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the shadow is the gap

The gaps are irregular around window frame, but the widest are no than half inch (1.3cm) and the frame is flexible metal because the window is uPVC. I can use backer foam that's half inch wide and since gap is deep I can stack them on top each other for tight fit. I haven't used spray urethane foam. Which is better? I also have rope caulk, it's like putty.

Also how does one apply silicone for such big gaps? I have silicone but also the synthetic stuff like Lexel which allows you to reapply without removing old caulk

  • Be very, very careful with spray urethane foam around window/door frames. It can easily bow frames when it expands, even the "low expansion" foams. Feb 6, 2018 at 15:20

1 Answer 1


There are pros and cons to each, making this a subjective question. Here are some comparison points.

  • Rope caulk is easy to work with. You can take your time and not worry about anything curing too soon. Urethane is potentially disastrous, as it bonds very well to almost anything it touches. Permanently. It cures in a few minutes or slightly longer, depending on environment.
  • Urethane seals better due to the bond it creates. Rope caulk also seals well, but in cases where size isn't quite right you may have gaps.
  • Urethane fits a wider range of gaps. You can easily fill narrow slits to voids several inches or more wide. Rope caulk comes in various sizes, but there are often compromises during installation.
  • Rope caulk is cheap, and you can use it now and later. Urethane foam isn't much more expensive, but you have to use the can in a fairly short time since the nozzle will seal itself shut.
  • Rope caulk can be removed for repositioning, changes, etc. Urethane would need to be cut and scraped away, usually leaving a residue.

Silicone and other topping caulks would need to be tooled to overlay backer in wide gaps. A combination of putty knives and other items can come in handy, along with a finger.

  • 3
    I've been able to reuse spray foam cans (once usually). As soon as you're done, take off nozzle/handle and set aside. Some foam should be in the can nozzle. Insert toothpick if you like. Let foam remnants cure for 2 days. Pull dried foam out of can nozzle. The long handle requires same- use toothpick in end for grabbing later, let cure for 12+ days. Use long wire or something to clean cured foam out of tubes. Feb 5, 2018 at 18:11
  • You favor rope caulk? It's easy to apply but in some cases it might come off and not stick well?
    – Altoban
    Feb 8, 2018 at 11:18

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