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Finishing rigid foam against an innie window

So I read the innie vs. outie window article on GBA and decided to go with the innie route as I had already made my R/O to size and didn't have time to fir out the windows before my window installer (friend) showed up. So I have my windows in with flanges against sheathing and they stick out just over 2" from the sheathing. I am putting 2' rigid polyiso on the exterior and butting it into the windows. This means there is no foam showing. Do I need to put peel and stick flashing on the exposed foam before installing it? should I leave it away from the window slightly and spray foam that gap? Wondering what the flashing detail will be. I will then put 3/4" vertical strapping and then siding. I have put my WRB against my sheathing. The polyiso is fiberglass faced not foil faced. Thanks in advance.

Asked by jordan Saunders
Posted Sep 1, 2014 9:21 PM ET
Edited Sep 2, 2014 6:21 AM ET


6 Answers

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You wrote, "Wondering what the flashing detail will be."

Indeed. Unfortunately, Jordan, it's a little late for that.

Your rough opening needs to be flashed and waterproof before your window is installed. Moreover, innie windows usually require a waterproof exterior sill flashing to convey rain that strikes the window to the exterior.

Finally, many windows include exterior casing. Does yours?

If I understand your plan correctly, your 3/4-inch-thick vertical furring strips will be exposed at the window sill area. This is the area where you need a weatherproof sill. Similarly, your window opening needs weatherproof exterior jamb extensions. All of these details needed to be thought out and installed and flashed before your window went in.

Answered by Martin Holladay
Posted Sep 2, 2014 6:20 AM ET


Hi Martin,
I suppose I wasn't completely clear. I have wrapped my rough opening with 30 # (not minute but lb) asphalt felt bottom to top and tied it into the WRB. Then installed the window flanges on top of that and tuck taped the flanges to the WRB as pictured below. There is no exterior casing on the windows. What I meant by flashing detail is whether there needs to be flashing around the rigid foam/furring strips or not.

I feel like so far I have followed the step by step info in the innie window installation you wrote so I hope I have not gone past the point of no return. I have not foamed in the windows yet either and they are just screwed in by the interior hurricane straps to the R/O.
Thanks again for any help.

photo (7).JPG
Answered by jordan Saunders
Posted Sep 2, 2014 12:26 PM ET


I'm glad that you flashed the rough opening (although asphalt felt isn't a great choice for the flashing on the rough sill, since asphalt felt won't be waterproof in the vulnerable corners).

Do you have a plan to flash the sill so that the water is conveyed beyond the furring strips? Needless to say, you don't want the tops of these furring strips to be visible or the spaces between the furring strips to be open to the atmosphere.

Answered by Martin Holladay
Posted Sep 2, 2014 12:54 PM ET


If it is too late to do other wise,
you could "double WB " it .
Just install a layer of tyvek from the top of the window, extending at least double width, down to the height of the window or lower.
( you obliviouly need to do this in bottom to top flashed layer fashion if multiple layers are to be used )
And then flash the window as you would do normally , on the flange, with the tyvek/WB.
Then go on with your foam layer and do your knew WB as you would do it .

As Martin said, you will need a bottom sill to flash out from the insulation layer.

And do not rely solely on tuck tape to hold agaisn't water, it will end up bulging slowly from the top
untill water makes it way through it ( it it sees enough water ... if it will be well protected it should be ok )

Have fun with your innie!

Answered by Jin Kazama
Posted Sep 2, 2014 9:32 PM ET


Hey Jin,
I am unclear as to where you are suggesting putting the double WB. Is it over the window flange? and I do plan on using metal flashing around the entire window. If you could clarify that would be great.

Answered by jordan Saunders
Posted Sep 3, 2014 12:25 AM ET


I think the best advice would to remove the windows, create a membrane covered slope sub-sill that drains to the outside. Then re-install the windows.
I think the second best advice would be to not tape the flange across the bottom.
If by some fluke, some water gets through the window, not taping the bottom (or cutting the tape that is already there) gives the RO a chance to dry, perchance even drain to the outside.
To provide an airseal i would seal the roomside edge face of the window to the RO.
Whatever your approach at the window, Martin is correct in suggesting in the sill area in front of the window, you need to make sure water that gets past your ?alum'n? sill encounters a flashing that protects the building against water damage.
Good Luck
Stephen Thwaites
Thermotech Fiberglass Fenestration

Answered by Stephen Thwaites
Posted Sep 3, 2014 3:28 PM ET

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