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Community and Q&A

Boarding up basement window from the inside with foam

noew | Posted in Energy Efficiency and Durability on

I don’t need our 4 basement windows and would like to cover them up from the inside with 2″ of Thermax polyiso, and seal the perimeter with canned spray foam, so that absolutely no interior air would get in contact with the windows. I just finished insulating the rim joists with the cut-and-cobble approach presented on this site, which got me thinking about using a similar method for the windows. Is this a viable approach, or will the wooden windows rot out in no time?

In his reply to another post, Martin suggests bricking up unwanted basement windows, but I don’t feel as confident with a trowel as I do with a foam gun and glue. There is also lead paint that I don’t want to worry about. I like that the process of installing foam in this way would be potentially reversible.

Details: Climate zone 5, 100 year old unconditioned basement with cinder block walls. No apparent mud sill seal. Windows are wooden single pane with storm. They start about 8″ above grade, up to the sill plate at 2′. They currently don’t show signs of rot.

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  1. Expert Member
    DCcontrarian | | #1

    Here's what I did: take out the wooden windows and their frames. Put in a new frame of pressure treated 2x4's. Cut a piece of cement board tile backer and screw it to the outside of the treated frame. Then seal with mortar. Skim coat if desired. Insulate from the interior.

    1. noew | | #5

      Thanks DCContrarian. Does the frame need to be anchored to the foundation wall somehow, or can I just friction-fit it into the window opening?

      1. Expert Member
        DCcontrarian | | #6

        I used two Tapcons in each side.

        It wasn't hard at all and I'm so glad I did it. The old windows were cheap when they were new and nothing but trouble. Taking them out was five minutes work with a demolition bar. I thought about leaving the frames and just taking out the sashes, but they weren't treated and had started to rot. Screwing in four pieces of pressure treated was minimal work and allowed me to work with a solid surface.

        If you leave the windows in they will rot, the glass will get damaged and mice and other things will start coming in.

  2. Expert Member


    I agree with DC that building a wall in the openings should be the eventual solution. But if it takes a while to get to that, there is no harm in doing what you suggest in the meantime. If the windows are going anyway there isn't much harm if they deteriorate a bit quicker with foam board on the inside.

    If you decide to add foam, I'd paint the glass first.

    1. noew | | #4

      Thanks Malcolm. If I leave the windows in place for now, and they do start to deteriorate, do you see any risk to the sill plate (since there is no seal between window frame and sill)?

      1. Expert Member
        MALCOLM TAYLOR | | #7


        Yes it's certainly possible.

        If you do decide to remove the windows, what replaces them should contain all the same elements as any wall. Cladding, a WRB, air-barrier, insulation, and interior finish.

  3. Expert Member
    BILL WICHERS | | #3

    Another vote for removing the window and placing a proper wall-like material in place of it.

    Note that if you have any egress windows though, those should remain operable, or be replaced with something else that will meet code requirements as an emergency exit door.


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