Dr. Joseph Lstiburek needs little introduction. The well-known Canadian engineer is a principal of the Building Science Corporation in Massachusetts. He’s also a regular GBA podcaster and Fine Homebuilding author.
On Wednesday, June 6th, I attended an all-day building science class presented by Dr. Joe in Westford, Massachusetts. As usual, his presentation combined salty language, corny jokes, light-hearted insults, and rock-solid building science information.
Although I’ve been listening to Joe’s presentations for at least 13 years, I learn something new each time I hear him speak. This time around, I harvested two news stories, one six-digit idea, and at least 16 interesting quotes.
News story #1: Don’t trust published perm ratings for vapor-retarder paint
Every now and then, someone posts a question about vapor retarders on GBA’s Q&A page. Over the years, I’ve often recommended the use vapor-retarder paint on interior drywall. Vapor-retarder paint is less problematic than interior polyethylene; moreover, the paint should satisfy a building inspector looking for a code-required 1-perm vapor retarder on a home’s warm-in-winter surfaces.
I have also recommended the use of vapor-retarder paint on cured open-cell spray foam installed against the underside of roof sheathing in cold climates. This method of vapor control was suggested to me several years ago by Joe Lstiburek, who told me that some type of vapor retarder is necessary in cold climates to prevent moisture accumulation in the sheathing.
Here’s the news: after delving into the nitty-gritty details of vapor-permeance testing of vapor-retarder paints, Lstiburek has concluded that the vapor permeance rating listed in the manufacturers’ specs (usually 1 perm) only occurs in a lab, not in the real world.
The 1-perm value is an artifact of the laboratory testing method usually used by paint manufacturers — a type of test that doesn’t…