Taping base flashing
Hi all,
Building out in Nova Scotia. We have a frost-protected shallow foundation so the solution that I settled on was EPDM covering the skirt foam (acting as a ground gutter) with coil stock extending up onto the sheathing. So, when backfilled, you’ll see a ~6″ strip of metal.
The coil stock runs up the sheathing 8-10″. Overkill, likely. Anyway, here’s the question:
Should I tape the top of the coil stock to the sheathing? Any moisture concerns with doing so? I’m thinking about the benefits of diverting any water that gets behind the wrb versus allowing some outward drying of the lower sheathing. The coil stock will be punctured by rain screen and siding nails.
Many thanks!
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Replies
Running the coil stock 8-10" up the wall isn't overkill, it's a mistake. Your vapor drive is primarily outward and any moisture that reaches your sheathing won't be able to dry to the exterior. If you have cellulose insulation, the moisture might be able to migrate up and out before it causes problems, but I wouldn't bet on it. I would stop the metal at the sheathing or maybe 1" up at most, and seal the connection with a vapor-permeable tape such as Siga Fentrim.
Ry_Buc,
I would tape the transition between the concrete and framing to air-seal, but not to stop water getting behind the flashing. As some point we have to be confident the exterior layers - cladding, rain-screen gap and WRB - will keep out water from the outside. Maybe not entirely, but enough that while it might occasionally add some small amount of moisture to the sheathing, there won't be enough to require draining. If water is flowing down the surface behind the WRB, the whole assembly had failed, and the presence of the tape won't save it.
Well, dang. I based the solution off this BSC article. https://basc.pnnl.gov/resource-guides/flashing-bottom-exterior-walls
Perhaps I took the coil height too far?
I should add that the first floor is primarily unheated garage + small utility room. Vapor barrier is intello with tescon vana taped seams. You still believe there is risk of vapor drive outward at the base of this wall?
Appreciate you calling it out before I moved on to the next step. Not too late to bring out the nibblers.
It's impossible to say. There would likely be less internal moisture generated than in a home, but you're also closer to grade than ideal--the BSC article shows 8" minimum. Vertical legs on the type of flashing shown are usually 1-2".
Okay. Shorten up the vertical legs and no / vapor permeable tape. Thank you guys.
I don’t think it’s that big of a problem. Step flashing on roof to wall is at min 4”, and often times has a peel and stick detailed underneath at the corner, effectively negating the drying potential. I don’t think it needs to be that high, but I find it hard to believe that this will cause a moisture problem from a drying potential point of view. I agree that you should install a tape at the bottom of the sheathing - concrete.
This is all said from a gut reaction and not with any data to back it up.
freyr_design,
Where is that line? Peel & stick around window openings, under flashings, or around wall penetrations. What width becomes problematic? I don't know either.