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Insulation sheeting around a window

I am replacing a window on a 12 foot long single story wall on my house. I am debating installing 1/2 inch insulation sheets on top of my osb. I am in climate zone 4.

Anyway, somewhere on here I downloaded from the department of energy an information packet on the subject. Under "wall 1" example it showed putting housewrap or tar paper (#15) on top of the osb. Installing window rough opening flashing, then installing the window on top of the osb/tar paper. This serves as the drainage plane.
Insulation sheeting goes on top of this and on top of the window nailing flange I assume. I have a few questions though,

1. If the tar paper is the drainage plane, should the insulation sheeting seams be taped? (And what kind of tape should I use on the tar paper seams?

2. Most vinyl windows used with vinyl siding like I will be using have a built in channel to hide the siding. When I held a piece of insulation sheeting up to the window yesterday, it looks as if the sheets completely fill the channel making it difficult to put the vinyl siding in the channel. The only way I see around this is to somehow sand the thickness of the insulation sheet down so that the ends that go into the channel are a smaller diameter, or to cut away the insulation entirely so that the window nailing flange is exposed. (I assume the vinyl siding will "bend down" some enabeling it to go into the channel) I am not sure if this option will create a water problem or not if the window is flashed correctly.

The only other option I see is purchasing additional vinyl j channel and install it around the window. This seems the best option but my siding has been on the house since 1998 and there is no way to find anything that matches. (And I live in a rural area, my options are limited and I need to get this done this weekend.

So, what do you guys and gals recommend?

Asked by don gilbert
Posted Thu, 05/29/2014 - 08:45

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3 Answers

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1.
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Don,
Q. "If the tar paper is the drainage plane, should the insulation sheeting seams be taped?"

A. It's always a good idea to tape the seams of your rigid foam, to limit water entry and to improve airtightness.

Q. "What kind of tape should I use on the tar paper seams?"

A. Asphalt felt cannot be taped. If you want to have a water-resistive barrier (WRB) that can be taped, you should choose housewrap, not asphalt felt.

Q. "Most vinyl windows used with vinyl siding like I will be using have a built in channel to hide the siding."

If you want to install windows with a built-in J-channel, then you need to install the windows as "outies." This can be accomplished by installing a picture-frame around each window rough-opening, using material that is the same thickness as your rigid foam. (The picture frame is basically a shim to bring your window into the correct plane.) Your other option is to buy different windows -- without the built-in J-channels.

Whatever you do, make sure that you understand the flashing principles, and that your WRB is integrated with your flashing. For more information, see How to Install Rigid Foam Sheathing. I strongly suggest that you take advantage of the many links in that article -- the links will help you understand the steps and will answer many questions.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 05/29/2014 - 09:15

2.
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Wouldn't installing picture frames on the outside of the rough opening cause a gap between the window and the inside frame of the window? If I understand this correctly, you cant put insulation over the picture frame, so that part would be uninsulated?

Answered by don gilbert
Posted Thu, 05/29/2014 - 10:49

3.
Helpful? 0

Don,
Q. "Wouldn't installing picture frames on the outside of the rough opening cause a gap between the window and the inside frame of the window?"

A. I'm not sure what you mean by "the window" and "the inside frame of the window."

The window consists of a window frame holding a window sash. The window frame is designed to be mounted in a window rough opening. Where would the gap be?

This article might be worth reading: ‘Innie’ Windows or ‘Outie’ Windows?

I should also mention that it's possible to mount a flanged window over the rigid foam insulation, and to nail the flanges through the foam, as long as the foam isn't too thick. For more information on this approach, see Nailing Window Flanges Through Foam.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 05/29/2014 - 11:05

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