I put in a new window and need to patch up the wall around the installation. I noticed that the cellulose insulation in the walls doesn’t completely fill the cavities. Can I just spray some cans of spray foam or do I need to do something else?

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I also opened up more wall where I saw no insulation. Could I hit this with some foam also ? enter image description here

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    They make window specific spray foam (blue cans of the major brand). That foam expands less and can cause less problems with the window. I would use is in all gaps around the window. If the gap is too small (1/4") then caulk it shut. This areas is a major are for air leaks. Also, was the window flashed at the bottom? Jan 23, 2023 at 22:26
  • The window was professionally installed into an existing rough opening. I’m just doing the interior work. Those gaps are outside of the framing looking into the stud bays of the wall. Only about an inch below the window, but maybe the whole stud best to the right. I don’t see any insulation there. Definitely cold here. Jan 24, 2023 at 2:12
  • I think you updated the photos, these are much better. Jan 24, 2023 at 19:36
  • Yeah. I cut more of the wall away to see what I’m dealing with. Jan 24, 2023 at 20:19
  • If you plan to do work like this often, get a wireless endo/boroscope camera. You can find one for $25-50. Mine connects to my phone and has about 20ft of cable. Very useful. With a 1/4 hole, you can see behind the wall. This is how I determined that the bay window on our 2nd floor had zero insultation. It is over the porch, to I drilled a hole in the porch ceiling and put the camera in. I could see the back of the plaster wall -- showing the problem was not the windows but the wall itself. Jan 24, 2023 at 23:20

2 Answers 2


You absolutely should fill that void. Whether you use up your can of foam or fit some fiberglass is your call. Judging by the size of the window jamb gaps you'll have foam left over to do it.

Leaving well enough alone is actually leaving the job incomplete.

  • I was just thinking spray foam bc I’m familiar with it and can imagine sticking the hose in the gaps and filling it up. How would I get insulation in otherwise? I think maybe they missed the stud bay to the side of the window. Jan 24, 2023 at 2:06
  • I can't begin to see the size or shape of that cavity. I assumed it was only missing part of the insulation within it. Are you sure your home is insulated? It's common for retrofit cellulose to leave gaps. It's not a great technique.
    – isherwood
    Jan 24, 2023 at 13:38
  • The house is insulated. I can only peek a little into that stud bay and I don’t see any insulation. The other gaps are where the cellulose settled an inch or so. Jan 24, 2023 at 15:49
  • I would poke fiberglass insulation into the gaps. With foam, you open up the possibility of a big sticky mess. Jan 24, 2023 at 16:03
  • Use an insulation bat on the stud bay. Then under the window fill the insulation to about 1" of the framing. You can just do this by hand pulling it from your bat. I would then Spray foam the rest of the space. Make sure you can get the foam all the way to the outside sheathing. I like foam in these areas as it can fill voids between the framing and sheathing better than fiberglass or mineral wool. Jan 24, 2023 at 20:03

Beware fire!

Do some flame-testing of anything you put in your house like that. DIY canned foam products tend to have much, much worse fire characteristics than the commercial mixed-on-site 2-part stuff used by professionals. Not to suggest that stuff is particularly good.

It isn't just that it's a fire accelerant. When it burns, even if exposed to flame source from elsewhere, it emits toxic fumes that will incapacitate you and prevent your escape. This usually involves cyanates, that stuff that caused all the trouble in Bhopal, India.

Test the stuff yourself, outside, with the wind blowing away from you. Or just watch this video

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    This is valid info, but doesn't actually answer the question...
    – FreeMan
    Jan 24, 2023 at 13:32
  • @FreeMan It is something to think about if OP is planning to use a few cans to fill one opening/gap, instead of using one can to seal a brunch of windows. OP should give the size of space the old insulation has left empty. From what OP said these gaps are quite big.
    – crip659
    Jan 24, 2023 at 14:30
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    I edited the original question to try to answer that. Is it when possible for several cans of insulation to work? We’re talking about 14”x3.5”by 60”(?). It’s possible the bay is half filled, but I can’t tell Jan 24, 2023 at 15:57
  • On closer inspection, the area is probably 24”x14”x3.5” Jan 24, 2023 at 16:04
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    @manassehkatz thank you for the edit. Battleship Texas recently filled all their ballast tanks and bilges with closed-cell spray foam, to reduce the probability of the ship sinking in the Galveston Ship Channel (an outcome exceeding the wildest hopes of the IJN or Kregsmarine, though, 80 years late). At the drydock they blasted it into dust with 10,000 PSI water jets and inspectors certified it 100% gone before welding/cutting could begin. They wound up opening up and re-skinning most of those spaces. Jan 24, 2023 at 20:14

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