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I have a front window that has about a 1/32 inch gap in the summer, but about a 1/4 inch gap in the winter. The original caulk completely came loose and allowed condensation from the window to get into the wall. Worried about this, I removed the caulk and re-caulked it this fall when there was only the small gap. The other day I noticed that the caulk had come free on one side already. I had scraped all the old caulk off and cleaned with alcohol before applying the new caulk. I used silicone caulk.

What is the trick for this? I have noticed that wherever a surface that needs to be caulked touched an outside wall that this is common. Any tips?

EDIT: the surfaces involved are the plastic window frame and a plastic window sill. The area needing caulked is similar to what is shown here (the lower right arrow) except the sill-surface is plastic. Or similar to this pic where the water is puddling (this is not my window)

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  • Could you clarify exactly which surface you are talking about? Is this actually against the glass, or on the jams or sill? I pic would be very helpful. I different solution may be needed depending where this problem exists. – shirlock homes Jan 10 '11 at 17:07
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I think the real problem here is that as things get colder and hotter they expand and contract. If you fill the gaps and make it look perfect when it's warm out and the crack is small, it will not look very good once it gets cold and that crack grows again.

I would scrape old caulk and fill it in when it is cold out and the crack is the biggest. This may buldge some in the summer when it's hot, but that's probably better than having a crack.

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  • This is what I ended up doing. I applied it after the gap had widened and used a large bead. Come spring everything looked good. – Nemi Dec 9 '13 at 15:11
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Sometimes pure silicon will not stick well to vinyl. I suggest you use a siliconized Acrylic caulk that matches color or could be painted if needed. Be sure to force the caulk into the void with your gun, then round it off smoothly. Be sure to fill deeply into the gap between the window unit and jams. If they are recent model vinyl cased windows, there is no reason to consider replacement.

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  • Thanks for the suggestion. I was under the impression that acrylic caulks hardened over time? – Nemi Jan 11 '11 at 15:33
  • Many of the new acrylic/silicon caulks stay supple for many years with minimal shrinkage. I have had good results with DAP, bad results with Red Devil and Liquid Nails brands. BTW, I don't have stock in DAP. lol – shirlock homes Jan 11 '11 at 21:33
  • Should say, for larger gaps, the first application may shrink a bit, let it cure a few days and skim coat it with a second coat if that occurs. – shirlock homes Jan 11 '11 at 21:35
  • Ok, it is alright to apply acrylic over cured acrylic I take it? I know that nothing will stick to cured silicone, including silicone. – Nemi Jan 12 '11 at 16:25
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You already stated that you are using Silicon based caulking, so my only real suggestion would be to try a wider bead of caulk which will hopefully allow a little more flex and not separate from one source. You later describe the issue as being from the plastic frame to the plastic window sill, almost sounds as though this window might need to be replaced. Barring that Duct Tape will fix just about anything :)

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  • I only down vote if the answer is wrong or unsafe. This forum is designed to help DIYers with informed good advise, not guesses or "Duct Tape" fixes. If you don't have constructive answers, then ask questions. Sorry to dis you on your first attempt. – shirlock homes Jan 10 '11 at 22:41

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