Builders who follow the prescriptive requirements of the 2012 International Residential Code (IRC) in Climate Zone 6, 7, or 8 are required to install a minimum of “20+5 or 13+10” wall insulation. What does this mean? According to an explanatory footnote in the code, the “First value is cavity insulation, [and the] second is continuous insulation or insulated siding, so ‘13+5’ means R-13 cavity insulation plus R-5 continuous insulation or insulated siding.”
Here in Vermont (a Climate Zone 6 state), builders have been framing walls with 2x6s for at least 35 or 40 years. Nobody installs R-13 insulation in walls in Vermont, so the most likely way that Vermont builders will comply with this code provision is to install R-20 insulation between the studs and R-5 rigid foam insulation on the exterior side of the wall sheathing.
While this approach meets minimum code requirements, it violates a tenet of good wall design: namely, that any rigid foam installed on the exterior side of wall sheathing needs to be thick enough to keep the sheathing above the dew point during the winter. (For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)
If you want to eliminate an interior vapor barrier, the code requires thick foam
Cold wall sheathing is more likely to be damp than warm wall sheathing, so the “20+5” requirement is problematic. There’s more, however: the 2012 IRC still maintains antiquated vapor barrier requirements. In section R702.7, the code notes that “Class I or II vapor retarders are required on the interior side of frame walls in Climate Zones 5, 6, 7, 8 and Marine 4.” That requirement has always been unfortunate, but it has proven to be hard to change.
Of course, if you’re building a wall with exterior rigid foam, the wall can no longer dry to the exterior; it needs to be able to dry to the interior. An interior Class I vapor barrier prevents inward drying, and a Class II vapor retarder reduces inward drying to a considerable extent. Fortunately, code writers have come up with an effective but clumsy way to address walls with rigid foam; since 2007, the IRC and IECC have allowed builders in some cases to omit an interior Class I or Class II vapor retarder (substituting a Class III vapor retarder — in other words, ordinary latex paint).
In Climate Zone 6, for example, the code allows builders to omit the Class I or Class II vapor retarder if the walls include “insulated [exterior] sheathing with R-value ≥ 7.5 over [a] 2 x 4 wall” or if the walls include “insulated [exterior] sheathing with R-value ≥ 11.25 over [a] 2 x 6 wall.” (The provisions are found in Table R702.7.1 of the 2012 IRC, which is reproduced as Image #2 in the photo gallery at the top of the page.)
These code-mandated minimum R-values for exterior rigid foam correspond to the recommendations I made in my article, Calculating the Minimum Thickness of Rigid Foam Sheathing. Unfortunately, though, the code doesn’t allow builders to eliminate the Class I or Class II vapor retarder if they’re building a code-minimum 20+5 wall.
Why does the code’s prescriptive table (Table 402.1.1 in the 2012 IECC and Table N1102.1.1 in the 2012 IRC) require an insulation method — 20+5 — that most building scientists consider risky? (The table is reproduced in the photo gallery as Image #3.)
The short answer is that the code was not written by building scientists; its provisions are historical accidents resulting from political battles and compromises.
I first pointed out the serious problems with the current code’s wall insulation requirements for Climate Zones 6, 7, and 8 in a 2011 article, An Overview of the 2012 Energy Code. Unfortunately, in the three years since that article appeared, there hasn’t been any organized effort to fix the “20+5” problem.
To summarize the problems:
- The code establishes a minimum prescriptive requirement for walls that mandates a layer of rigid foam that is too thin to keep the wall sheathing above the dew point during the winter. If builders choose to follow this minimum code requirement, wall sheathing can accumulate moisture and eventually rot.
- The code requires the installation of an interior Class I or Class II vapor retarder — often interpreted to mean a polyethylene vapor barrier — on walls meeting the minimum code requirement of 20+5. In many ways, this is the worst-case scenario: you end up with a wall that can’t dry outward and can’t dry inward, with exterior foam that is too thin to keep the sheathing above the dew point.
- The code does not include any provisions for the use of smart vapor retarders.
Imagining a better code
Vermont regulators recently held public hearings to discuss possible changes to Vermont’s building code. Although I was unable to attend the hearings, I heard that one of the topics under discussion was: How do we reconcile the requirements of the 2012 IRC and 2015 IRC with building scientists’ warnings about the dangers of thin exterior foam?
Fortunately, the energy consultants who have been attending these hearings understand the dilemma they face. One of these consultants, Richard Faesy of Energy Futures Group, told me, “We want to get the exterior insulation detailing and building science right.”
Several approaches are possible. Here are some suggestions.
