the exterior side of the rigid foam (i.e., the furring strips and the siding) should be able to dry to the exterior.
This approach is spelled out in the following article: Calculating the Minimum Thickness of Rigid Foam Sheathing.
Fortunately for you, your climate zone (Zone 4) is mild, so your wall sheathing won't ever get cold enough to get you in trouble. You have wide latitude when you choose the thickness of your exterior foam. You will be choosing a foam thickness that helps you meet your R-value goals for your whole wall assembly, rather than worrying about dew points and your sheathing temperature in January.
Thanks for your reply, Martin.
My sheathing condensation fears stem from the Ted Cushman article 'Robust Walls' in JLC. Lstiburek outlines a method for estimating condensation potential in which the average temperature of the coldest three months is compared to the dewpoint of air at interior temperature and humidity (70F and 35% humidity gives 40F dewpoint). When I run the numbers for my area, I get an average temperature of 35.5F.
I've read the 'Minimum thickness of Foam Sheathing' article before, and was surprised that the recommendation in that article differs from the calculated value, which suggests placing 20-25% of the insulation outside of the sheathing.
Perhaps I'm splitting hairs here, but I do like understanding the reasoning behind the recommendations. The outside design temperature is an average, so there should be warmer periods during the winter that offer drying potential to the exterior sheathing and thus limit net moisture accumulation. I think I've read some posts by Dana Dorsett suggesting this mechanism. Do I understand this correctly?
Jeffrey,
The method mentioned in the Ted Cushman article (which is also explained on the GBA site here: Are Dew-Point Calculations Really Necessary?) is more precise than the "consult the table" method provided in the International codes. Assuming that you have correctly performed the calculation, it makes perfect sense to prefer the results of the calculation over the guidance provided in the table.
There are many variables, of course. Outdoor temperatures vary from year to year, as does indoor RH. These calculations help builders make judgments, but they aren't absolute determinations. When in doubt, thicker exterior rigid foam is safer than thinner foam.
Thank you, Martin.
Higher foam-R is always going to trump higher-perm foam, but 1.5lb (Type-II) EPS is about 2.5-3x more permeable than XPS at any given thickness, if only 85% of the R-value at that thickness.
A vapor permeance of 1 perm is not super-fast drying for bulk water incursions, but enough to mitigate seasonal diffusion moisture issues coming through 3-5 perm paint from an air-tight interior. If there is even small amounts of air-transported moisture due to leaky drywall it's much better to have sufficient exterior R to manage it- 1 perm on the exterior by itself is going to be too vapor tight. Its the boundary between Class-I and Class-II vapor redardency- if you had 1 perm on the interior you wouldn't have much risk to the sheathing from interior side seasonal vapor drives, but it would limit the drying rate toward the interior by 70-80% compared to latex paint. It's still a fairly powerful vapor retarder.
The fact that XPS performance falls to about EPS levels within a 50 year lifecycle of a house, and the fact that it's more vapor tight, using EPS at the appropriate R is a preferable way to go (since it's performance is fairly stable over time.)
Dana,
Thanks for clarifying the greather importance of foam insulative value relative to foam permeability. I like the suggestion for EPS because it's also a more environmentally-friendly foam.
I'm very interested in exterior mineral wool because it provides the necessary insulation with high permeability. The rock wool projects documented on GBA are interesting, but the installation seems less straighforward and more expensive than foam sheathing.