Problem: 6 months ago, I measured my windows so that I could order insert style windows from a building supply company. I did not know at the time that you are supposed to subtract a half an inch to three quarter of an inch from what you measure, in order to provide for space for insulation. The windows arrived and now my only option seems to be to sand my sills down so that they fit top to bottom. I got one installed by doing this. The left to right width seems to fit okay with about an one eighth to one sixteenth of space between the window and the frame wood. The top is incredibly tight. I can't get the straw to the loctight in there.

I have an explicit question and then I will also ask for ideas more generally. How do I get the loctite into such a narrow space? THe straw doesn't fit unless i flatten it a bit and then it's difficult to keep the straw at the right depth into the crevice.

Quite obviously, I am a newb and did the wrong thing. I would really appreciate any ideas or advice given the circumstances.

  • 2
    The top needs to be loose. If any movement of the building, then weight will be put on the window. Windows do not like weight on them, since they don't bend well. Best idea is to lower the bottom frame a bit to give space.
    – crip659
    Jun 11, 2022 at 22:01
  • It's a 1983 home so hopefully it's done settling. I'll sand down enough to try to provide an eighth of an inch of space on the top.
    – tnk479
    Jun 11, 2022 at 22:02
  • 1
    Agree that a decent gap at the top is a really good idea. Depending on soils, even old houses can heave seasonally. Jun 11, 2022 at 22:18
  • 2
    This is a common DIY failure, how I have solved it for several customers is to use a power hand plane and cut the top and bottom rough Sills down, you need to set the nail heads or pull exposed nails until you cut both down, houses move more than an 1/8 of an inch over a year in many locations and if the frames are rigidly installed there will be problems later and with seasonal changes.
    – Ed Beal
    Jun 11, 2022 at 22:33
  • 1
    You could remove the sill, but imho, it's more trouble than it's worth. A belt sander with a 60 or 80 grit belt will give you 1/4" with very little trouble. Jun 12, 2022 at 17:31

1 Answer 1


Don't worry about insulating the gap -- 1/8" of air isn't going to make a difference.

What you do want to do is use a high quality caulk (my personal preference is dynaflex 230, but there's lots of other choices) and make sure you seal both interior and exterior sides.

  • What about foam rod (it says it's a caulk saver on the packaging). Is that worth pushing into there? I caulked all around - two rows of caulk on the sill and caulk all around the sash stops. Then I caulked the outside all around. There should be a nice seal around the window.
    – tnk479
    Jun 11, 2022 at 22:04
  • Foam rod would work, though I'd say caulk (air seal) is the most important thing. Jun 11, 2022 at 22:18
  • I like backer rod and chinking ( log cabin sealant ). I use energy seal and got my house to 1.1 ACH50. Jun 12, 2022 at 5:34

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