How To Install Kraft Faced Insulation In Walls

If you install fiberglass batt insulation* with a kraft newspaper vapor retarder in a abode, which style do you face the vapor retarder? To the inside of the habitation or the outside of the home? For many building science questions, the answer is, 'It depends.' For this one, the answer is articulate.

SPOILER Warning: The answer is right below this photograph so if you'd rather wait and discover out when you see the motion picture in the theater, don't read whatsoever further.

The answer is that it doesn't matter . Nope. Y'all tin can install the paper facing nevertheless you want—as long as the building inspector † lets you, of course.

Why doesn't it thing?

Kickoff, the kraft paper is a vapor retarder meant to reduce the potential for moisture problems caused by diffusion. Sounds like a good idea, but the vast bulk of wet issues are acquired by air leakage, not improvidence, even in places like Maine. Do the air sealing; stop worrying so much about vapor retarders.

Second, if yous install it the wrong way, it's unlikely that you lot'll accept a trouble. I suggest you read Joseph Lstiburek's paper, Heed the Gap, Eh! The graph below, from that paper, shows water vapor permeance as a role of relative humidity.

As you can see, the permeance rises as the relative humidity rises and hits 10, the point at which we depict a material as vapor permeable, when the RH is 60%. The issue here is that If you lot put the kraft paper on the wrong side and it gets moisture, information technology won't trap moisture. The wetter it gets, the ameliorate information technology dries. If you put information technology on the right side, where the humidity is, it'due south not much of a vapor retarder.

Also on the graph is the permeance of polyethylene. As Dr. Joe says in the article, "Plastic vapor barriers really are vapor barriers when things get wet. Not so asphalt saturated kraft paper. And most walls with asphalt saturated kraft paper thank the building science gods for the deviation."

What about the advice to put it on the 'warm-in-winter' side?

If that really mattered, exercise yous think the US building code would take dropped the requirement to utilise paper-faced batts? The warm-in-winter suggestion says that if you're trying to limit the diffusion of water vapor, put the vapor retarder on the humid side of the wall, where…uh…information technology's not able to retard much vapor.

In a actually, really cold climate, it may matter, just even in Maine and Ontario, vapor retarder pigment would be a better manner to go. If y'all want to irksome downward the vapor diffusion, why non do it before it hits the drywall?

So but relax. If your building inspector wants you lot to put the kraft newspaper on the 'incorrect' side, take another await at the graph above and exist comforted that it doesn't actually matter. Then become get yourself some unfaced batts (and do your best to install them to Course I quality).

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*Speaking of fiberglass batt insulation, Carl Seville (a.m.a. the Green Curmudgeon) wrote another article on poorly installed batts recently, with photos of Knauf fiberglass. Rather than being a dandy, similar Guardian did to me a couple of years ago, they commented on the commodity and asked for a dialogue on how to become better installation in the field. Kudos to Knauf!

† My friend Abe Kruger likes to say that you lot should treat building inspectors like wild animals. Y'all accept to approach slowly and quietly because they spook easily. In the HERS rater course I taught in Toronto this calendar week, I used that analogy but earlier I could get to the reason why they're like wild animals, ane of the students said, "You mean we can shoot them?"

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Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and the founder of Free energy Vanguard. He has a PhD in physics and writes the Energy Vanguard Blog. He is also writing a volume on edifice science. You can follow him on Twitter at @EnergyVanguard.

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