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,…