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Outie Windows with Exterior Rigid Foam Insulation

WilliamC | Posted in General Questions on

I am remodeling the exterior of my house and am going to add 1” of rigid foam continuous insulation.

I am a little unclear about the process of adjusting the windows for CI, however, and was hoping someone could confirm if either of the following methods would work for installing “outie” windows with 1” of foam CI. I am using new construction windows with nailing fins.

The first method I am considering using is the one described by Matt Risinger in the video linked below. It seems much simpler than building a plywood box. He simply installs pressure treated 2×4’s ripped to the correct depth (in my case 1”) on the face of the zip sheathing around the window frame. This methods seems like it would be effective and fairly simple.

The second method is described in the article linked below. It starts on “Figure 6”. If I understand it right, nothing unusual needs to be done if the rigid foam is under 1 1/2”. You simply sit the window on the sill and over the foam itself and no extra support is needed as long as the CI is under 1 1/2”. I feel like I must be misunderstanding this but that is how I read it. If this is true then it would seem that I do not need to buck out the window at all.

I appreciate any help in sorting out how I should install windows with 1” of rigid foam.

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  1. Expert Member
    Akos | | #1

    I like and have done option 1 with the face nailed window bucks, simple and solid. They don't need to be PT, regular lumber works. Run the WRB over the foam and it turns into a pretty standard install as everything is plane. The nice part about this is the siding buts up to window so no exterior extension/extra trim work needed.

    Make the bucks wider than the window flange so you have something solid to nail the strapping/trim into.

    Option 2 is technically higher effective R value but I doubt it can even be measured.

    1. WilliamC | | #3

      Thanks for the advice!

      I’ll make sure to make the bucks wider than the flange. I like that this method doesn’t require anything unusual since the windows will but up to the siding.

      Option one also seems a bit more solid than option 2. Option two seems to have a higher R value like you said and might be a bit easier but I’m a little unsure about nailing the window to/through the foam.

  2. AndyKosick | | #2

    The first thing is to follow manufacturers instructions for warranty purposes. That said, I was recently introduced to the report below which is probably what the BSI article is based on.

    Furthermore, my local Habitat for Humanity has been nailing vinyl windows through 2” inch CI alone for a decade now. Couple of shims to align it, nails in flange, single component foamed in place. All fine to this day. I actually replaced the windows in a home installed this way in 1” CI from the 80’s. Can foamed in place, and after removing all fasteners I could throw my body against the window, wouldn’t move. Pry bars, nope. I had to sawzall the entire perimeter of each window to get them out.

    Another local builder uses preformed pan flashing but nothing additional and then just nails through 1” CI. Hundreds of homes at this point and no problems.

    The only consideration may be the type of siding and fastening requirements around the windows.

    Hope this helps.

    1. WilliamC | | #4

      Wow that’s interesting thanks for the information. I’ll read through that report.

      The siding in this case will be vinyl. I’m curious what would be the benefit in this situation of using preformed pan flashing instead of something like zip’s stretch tape or liquid flashing.

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