Installing a second layer of Tyvek Homewrap over the over exposed Tyvek?
orange_cat
| Posted in General Questions on
Due to delays Tyvek Homewrap reached the 4 months of exposure mark. The builder proposes installing another layer on top, without removing. DuPont permits either but advises reduced permeability if installed as a second lawyer.
Zone 5A, wall assembly from the outside in:
– Brick
– Ventilated cavity
– XPS rigid CI
– WRB (Tyvek that got overexposed)
– Plywood sheathing (SSD)
– 2×6 wood frame (SSD)
– mineral wool batt insulation
– polyethylene vapor barrier
– drywall
What would you recommend?
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Replies
You may be able to just use the XPS rigid CI as the WRB with seams taped/etc. It depends the XPS specific manf brand being used.
Detailing Tyvek to Tyvek sounds like a nightmare on a build with lots of windows/etc.
If the house wrap is behind the rigid, it is not exposed. I would not worry about it.
I have a neighbor that has had exposed house wrap for about 3 years and counting, not in the best shape but still there, this stuff holds up pretty well.
Unfortunately XPS is not installed. It is been just Tyvek over the plywood exposed to sun for 4 months. Windows were taped to the current layer of Tyvek. DuPont states that either replacement or laying a second layer on top is permitted, but there is reduced permeability with the second layer.
I do not have the exact brand of XPS.
I think they are saying that since you are going to put xps on, the Tyvek is just acting as a secondary WRB, so you shouldn't need another layer of Tyvek.
Also rigid xps boards when thick enough are impermeable anyways, so therefore your building envelope is already impermeable.
orange_cat,
The reduced permeance isn't a problem. Tyvek is 77 perms. Compare that to Typar at 12, and Tyvek Commercial at 28. You could argue regular Tyvek is too vapour-open.
It's the plywood ( (o.5 to 20 perms), and the XPS (0.5 to 1 perms) which will govern how much drying to the exterior you get.
Is this assembly a vapor sandwich? Poly on the inside and XPS on the outside?
matthew25,
It depends on the climate, and how thick the foam is.
https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness?oly_enc_id=7565D0080934G5L
Thank you all and especially for asking the question about polyethylene.
I have reread the linked (and linked from their) write ups, and I am still confused.
The wall assembly is (from the outside)
Brick (modular molded)
Ventilated cavity
XPS Rigid CI 1 inch [see the note below]
WRB (Tyvek Homewrap that will now will be two layers)
Plywhood Sheathing (SSD)
2x6 wood framing (SSD)
Mineral wool batt insulation (R-24)
Polyethylene vapor barrier
5/8 GWB
For XPS - 1 inch XPS was specified originally with R-value of R-7.5 which apparently is not easy to find in Ontario so it still 1 inch of XPS, with R-5.
Zone 5A.
What is the verdict on Polyethylene in this scenario? (Cannot have more than 1 inch XPS for architectural reasons).
orange_cat,
If you had room for the recommended amount of exterior foam for your climate, then you should not include an interior vapour-barrier. With just 1 " of XPS you need the poly to keep the wall safe, and it is required by code. What you are proposing to use a very common wall assembly in Ontario, and it appears to work fine there.
Pretty much what Malcolm said. This works well enough:
http://effectiver.ca/calculator/wall.php?id=8843
If you want better, have them use a variable perm membrane instead of poly. This is pretty pricy up here in the great white north for some reason but compared to the cost of a custom build, it is pretty much noise.
http://effectiver.ca/calculator/wall.php?id=9343
You can also ask if the builder can get you either fiber faced polyiso (stock item at most roofing places) or permeable polyiso such as EnerAir. These allow for a bit of drying and are higher R/inch, 1.5" is ~r9 which would let you skip the vapor retarder completely.