More Musings of an Energy Nerd
Wind-driven rain can penetrate almost all types of siding, including vinyl siding, wood clapboards, cedar shingles, stucco, and brick veneer. To reduce the chance that exterior moisture will cause sheathing rot, every wall should have a water-resistive barrier—that is, a layer of asphalt felt, plastic housewrap, or a similar product—between the siding and the wall sheathing.
Back in the 1980s, I would sometimes drive by a construction site and see vinyl siding being installed directly over OSB, with no signs of any housewrap or asphalt felt. Fortunately, that type of reckless behavior is a thing of the past. For most types of walls, installing a water-resistive barrier (WRB) behind the siding is not optional; it’s a building code requirement. The traditional material of choice for this layer is asphalt felt; these days, however, most U.S. builders use plastic housewrap. There are other options as well. Whichever material you use as a WRB, it must be installed carefully to perform as intended.
What does a WRB do?
The main purpose of a WRB is to resist the entry of rain that gets past the siding. It isn’t particularly useful to think of a WRB as an air barrier. Most WRBs are not sold as air barriers, although some products, when carefully installed, can reduce air leakage through a wall. It’s also important to emphasize that a WRB is not a vapor barrier; in fact, the vast majority of WRBs are designed to be vapor-permeable. If an ignorant builder installs a vapor barrier like polyethylene under siding, the result is often rapid sheathing rot.
Finally, it’s important to remember that no WRB is waterproof. At best, when installed properly, a WRB can reduce (rather than eliminate) the chance that wall sheathing gets wet.
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4 Comments
Martin,
The photo illustrating this blog of Mike Guertin installing house-wrap around a window with no head-flashing installed, echoes others I have seen on GBA over the years. It's not an option under our code here, but do you know if it is common practice to rely on tape instead of metal head-flashing in many climate zones - and I wonder what you think of that?
Malcolm,
I've always been a fan of metal head flashing. For more information, see my 2019 article, All About Flashing.
Malcolm, Mike goes into more detail about this method here: https://www.finehomebuilding.com/2024/03/06/windows-before-housewrap. He often writes about flashing, including here https://www.finehomebuilding.com/project-guides/windows-doors/drip-cap-design-installation and here https://www.finehomebuilding.com/project-guides/windows-doors/adding-head-flashing.
Michael,
Yet again I’ve worded my question poorly. I’m interested in the regional differences, not implying Mike isn’t doing things correctly. My anecdotal experience up here is that in many areas of Canada head-flashing isn’t used. I was more wondering whether it may be a bit like rain-screens, in that it is very useful in many climates, but perhaps not necessary in some.
Head-flashing deals exclusively with bulk water intrusion, not drying the wall to the exterior. If you have no rain-screen gap to provide drainage for, and the window frame is protected by trim, is head-flashing always useful?
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