GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Basement windows in new construction

Nat_T | Posted in General Questions on

Questions about installing basement windows in new construction, cast-in-place concrete walls.

The basement will have an apartment so I can’t omit windows. One window will be an egress window to a lightwell, the others as minimal as possible to get light and air. Other than the egress window the others will be small sliders that are slightly above grade.

Most info I find about basement window detailing deals with replacement/retrofit.

From my research it appears typical to have a basement window tight to the underside of the sill plates. Any info/resources on how to detail this? It seems to throw a wrench in keeping a consistent base of wall detail for flashings, air sealing etc. The other option is to push the windows down lower and keep a beam of concrete above the window to keep the detailing consistent. In my case this would push the windows a bit below grade, not sure which is the lesser of two evils.

Any good articles/resources for detailing windows in new construction below grade concrete walls?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. user-2310254 | | #1

    This article might be helpful:

    As for your question about the opening, it typically has a lintel at the top to support the building load in that area.

    1. Nat_T | | #6

      Thanks, Steve - that article is for replacing basement windows which is where most of the I find is focused. Not a lot of info on doing it in new construction.

  2. Expert Member
    Akos | | #2

    If you want larger openings you can actually push the top of the window up closer to the bottom of the wall. This really works well if you have larger floor joists such as TJI or trusses.

    What you have to do is put in a small 2x header into the bottom of the wall above the window to carry the wall loads to the side of the window.

    You can than frame the floor joist in this area similar to a stair opening. Now inside this opening you can run the joist at 90 degrees to support the floor load above but because of the shorter span, these can now be 2x6.

    A higher window means way more light and lower window well.

    All basement windows I've done have been pretty close to this:

    Spray foam and caulk no other flashing.

  3. Expert Member


    I like to 0ver-size the opening in the concrete to accommodate at least 3" rough framing. That allows you to include adequate insulation on the inside at the jambs and sill.

    1. MattJF | | #4

      Malcolm, do you have any photos of this configuration? I am thinking about something similar, but in a retrofit, so the insulation will be probably only be 1" thick, wrapping from the interior insulation into the inside of the concrete opening.

      Any guidance on how you are trimming or finishing the exterior side?

      1. Expert Member
        MALCOLM TAYLOR | | #5


        If you are only using 1" of rigid insulation inside, just over-size the concrete opening enough so there is room to provide a pt 2"x4" on the jamb and sills to provide a rough framed opening for a flanged window.

        Set the framing 2 1/2" from the outside edge of the concrete, and trim the head and jamb with painted 2"x2" pt lumber. Use a pt 2"x4" sloped 6 degrees as the sill, which should overhang the foundation by an inch or so.

  4. Nat_T | | #7

    I am fine with sizing/detailing a header to carry the load over the wall.

    My questions are more related to the waterproofing, air sealing, and general detailing:

    - My base of wall sheathing is typically going to get taped to the exterior top of concrete wall. If I frame my window tight to the u/side of sill that detail disappears here. Do I just forget that detail over this small area and chalk it up as good enough? Or do I still tape but wrap the tape to the underside of sill plate and then cover/bury the tape with a piece of p/t trim after window install?

    - After installing a flanged windows to the p/t buck how is that flashed? I tape the flange and turn the tape onto the concrete? Is that durable and robust enough? I guess once you trim the window you bury that tape thereby protecting it?

    Both of the articles that are linked in the responses above are for replacing basement windows. I can't find anything about my detailing questions above for new construction.

  5. Expert Member
    Akos | | #8

    Above the window you normally have the J trim for your siding.

    So the layers go.
    Peel and stick over the buck up under the header up to the wall sheathing. J trim over this which gets lapped by the wall WRB. Window trim over the peel and stick. Basically the siding J trim acts a the head flashing for the window.

    On the bottom, wall rigid insulation taped to the buck, sill flashing over that.

    The air barrier is either a bead of caulk on the window flange or tape the inside to the peel and stick of the buck.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |