If you want to slow down the vapor diffusion, why not do it before it hits the drywall? In a really, really cold climate, it may matter, but even in Maine and Ontario, vapor retarder paint would be a better way to go. building code would have dropped the requirement to use 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 … it’s not able to retard much vapor.
If that really mattered, do you think the U.S. What about the advice to put it on the “warm-in-winter” side? And most walls with asphalt-saturated kraft paper thank the building science gods for the difference.” Joe says in the article, “Plastic vapor barriers really are vapor barriers when things get wet. If you put it on the right side, where the humidity is, it’s not much of a vapor retarder, because that’s where it becomes vapor-permeable.Īlso on the graph is the permeance of polyethylene. The upshot here is that if you put the kraft paper on the wrong side and it gets wet, it won’t trap moisture. I suggest you read Joseph Lstiburek’s paper, Mind the Gap, Eh! The graph below, from that paper, shows the water vapor permeance of kraft paper as a function of relative humidity.Īs you can see, the permeance of the kraft paper rises as the relative humidity rises and hits 10, the point at which we describe a material as vapor permeable, when the RH is 60%. Second, if you install it the wrong way, it’s unlikely that you’ll have a problem. Joseph Lstiburek: Air Barrier or Vapor Barrier? Joe Lstiburek Discusses Basement Insulation and Vapor Retarders
Forget Vapor Diffusion - Stop the Air Leaks!
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