Remove code requirements for interior vapor retarders. Any attempt to come up with rules to regulate the vapor permeance of building materials is fraught with problems. On houses with certain (forgiving) types of wall assemblies, an interior vapor barrier may not cause any problems. However, houses with less forgiving wall assemblies, especially air-conditioned houses, may develop problems if they include an interior vapor barrier.
Since interior vapor retarders (especially Class I vapor retarders) on walls and ceiling cause as many problems as they solve, there is no logical reason for building codes to require them. All such requirements should be eliminated — especially in light of the fact that an increasing number of Climate Zone 6 homes include air conditioning.
Would this code change allow some builders to design bad wall assemblies? Of course. But they’re already doing that. To minimize potential problems, building codes should do a better job of regulating the airtightness of building envelopes.
Require builders who install exterior rigid foam to specify foam that is thick enough to keep the wall sheathing above the dew point in winter. This principle is the basis for existing requirements found in Table R601.3.1 in the 2009 IRC and Table R702.7.1 in the 2012 IRC (Image #2).
If code writers want to retain the format of the prescriptive table (Table N1102.1.1 or R401.1.1 in the 2012 codes) listing minimum R-values for walls (Image #3), then a logical way to describe minimum R-values for walls with rigid foam in Climate Zone 6 would be: “20+11.25 or 13+7.5.” (The corresponding requirement for Climate Zones 7 and 8 would be: “20+15 or 13+10.”)
Even better would be a paragraph that explains what this means this in plain English.
Needless to say, the code needs to distinguish rigid foam products that have a low vapor permeance (foil-faced polyiso, XPS, and EPS) from mineral wool panels that have a high vapor permeance. There is no need for exterior mineral wool insulation to adhere to minimum R-value requirements.
One possible objection to my proposal: by requiring thicker exterior foam, this proposed code change will increase minimum R-value requirements for walls. There are two possible responses to this objection:
- So what? Is that a bad thing?
- The use of rigid foam shouldn’t be mandatory. If code writers want to allow R-25 walls, that’s fine. There are ways to build R-25 walls that don’t cause rot. But code writers will have to come up with consistent method for calculating the R-value of walls — one that is easy for code enforcement officers and builders to understand and use. Up until now, no one has wanted to do that. (This task is complicated — but it’s possible.)
Include a “fanfold” exception. Thin rigid foam is sometimes acceptable, especially if it is somewhat vapor-permeable. Some brands of 1/4-inch-thick or 3/8-inch-thick fanfold foam insulation that are typically used under vinyl siding are made of uncoated EPS and are therefore fairly vapor-permeable; there is no reason to ban the use of such products. Unfortunately, enforcing this fanfold exception would be tricky, because it would require building inspectors to know how to research the vapor permeance ratings of building products.
Simplicity is a virtue
The code changes I have proposed are fairly simple, and that’s good. A prescriptive code should strive for simplicity. Any builder who wants to build an assembly that doesn’t meet prescriptive requirements should be free to comply with the code by following an alternative performance path. (In Vermont, regulators propose to establish maximum HERS Index goals for builders who prefer to pursue a performance path.)
It might be argued that thinner exterior rigid foam could work as long as a home is equipped with a smart vapor retarder. But any code that tries to reflect that fact will need to define a smart vapor retarder, and that’s more complicated than it appears.
It can also be argued that thinner exterior rigid foam will work as long as homeowners successfully maintain a low level of interior relative humidity. That’s true — but good luck with enforcing that method of compliance.
Readers are invited to propose their own code changes
Code writing is tricky. I’m not an expert at code writing, but I know bad writing (and bad advice) when I see it.
Here are the hallmarks of a good building code:
- It is written in plain English, using the same vocabulary usually heard at construction sites. (And I don’t mean, “Remember to install the friggin’ vapor barrier, you idiot!”)
- It is flexible enough to allow designers and builders to come up with new ways of achieving desired goals.
- It achieves regulators’ goals without any undesired side effects.
- It is simple enough for builders and code enforcement officials to understand.
- It is easily enforceable, without the need to look up permeance values, R-values, or U-factors on manufacturers’ web sites.
My proposals are probably flawed and are unlikely to be implemented. I look forward to reading better suggestions from readers.
In the meantime, one fact is undeniable: any building code which describes the prescriptive requirements for a wall in Climate Zone 6 as “20+5” while simultaneously requiring this wall to have an interior Class I or Class II vapor retarder is nuts.
It’s time to fix the code.
Martin Holladay’s previous blog: “Every House Needs Roof Overhangs.”