I recently cut two holes in my foundation and installed new windows. I lined my hole with 2x8 PT wood bucks that I attached to the masonry with Tapcon screws. Then I attached my window nailing flange to the wood buck. All seemed to work fine.

Now I would like to replace some of the other windows in the room (basement). The existing windows are a reasonable size, but because they are old steel frame, and the frame is very thin, the effective window area is very close to the same size as the hole in the block wall. If I follow the same strategy as I described above, then by the time I add the wood buck and a modern vinyl window, my effective window area is reduced by about 4.5" on each side! (1.5" for the buck, 3" for the vinyl window frame). The resulting window would be tiny. I am looking for a way to maximize the final size of the window without having to cut away any more block.

Does anyone here have experience attaching the window directly to the masonry opening, and skipping the wood buck? For small windows, I have heard of people essentially floating the window in the rough (cmu) opening with spray foam - no nails or screws! - but I am a little hesitant to pursue this option...

For reference I am in Anchorage AK.

  • 1
    There are a bunch of factors - hard to go wider because the blocks on each side were poured full, so harder cutting, and if I remove the entire poured column, I need to re-fill the new side column. Also wider brings up potential structural issues in needing a different header system which drops the top of the window closer to grade. If I go deeper, it is easy to cut, but then I have more below grade window that I have to worry about drainage in front of, stuff like that...
    – tbc
    Commented Oct 24, 2013 at 16:46

5 Answers 5


Just get replacement windows instead of new construction and use tapcons to fasten the windows through the sides. Caulk outside good to keep out water. Foundation should also be graded away from the window to keep out water.

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  • you don't think the tapcons will blow out the corners of the masonry (screws/holes may only be 1/2" to 3/4" in from edge of the block wall)?
    – tbc
    Commented Oct 22, 2013 at 22:09
  • Are you talking about the nailing flange of a new construction window? I was talking about a window without the flange and would install in the middle of your foundation wall like your old window now is.
    – Justin K
    Commented Oct 22, 2013 at 22:59
  • The window vendor I was planning on using does not sell a window without a flange. For retrofit windows, they recommend scoring and snapping off the nailing fin and fastening through the brick mould into the buck (vinyl window with built in brick mould). Supposedly works well to replace windows in a framed wall without needing to remove siding.
    – tbc
    Commented Oct 23, 2013 at 15:02
  • You said you were replacing an existing basement window. Almost every basement window i seen is install in the middle of the foundation and on the outside of the foundation the concrete is tapered so water drains away. I added a picture to the above post. (Admins keep editing my posts with wrong information) This makes is hard to install a window flush to the outside with nailing flange.
    – Justin K
    Commented Oct 23, 2013 at 23:00
  • If your foundation doesnt have this tapper then use the flange and cover with trim but you will be using concrete anchors to attach trim. Or remove flange like you said and install were old window was and chaulk both sides good. Either way I would still use shims and tapcons through the sides of the window into the foundation to hold the window in place square and level.
    – Justin K
    Commented Oct 23, 2013 at 23:03

I work in Manhattan, and windows are installed like this all the time. Snap off the nailing flange and attach a strap to the window. The straps can be anchored further back. As for a slope to drain away water typically in the city we see stone sills with a bevel to drain out, it's typically only a 5 to 8 degree angle. I would confer to your original conditions.

If you had a steel window and it was in good shape with the original grade then as long as you have a solid caulk joint you should be fine. Basement windows are usually buried in snow and will get a large amount of water during a thaw; I would recommend an awning type window as it will seal a lot better and deal with water issues more so then a slider.


Without seeing the issues, here in south fla installing Windows is a different world but the supplier should be able to get an equal leg window vs a flanged window. And yes there is an edge distance for tapcons, more so in our area, they should be able to tell you depending on the diameter of tapcon. Hope it helps


All you need is shims and Spray Foam. Shim the window on all four sides Square in the opening. The Sims will hold the window tight, let the spray foam dry and take out the shims and fill the holes with spray foam. No use some round backer rod and place that around all four sides on the exterior and then caulk the window to the concrete with backer rod preventing three-way sealing or attaching of the caulking. Spray foam alone can handle 150 mile an hour cross winds that's 25 miles an hour more than just screws.

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    Spray foam has more shear resistance than tapcon screws? Do you have a source for this?
    – BMitch
    Commented Apr 20, 2016 at 11:52
  • Spray foam does adhere well, and since it flexes to distribute the force across w wide area many folks do seem to consider it sufficient mechanical attachment. I'd consider tapcons as well, given how much hassle already goes into a "proper" basement window retrofit; belt, suspenders, and helium balloons... My question is still whether to try to cheat by installing new windows as inserts into the old cast-in-place metal frame rather than fighting with removing those and doing it "right".
    – keshlam
    Commented Jan 6, 2023 at 18:19

I would just say to use Door and Window foam to seal the gap around the window because it is flexible and does not exert pressure on the frame during the curing process. The foam bonds to the frame of the window and to the surrounding masonry or concrete.

If it is a concrete foundation and you would like to fasten directly into the concrete without a window buck then you would need a concrete screw anchor.

You need to order the right length of screw.

You want the screw to be embedded into the concrete around 3/4" to 1".

You take the embedment depth and add the thickness of what you are fastening to arrive at the length of the screw that you want to order.

If you are using a 3/16" diameter Tap-Con for example, you will need to order a 5/32" masonry drill bit to drill the pilot hole.

If you have an SDS rotary hammer, then order a 5/32" x 6" SDS drill bit.

If you have a conventional hammer drill then find a bit that is long enough to fit in your drill chuck and drill through the frame and into the concrete.

Make sure that you drill 1/4" deeper than the embedment depth of the screw.

Typically concrete screw fasteners come in either a flat, countersunk head or a 5/16" hex head so before you order your window check to make sure that your fastener will work in the application. Otherwise, just use a hex head concrete screw and remove it after the foam cures.

Whoever said that the foam with keep the window in place is right. You will not be able to budge it.

  • I don't see how this is adding much to the other answers that have been here for years. We've already had "just spray foam" with the very reasonable question of "really??", also Tapcons™ have already been recommended, too.
    – FreeMan
    Commented Mar 26, 2022 at 21:27

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