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Community and Q&A

Ideal window rough opening gap for good air sealing

DarkNova | Posted in General Questions on

The manufacturer I’m ordering windows from allows a rough opening gap around the window of 3/8″ to 1/2″ from all 4 sides (so the rough opening would be 3/4″ to 1″ wider & taller than the window).

For those of you who have experience installing windows with good air sealing, would it be preferable to do 3/8″ or 1/2″ as the gap? The argument I can think of for 3/8″ is that starting with a smaller gap leaves you less you need to fill, but the argument for a bigger gap is that it might be easier to do the air sealing and if the window shifts say 0.10″ over time that is a smaller percentage of the gap so there is less likelihood for sealant separation. Thoughts? Thanks.

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  1. davidmeiland | | #1

    If you are framing nice clean openings that are level and square, you can easily go window + 3/4", for 3/8" gaps all four sides. Then, use 1/2" backer rod in the gaps. If you're giving instructions to a framing crew whose might be hurrying, tell them window + 1".

  2. DarkNova | | #2

    Thanks David.

  3. milwaukee | | #3

    I prefer 3/4 inch myself all around the window on all sides. I never install windows with the sill touching the rough sill. I like to have a lot of spray foam thru the entire cavity, not part way. Inorganic shim (like pvc shims, etc) go under the window sill unit for shimming. But for most, 3/4 inch sounds huge. But a 1.5 inch wider rough opening (window exterior dimensions + 1.5 inches) is great for getting the nozzle of the spray foam gun into the cavity and making sure it is filled fully. In my opinion it yields a better long term air tightness.


  4. davidmeiland | | #4

    3/4" on all sides IS huge. A lot of window flanges are in the range of 1-1/4", so you'd be nailing your window into the edge of the framing.

  5. Expert Member

    The RO is only larger than the window to get it into place. Commercial windows are often site-built with no clearance. I don't see the virtue of creating a gap and then having to insulate it. The ideal window install would be into a RO ever so slightly larger than the frame, and sealed with caulk.
    Remember that after the window is in the interior needs to be cased. With gaps in the region of 3/4" drywall returns become problematic, and interior wood casing has to by installed and shimmed so that it too has a substantial air space between it and the framing that then needs insulating. It seems like creating a problem and then having to deal with it.

  6. exeric | | #6

    No expert here, but from my own experience the problem comes from the fact that it's very difficult to get the RO completely level, square, and true. Most people would be surprised just how much framing is about managing the dimensional errors so they don't become a problem. It's not about dimensional perfection. If you have 10 rough openings in a house for windows and doors and you aim for a perfect fit with windows it will take roughly 3 times as long to make those openings. And if you mess up on one, then what do you do? Plane it down.

    1/2" on each side is a good real world compromise.

  7. DarkNova | | #7

    Didn't know this question would create a debate :-)

  8. Expert Member

    When I am specifying window dimensions on drawings for another contractor to build I give the RO and let the manufacturer decide on the gap. When I'm doing the building, I specify the frame size and make the RO tighter. As Eric H. said: It's then on me if they don't fit.

  9. Trishamegan | | #9

    Manufacturer's often have the rough opening 3/8" bigger on all sides for vinyl windows.

  10. milwaukee | | #10

    I hate when installing windows I have seen the famous 3/8 gap all around and the rough frame is so out-of-wack (not straight, warped, humped, etc.). Then the window install can have spots where the 3/8 gap becomes non-existent! In these cases air sealing and insulating become hard to do as getting the window straight and plumb etc yields inconsistent voids. It does not always happen, with 3/4 inch larger in all directions, you can most always get a nice air sealing / insulating factor around the window. I guess I like this method cuz I hate flanged windows. I like to install Windows from the inside so it always guarantees to the flush with the drywall (sometimes window jambs are not correct depth and I have to take them apart and fix them. And if the non flanged window does not stick out far enough for the exterior cladding I can always add details to make the window look deeper from the outside. Thanks

  11. Expert Member
    MALCOLM TAYLOR | | #11

    I'm curious as to how much experience you have installing windows beyond your own house?

  12. morganparis | | #12

    "The ideal window install would be into a RO ever so slightly larger than the frame, and sealed with caulk".
    No. Good luck with getting caulk into the gap, as opposed to sitting in a thin bead across it where it will eventually stretch and crack. The wider gap (though not 3/4" for the functional reasons cited) is essential to install a proper compressible air sealant. Air infiltration control trumps insulation.

  13. user-4524083 | | #13

    I'm with Malcolm - When I was building energy efficient houses,I often used the masonry rough openings in the framing.An 1/8" to a 1/4" is fine to caulk. I figured that 20 windows times 1" extra R30 insulation in the walls was worth it.There probably is some ideal but I feel that the suggested R.O.'s are a holdover from the days of rougher framing materials, and are excessive.Interesting thread on a question that seems so straightforward.

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