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Old 09-11-2011, 03:57 PM   #1
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Installing foam board around windows ?

I am near the point of installing vinyl siding on my new construction house. The location is southwestern Arkansas.
The walls are 2x6 with taped Zip panels for the sheathing.
The insulation between the studs will most likely be Roxul.

For my climate zone, according to the advisors at , I should not use a poly vapor barrier beneath the drywall, but instead just let the painted drywall act as a vapor retarder so that the wall can dry to the interior.

I was concerned that I might need to have some foam board on the exterior of the sheathing to keep the sheathing warm enough to prevent condensation on the interior of the sheathing in the colder months.
However, again according to the advisors at, it would not be necessary in my climate zone since they say the interior of the sheathing would not reach the dew point.

Now that I am at the point of siding the house, I am toying with the idea of installing about 1" of foam board anyway to add to the thermal envelope of the house.

If I do so, the concern is how do I transition at the windows.
The windows are already installed with the nailing fins directly over the Zip sheathing. The window sills are flashed, using a 1/4" dam along the interior edge of the sill, and the side and top nailing fins are flashed to the sheathing.
The windows are vinyl with a built-in 'j' channel.
If I add the 1" foam board, the vinyl siding will no longer, of course, fit into the built-in 'j' channel.

What are my options for weatherproofing this juncture of the siding to the windows when the 1" foam board projects the siding outward 1" from the sheathing ?


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Old 09-11-2011, 07:22 PM   #2
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you can use a wide pocket j channel wide enough to recieve both foam and siding,you may have to make this from aluminum

the only way to use the pocket on the window is to build out your windows the thickness of the foam

all your ''weather proofing'' needs to be done at the wall surface


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Old 09-12-2011, 06:12 AM   #3
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Is there an exterior projection to the windows that will go beyond the depth of the foam board?
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Old 09-12-2011, 04:13 PM   #4
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Hi Arkansas, Do a search on "Innie or Outie Windows" - this is good site to start:
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