Christopher Solar had a simple plan for an addition to his Ottawa home. The one-room structure would have a shed-style roof with a cathedral ceiling and vertical board siding. Solar liked a wall assembly he’d read about at GreenBuildingAdvisor, which consists of exterior foam, batt or blown insulation in the stud cavities and airtight drywall on the interior. An interior polyethylene vapor retarder never entered the picture.
And that’s where his story gets complicated.
“I have been warned by my designer that the building department here is not likely to approve anything that does not include the poly [vapor retarder],” Solar writes in a post at GreenBuildingAdvisor’s Q&A forum. “If that’s truly the case, what are the options for a better-than-average insulation system (average around here being stud cavity insulation only thermal bridging be damned)?”
If Solar is forced to install interior poly, he wonders whether it’s safe to install any amount of exterior foam on the walls or the roof for fear of trapping moisture inside wall and roof cavities.
“I know some foams are vapor-permeable, but if I have say 1 in. or 2 in. of exterior [extruded polystyrene] foam, is that really a wall assembly that can dry to the outside?” he asks. “My instincts say ‘no’ but if I’m wrong please let me know.”
Solar’s dilemma is the topic for this Q&A Spotlight.
Try a different type of interior sheeting
It may be tempting to think the local building inspector could be educated about the risk of trapping moisture inside walls with a poly vapor retarder. But, says GBA senior editor Martin Holladay, “many builders have found that it’s harder to educate a building inspector than it is to train a cat.”
An alternative, Holladay suggests, would be to use another material in place of the poly, something like CertainTeed’s MemBrain, a “smart vapor retarder” with variable permeance.
“It seems that most building inspectors just want to see something that comes in a roll installed on the interior side of the studs, so this solution often satisfies them,” Holladay says. “While the expense is unnecessary, it’s often cheaper and faster than battling city hall.”
Excellent idea, says Anthony Ratliffe. “The problem is that building inspectors in most provinces of Canada are rather simple followers of the law, over which they have no influence other than asking their local municipal authority for a change in the relevant by-law,” Ratliffe writes.
Inspectors are often well qualified technically, he says, but don’t have much discretion for allowing significant variations to local codes. “What is significant?” he adds. “That point is indeed discussable with the inspector, and may offer a way forward in a given case! Normally, the local code incorporates all or many parts of several national codes/standards by reference (e.g. electrical, plumbing, insulation, ventilation, structural etc.). At the local level, these are considered ‘safe’ and thus practically sacred to individual more or less technically ignorant elected councillors (I am one, so know how the system tends to work).”
Given that reality, Ratliffe says, go ahead and use a product like MemBrain and convince the inspector that amounts to a non-significant variation, or hope he doesn’t notice the switch.
Using MemBrain shouldn’t be a problem
Marc Labrie found himself in a similar situation. He wanted to avoid the use of a poly vapor retarder, and so he met with the local inspector to discuss his options.
As long as the product has an approval number from the Canadian Construction Materials Centre, Labrie writes, there shouldn’t be a problem with using it. And in fact, MemBrain is the subject of a CCMC evaluation report, which found that it “can serve as an alternative solution that will achieve at least the minimum level of performance for vapour barriers and air barrier system with respect to condensation control…”
Ratliffe still thinks that the inspector’s point of view is key. “Provided you can persuade your local Inspector that what you want to do lies within the by-law as written plus his allowed discretion, you are fine,” Ratliffe writes. “It is just that most local by-laws are pretty inflexible in their implementation. They vary quite a lot between different locations. If you don’t want to use a poly vapor barrier, and the local code requires it, that is where the problem lies and one has to find a work-around.”
When using exterior foam, include a rainscreen gap
While not professing to be an expert, Allister Tobias says that if he were going to install 2 in. or more of rigid foam insulation over wood sheathing, he would leave a capillary break “between the two.”
“My read is that there is movement away from having [more than] 2 in. of rigid XPS or EPS (a vapor barrier) directly on OSB or plywood without some kind of avenue for moisture drainage between the two,” Tobias says.
Wrong, replies Holladay. “The reason that you don’t want a 10 mm (3/8 inch) gap at the location you propose is that you don’t want any air flow between the exterior insulation and the insulation between the studs,” Holladay writes. “With air flow, the exterior insulation becomes useless. As long as the exterior foam is thick enough to keep the wall sheathing above the dew point during the winter, then the sheathing will be warm and dry all winter long. The only danger occurs when the sheathing gets cold and damp.”
He adds, “There are two potential sources of moisture: rain and condensation (or accumulation) of interior moisture carried by leaking air or diffusion. The rain is handled by good flashing and a rainscreen gap (10 mm if you like, or more) between the siding and the rigid foam. The potential for accumulation of moisture from leakage of interior air or diffusion is handled by making the rigid foam thick enough to keep the OSB or plywood warm.”
Our expert’s opinion
We asked GBA technical director Peter Yost for his thoughts. Here’s his reply:
Using “smart” retarders: This is a perfectly good solution to the issue in this spotlight; see the GBA Product Guide for three listings for smart vapor retarders. In some areas of the country (including here in southern Vermont), these products aren’t easily available and have to be special ordered, which is a shame, given how well they work in cold climates and with many building assemblies.
Concerning the impact of exterior rigid insulation on drying to the exterior: For many building materials, including rigid foam, the perm rating of the material is inversely proportional to the thickness. So if a 1-inch thick sample of a material like XPS has a perm rating of 1, the permeance drops to approximately 0.5 for a 2-inch thick sample, and 0.33 for 3 inches. So, yes, it’s not a good idea to count on significant drying to the exterior with XPS insulation.
On the other hand, other types of insulation, such as mineral wool, are much more vapor-permeable, and should be considered in assemblies for which drying to the exterior is required.
Concerning building inspectors and the code: Every code has a section such as R104 of the International Residential Code that covers the authority of the building code official. This section gives any building official “… the authority to render interpretations of this code and to adopt policies and procedures in order to clarify the applications of its provisions.”
This means that any building code official can take data or other information and use this insight to grant reasonable exceptions to the letter of the code while maintaining the intent of the code. The trouble is that many code officials are very reluctant to use this authority because it puts their neck on the line. If building code officials understood building science as I think they should, many more would feel empowered to make exceptions to antiquated ways of thinking (and code language) about the ways that today’s assemblies work from a hygrothermal perspective.