Detailing a Window for Energy Efficiency
![](https://secure.gravatar.com/avatar/7885a92946b033e6daa8ffe26f5390d7?s=52&d=https%3A%2F%2Fimages.greenbuildingadvisor.com%2Fapp%2Favatars%2F200x200%2Fa.png&r=g)
I would very much appreciate this community to review my detail, and comment away. Anything I’m missing? Anything you would do differently?
I’ve opted to frame an extended opening for the window 1-1/2″ more than the wall framing. This of course interrupts the continuous insulation, but seems worth it. The sill is sloped, and everything is covered in Prosoco Fast Flash or Huber Liquid Flash, until it’s water-resistant and air-tight. Then a metal head flashing and some more liquid flashing on top.
1) On my sill, I have a level “rough sill” and sloped “bevel sill.” I flashed both of them. Is that overkill or otherwise incorrect?
2) Windows and doors are 6 months out! I am hoping to install everything including the siding. When I get the windows and doors, I am hoping to install the windows and doors > liquid flash > some 3/4″ plywood shims > install trim, including popping the top trim under the head flashing. Am I asking for trouble??
3) Any opinions comparing Prosoco Air Dam and good ol’ slow spray foam?
4) Other suggestions?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb1.png)
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb2.png)
Replies
1) You only need to flash your sloped sill. Don't worry about the rough sill.
2) Your plan is fine. Putting the trim under the head flashing is the best for water management, also makes the trim finish last much longer.
3) I only ever used low expansion foam backed by flex tape to seal windows. Doesn't leak air or water.
P.S. With the current material prices, a lower cost assembly might be to go with 2x4+CDX/OSB/Gysump+2" polyiso with the WRB (standard housewrap) over the iso. This is slightly lower assembly R value than yours but in your mild climate, it won't matter much. It is the same thickness as a 2x6 wall so you can use standard windows/doors. This also puts the WRB in plane with your window bucks so it make detailing a bit easier.
Hi Akos,
Thanks for the responses! A few follow-ups:
"1) You only need to flash your sloped sill. Don't worry about the rough sill."
~ I believe the rough sill flashing would be the air barrier, let me know if I'm getting that right.
"P.S. With the current material prices, a lower cost assembly might be to go with 2x4+CDX/OSB/Gysump+2" polyiso with the WRB (standard housewrap) over the iso. This is slightly lower assembly R value than yours but in your mild climate, it won't matter much. It is the same thickness as a 2x6 wall so you can use standard windows/doors. This also puts the WRB in plane with your window bucks so it make detailing a bit easier."
~I very much appreciate this. However, my structural engineering is FULL of 1/2" shear wall details. Zip R-sheathing doesn't come in 1/2" (just 7/16"). But more importantly, Zip R-sheathing or just plywood spaced off of the wall by rigid foam reduces the shear value ... and I have to go back to the engineer...
I'm stuck with the foam on the exterior (if want continuous insulation), and double stud walls are not practical for the space. Also, I am matching "outie" windows on the existing building, fyi.
The air barrier is between the surface of the flex flashing over the rough opening and the window. Having the flashing above the tapered sill doesn't change this in any way. It won't hurt to also tape the rough opening under the sill but it also won't do much.
My suggested assembly is a standard wall. I'm not suggesting Zip R or a DIY version of Zip R.
2x4 walls with standard sheathing nailed the studs (1/2" CDX if that is what the engineer wants) with the seams taped as your main air barrier. The 2" thick window/door bucks are nailed on the surface as Malcom suggests.
Over this you install the 2" polyiso. Cover the polyiso with house wrap and hold in place with 1x4 rains screen strapping. Since the house wrap is over the foam, you can use fiber faced roofing polyiso which is much cheaper.
The window bucks and the foam are the same thickness, the WRB is in the same plane as the window rough openings, so you can detail everything as if it were a standard wall. It just happens to be a much higher R value one.
Anukeen,
One suggestion: Frame your walls conventionally and add small bucks around the windows, rather than extended sills and headers. This allows you to tie everything together with the sheathing, and makes the air-sealing easier.