More Best Practices
The first time I used a self-adhered water-resistive barrier (WRB) was in 2019, on my own home, a 1950s Cape Cod fixer-upper. The walls had 15-lb. asphalt impregnated felt over board sheathing. I wanted to improve the water management and airtightness of the structure but didn’t want to upgrade the sheathing. I had been researching self-adhered (SA) WRBs and decided they were the right approach for what I was trying to accomplish.
I’ve heard a few building scientists say as much as 10% of the water that contacts cladding during a severe, wind-driven rainstorm can get behind the cladding. Of course, the primary function of the WRB is to keep moisture from moving to water-sensitive building components behind the barrier.
We know, too, that any moisture that ends up inside the wall—whether from a construction defect that allows water to bypass the WRB, or from air leaks that provide a pathway for water vapor to move into and assembly and possibly condense on a cold surface—needs a way to dry in at least one direction. This drying process by way of vapor diffusion is slow. When outward drying is desired, the wall needs to have some level of permeability but remain both water and airtight. Many self-adhered WRBs fit the bill.
Working with WRBs
Before we get into installing a window when using a SA WRB, we should first go over working with a self-adhered membrane. Basically, these products are large pieces of tape, with a slightly lower adhesive bond than you would find in quality construction tape. I used Henry Blueskin. Blueskin, which comes in widths up to 4-ft. by 100-foot-long rolls. It can be used either horizontally or vertically—the only difference being the overlap of the seams. Horizontal applications require a 2-in., shingle-style overlap; a 3-in.…
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10 Comments
Thorough, clear description, Randy. SAMs are in the growing category of products I haven't installed myself, which makes me cautious about specifying them on the projects I design. Is it possible for a single person to install it? I assume not, considering how challenging just installing 3' ice and water shield can be.
Another question: do you think the sealant behind the flanges is important? I feel like it's mostly a vestige left over from "the old days" and I usually just run tape over the flanges and onto the WRB and slightly onto the window frame, if the flange isn't integral. But your comment about potential wicking isn't something I had considered.
Hey Michael,
It is possible to install a SAM solo, I did it on my personal home. A vertical installation worked though I had to pick and choose my days based on wind speed. Personally, I like the product, they are more labor intensive and the WRB costs more, but I feel they are more effective and durable than mechanically attached. I recently tested a sample of Protecto Wrap's Wall VP which uses a synthetic butyl adhesive that supposedly works below 0°F. I'll be testing that products application in those low temps this winter.
As far as a sealant behind the windows, every window I've installed requires the sealant per the manufacturer's instructions. I recently had a conversation with Andersen, they are recommending staying away from the adhesive/sealant technology (like Lexel) because it makes future window replacement much more difficult. You're probably right, an old method, but an inexpensive belt and suspenders approach.
I'm curious as to how regional the practice of using a metal head-flashing is? Our code mandates them here in BC. Is their use not that widespread?
Hi Malcolm,
Personally, I use metal head flashings on all my projects, and I believe most window manufacturers recommend some sort of flashing, but I think it could be a material other than metal, such as plastic. The International Residential Code requires a flashing for all planned penetrations, though it also does not specify what the flashing is made of. That being said, a lot of the homes I visit to perform energy audits and building investigations are missing this detail. I think a lot of builders think the integrated nailing flange is enough.
Thanks Randy. I was curious as none of the builders I know in the Canadian prairies use them either.
Randy,
You showed a drip cap installation a while back that had a bend around the top corners of the window. This makes good sense and is easy to do by cutting the drip cap just a bit wider than the window. Show this again if you have it because water can find it's way in at the end of the drip cap.
Doug
Doug,
Here is a link to the head-flashing with end-dams. Particularly important when the wall includes a rain-screen gap.
https://hammerandhand.com/best-practices/manual/1-flashing/1-2-head-flashing/
Thanks for the details Malcolm!
Randy,
Our code requires end-dams too, and even specifies their height. I've no idea how you find out the 1-in-5 driving rain pressure.
"c) terminate at each end with an end-dam
i) with a height in millimetres not less than 25 mm or 1/10 the value of the 1-in-5 driving rain wind pressure in Pa."
Hey Doug,
Here's a photo of the turned down head flashing. Some of the photos in the article were taken on a mockup, I didn't install the head flashing for those photos, I probably should have. This photo is my home showing the flashing detail. I prefer the upturned detail more, but I'm not very good at bending the z-trim in that fashion. Aron Jones (#Bigdogconstruction) makes it look easy.
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