WRB Over Exterior Rigid Foam Insulation
Hey all. I have read arguments both ways but wanted a few first hand experiences and opinions if possible please.
Details:
I’m in zone 6 and it is a 2 story full retrofit inside and out. 2×4 walls with mineral wool insulation membrain then drywall on the inside(membrane details as air barrier). Board sheathing from the 50s on the outside. Then I plan to have 2-2.5” of rigid insulation (XP’s or polyiso) a wrb and a ventilated rain screen. “Outy” windows.
so do I need a wrb or is taped insulation enough? If you feel I need one as well is it better on the inside or outside of the insulation. I’m trying to balance the cost of al this. In an ideal world is use a peel and stick but not sure it’s worth the money.
thanks so much for your help and opinions!
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Replies
Adam,
If you are using foil faced polyiso, then I would would just tape the seams with Zip Tape and let that be your WRB.
I would avoid XPS as it loses some of its r value over time and is terrible for the planet. Also, most tape will not stick well to XPS so you would need a separate WRB over it.
The other alternative would be to add a WRB over the board sheathing and then add foam. I think this is a better option for 'innie' windows though.
Richard - FYI, polyiso also loses some 10-20% of its insulation value over the first few years, and also its R-value actually drops with lower temperatures. This is pretty well documented now. After about 5 years, XPS, Polyiso, and a higher density EPS are all very nearly identical in terms of R-value per inch. EPS has the advantage of actually performing better as the temperature drops. It is also environmentally more benign in its manufacture. For these reasons I'm specifying it in my projects for continuous wall and roof insulation.
https://www.greenbuildingadvisor.com/article/thermal-drift-of-polyiso-and-xps
https://www.greenbuildingadvisor.com/article/cold-weather-performance-of-polyisocyanurate
Insulfoam Type IX EPS has an R-value of 5 per inch at 25 degrees.
https://www.insulfoam.com/wp-content/uploads/2014/04/13007-Below-Grade-Type-IX-TDS_WEB_8-16.pdf
With board sheathing, you'll want to make sure you have a tight air barrier somewhere in the stackup. That could be peel and stick WRB on the sheathing, or taped foam. As Rick says, polyiso is much better for that than XPS. Just be sure that it's connected at the top and bottom edges to make a continuous air barrier.
As for which way to go, it mostly comes down to how you plan to detail the window flashing. Either way, the window flashing needs to work with the WRB location.
I recently designed a renovation for a house that had exterior foam and Tyvek over that. I found it harder to detail than my usual approach, which is treating the sheathing as the control layer for air and water. Windows can be inset, easily tied into that plane, or you can make bucks or use ThermalBucks to put the window at the exterior plane. I would be leery of using the foam as the only WRB because it requires any flashing to seal perfectly to the facing and stay there forever. I prefer mechanically lapped flashing when possible, or sealant systems designed to work as a system.
This is a tricky one isn't it! Exterior insulation is such a great improvement to the assembly but then the whole system gets more complicated. I am just finishing up a similar full retrofit of a 2 story home. Since we were also replacing the roof we removed all the soffits and did a continuous WRB over our existing structure. The roof got peel and stick the walls house wrap. The roof then got 4" of polyiso and the walls 2" of rigid mineral wool.
I like the WRB under the foam, that way if you do ever get any water back there you have protection for your sheathing.
If you are doing a retrofit, your air barrier should be at your sheathing. It doesn't hurt to have a warm side air barrier but it is hard to get that one continuous with all the transitions around your floors and ceiling. Since you have board sheathing, its best to tape the foam seams. Make sure this is continuous and ties back to your foundation on the bottom and the ceiling on the 2nd story at the top.
As for the WRB, with outie windows, I would put the WRB over the foam. You still need window bucks to bring the windows out but this avoids tape origami around the rough opening. House wrap is cheap and it works, I would not mess around with trying to detail foam as a WRB, too many reverse laps to deal with.
I would call thermalbuck since they make great products to fit your needs such as thermal tight. It’s foam boarding with a wrb on top of it pre installed.
Yes, that is a cool product---the flaps on the edges allow proper mechanical lapping at the seams to satisfy that concern with the taped foam approach.
Unfortunately ThermalTight is not available in Canada yet or I would be buying it for my cottage (new build) right now. Just going with ThermalBuck and EPS over the WRB on plywood sheathing instead. I am concerned about lack of drying to outside with the EPS but my builder isn't comfortable with furring (for mineral wool board) so I'm kind of stuck with 2" or less rigid foam for exterior. Using vinyl siding so can do without the furring that way.
My .02 is: if tape sheathing gaps or use canned window foam on every gap with pin point tip.
Wrap with WRB and air seal best possible.
Install Foam. Then install WRB just over door and window head cap flashing up to roof line. Run WRB 12"either side of drip cap. That way windows can be flashed as normal.
Or use WRB completly 2x
I did the same7 years ago on my casita, 2 layers of polyiso (3" total), WRB outside that and rainscreen. My windows were also outies. Taped rainscreen seams. Zone 9B. Great results.