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Community and Q&A

Mounting windows in concrete block

user-5631279 | Posted in Energy Efficiency and Durability on

We are finally moving along on our AAC house build and have selected Alpen for our windows. The block itself is 8″ thick and we’ll be adding a framed wall to the inside with more insulation. The outside of the wall is stucco. The entire wall will be one foot thick. Based on various things I’ve seen on this site I’d planned on mounting the window recessed a little bit from the outside to prevent wind washing but I’m open to a different approach.

What I’d like to know is how do you mount windows properly in block? Do you run boards around inside of the window, secured to the block and just use a nailing fin? Another suggestion I’d been given was to use the “mounting straps” of the window but that seems a bit more complicated. I’d appreciate any advice on this. Thanks!

-Mike and Jamie

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  1. user-2310254 | | #1

    Just to clarify, AAC (autoclaved aerated concrete) isn’t really the same as concrete block. Here is a quote from the Hebel manual.

    Windows can be installed directly to AAC depending on the finish of the exterior or buck strips may be used for fastening window fins (buck strips must be pressure treated wood and can be installed with 3” coated deck screws).

  2. GBA Editor
    Martin Holladay | | #2

    Mike and Jamie,
    What stage of construction are you at? Have you already framed the window rough openings? Have you already ordered your windows?

    There are at least two ways to install windows in your walls: (1) using masonry straps, or (2) inside a window buck. But these two approaches require different sizes for your rough openings. So if your rough openings have already been made, and your windows have already been ordered, you may have only one possible approach.

    It sounds like you will be installing these windows as "in-betweenies." That usually means that you need to order non-flanged windows -- although if you want to install flanged windows as in-betweenies, you can, as long as you plan ahead with really big rough openings. (For more on this last technique, see Windows in Double-Stud Walls: In-Betweenies.)

  3. user-2310254 | | #3

    Mike and Jamie,

    Also note that you may need to add insulation on the exterior or interior to achieve the level of insulation required by your local code. The eight-inch block has an R-value of R-8 to R-11. FWIW I like the product but think its performance characteristics are misrepresented.

  4. user-5631279 | | #4

    Thanks for the quick responses! The block walls are almost to the roof line now - we'll be pouring the bond beam soon. The windows are not ordered and the window openings don't have any buck strips or forms installed - just bare block currently. The openings are quite large so we have flexibility with mounting still. As for insulation, we are framing the interior against the walls which gives us another 3 1/2 of insulation in addition to the block itself.

    Are there any details on how to properly mount windows with masonry straps? I'm looking for the most durable and air-tight approach feasible. Also, any ideas on how to get it to look good with the stucco on the outside? I'm not sure how to bring it all together.

    Thanks again!


  5. GBA Editor
    Martin Holladay | | #5

    You need to follow all of the usual rules for installing windows, especially flashing rules. The entire window rough opening needs to be flashed. The sill pan (either a site-built sill pan or a manufactured sill pan) is especially important.

    I'll attach some photos of masonry clips below.


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