More Building Matters
There are lots of ways to install weather-tight windows. You might choose to install a sloped rough sill, a back dam, or both. Your sill pan could be site-made or manufactured from plastic or metal. You can protect the inside perimeter of the rough opening with house-wrap or a self-adhering membrane. For flashing, you
can choose between tapes and liquid-applied membranes. You even have options for fasteners—there are nails and screws that can do the job. If you’re following good window-flashing principles today, the window manufacturer’s instructions likely inform the process, and you probably improve on them for added reliability.
For many window installers in North America, though, one major step in the window installation and flashing process probably isn’t a decision consciously made: whether the mechanically fastened water-resistive barrier (WRB)—the housewrap—is installed over the walls before the windows are installed (housewrap first), or the housewrap is installed after the windows (windows first). If you scan through window-installation articles and videos by Fine Homebuilding and other building resources, you’ll notice that most of them profile the housewrap-first approach. This bias is reinforced by window manufacturers’ instructions. Many present the housewrap-first approach to window flashing exclusively; if they do include the windows-first approach, it’s often deeper in the guide or presented as an alternative.
We all fall into building habits based on what we learned early in our careers and how things are done in our areas. Many home-building practices are regional. The windows-first approach isn’t as widespread as the housewrap-first approach, but in some parts of the country, you’ll find it used regularly.
You may be skeptical of the idea of installing the windows before the housewrap, but it could be an approach that you find helpful when project conditions…
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4 Comments
Mike,
Thanks for the tips. I'm especially pleased with the description of the wind-driven rain deflector that I have never seen before.
One thing I notice with both many articles and manufacturer's instructions is the absence of metal head-flashing. Omitting it isn't an option here as it is required by our code, but I'm curious as to whether it isn't a standard feature in many other regions?
Hi Malcolm I'm surprised you haven't heard of the rain deflector. A couple window manufacturers (like Marvin) include it as an option at the end of their installation instructions.
There's nothing in the International Residential Code that requires a metal head flashing. The code references that windows be installed according to manufacturer's instructions. So if the window company calls for an additional head flashing then we'd have to install it. If the instructions did not call for the head flashing then we aren't required to install it.
I do think head flashings are a good practice especially since most window frame heads are flat so water lingers on top. A separate metal head flashing can easily be sloped to shed water faster. I probably should have included a discussion of head flashing in the video. Especially since there are different opinions how to integrate the wall leg of head flashing with the WRB (tucked behind WRB vs. taped to face of WRB vs. applied to face but not taped).
MG
Mike,
Thanks for the thoughtful rely.
I wonder if the absence of a rain-deflector and the use of metal head-flashing here may have something to do with rain-screen cavities being required in our code? Adding a gap makes the sill less vulnerable to wind-blown rain, but also makes the head more vulnerable to water entering the cavity at the head horizontally. Our code also mandates end dams on the head-flashing to mitigate the risk.
It would be nice if the IRC required back-venting cladding in high rain zones. It would certainly reduce the risk of wind-blown rain reaching the wall framing. End dams on head flashing make sense when the flashing top leg is fairly flat. It seems that when the top leg is sloped about 15deg or greater that the water drains off the front and less likely to move horizontally. I think if the IRC required head flashings with end dams on flanged windows builders' heads would explode.
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