Vapor Barrier or Housewrap?
Back when the idea of vapor barriers originated, they were primarily designed to stop the transmission of water vapor from inside your home into your wall cavities and into the joist space beneath your roof.
The traditional thinking has always been that, left to its own devices, the water vapor in your warm indoor air will migrate into these cold spaces and condense to form water or frost. Once that happens, structural damage is sure to follow.
What this means is that you probably don’t need a vapor barrier to prevent diffusion of moisture into your walls – at least not in areas with climates no colder than that of Spokane.
Even so, many housing experts still recommend the use of vapor barriers during construction, when they are cheap and easy to install. The reason? They help block air infiltration. And because infiltration can account for 30 to 40 percent of your heating and cooling costs, blocking it can save a lot of money and energy.
But are vapor barriers the best way to block infiltration?
Maybe not. Vapor barriers are always placed on the warm side of any wall, over the studs and just under the drywall. In this location, however, the barriers are pierced at dozens of places – at electrical outlets and switches, around doors and windows, and so on. As a result, air can seep in at each of these openings.
One answer to this problem is the ”housewrap,” an infiltration barrier consisting of a layer of plastic wrapped all around the outside of your house, either over or under the sheathing. There it can seal gaps in the sheathing, and at sills and soleplates.
And because it isn’t pierced by electrical circuits, it should have fewer gaps than a vapor barrier. DuPont, maker of Tyvek, the most popular infiltration barrier, contends that its product saves enough energy to have a payback time of around two years.
So the next time you have new construction, which you should choose, a vapor barrier or a housewrap? Right now, the jury is still out. Many of the builders I talk to are installing both. The way they see it, both are relatively cheap and easy to install, and both can work together.
In the meantime, what can you do in an existing home to help stop infiltration?
First of all, weatherstrip and caulk well outdoors. For the indoors, take a look at the accompanying sketch. You can run a bead of clear caulk around door and window trim and along the baseboards.
At electrical boxes, install foam gaskets under the cover plates. That should go a long way toward blocking infiltration. Of course, it will also probably increase indoor air pollution, but that’s another story.
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Tags: housewrap, vapor barrier