Background: The home I just bought had vinyl windows installed about 12 years ago. I've noticed some draftiness around the window frame, so I decided to pull back the trim and sill to check it out. It looks like it was previously sealed on the interior with a thin bead of caulk that is now failing. There is also a thin bead of caulk around the exterior as well.

exposed window caulking behind window sill trim

Photo is taken from the interior of the home, with the window on the left (you mostly see window sill here) and the wall on the right. The gap appears to be small, the head of my light is about 1/4" for reference. I think caulk was probably a reasonable option, although as I understand it, it should have been done with a backer rod for better longevity.

I live in CO, where we have drastic temperature swings. Air sealing methods with the most flex are ideal.

What's my best remedy?

Do I remove the sill and re-do this with the backer/caulk method and hope it lasts another 12 years?

What do I do if the gap is too small for a backer rod?

Should I use something like Siga Corvum® 12/48 air sealing tape instead of this, or in addition to? These tapes are used to air seal to passive house standards throughout Europe (interior as well as exterior). Not sure what an equivalent product in the US would be.

I have lots of windows in my house and they are likely all in a similar state. I want to make sure I am doing what with be most effective and last the longest. Taking off trim is no fun.

Extra, I'm familiar with the window/door spray foam and don't really feel like it is a good option for such a small gap, which is echoed by some other things I have read. Without the sill pulled away, the gap is no more than 1/8th of an inch.

Update: Adding a wider frame photo. wider image of window frame

  • When removing the old caulk on the external side, it's a good opportunity to take a good look at the state of the timbers, especially in the bottom corners as this is where water collects and can cause some rotting (especially on a poorly sealed window). If your timbers are black/ soft/ both, consider replacing the flashing first. Apr 12, 2022 at 15:20

1 Answer 1


That's such a tightly framed picture that it's hard to tell for sure, but since you said you removed the trim, I'll assume that the window is on the left and the wall on the right.

The normal method for filling the gap between the window and the framing is to use minimally expanding spray foam. This will expand to fill all the gaps, but won't expand too much and risk warping the side of the widow casing causing it to bind.

Once you've got the foam sprayed in to fill about 1/2 the space, let it expand and cure. Add more to fill in any gaps as necessary and cut off any that sticks out beyond the frame/wall, then reinstall your trim.

If this is the exterior side, then yes, you'll want to apply a good quality exterior caulk around the trim to keep water from getting behind the trim. You'll want to do the top & both sides, but leave the bottom open just in case any water does get back there - it'll give it an easy path to exit instead of sealing it into the wall. You say you're in Colorado where there's a tendency to get a fair bit of snow. If the window happens to be low and on the leeward side of the house, you might get drifting there, but it's not too likely that the snow will push up against the house and actually push into the gap at the bottom of the window.

If this is interior trim, then you only need to caulk it against the wall if there are unsightly gaps that need to be filled with wall/trim colored caulk just to make them less visible. The foam should be more than sufficient to stop any drafts getting through and will thoroughly insulate the opening.

Based on the added picture, I think what we were looking at in the closeup is this circled gap:

detail of OPs picture showing where I think the gap is

If that's the case, then yes, I'd simply caulk between the edge of the sill and the 2x4 framing in the window.

You indicate that this is a very small gap, roughly 1/4". I agree, it's difficult to get caulk into a gap that small, but it can be done. However, I'd cut the tip of the tube at roughly a 3/8" bead size, then put in a thicker bead, working it with the nozzle to try to get some pushed down into the gap and leave a reasonable amount above. Then, while the caulk is still soft & fresh I'd install the side trim, pushing it into the bit of a blob of caulk coming out of the gap.

By bedding the trim into the caulk, it'll sit exactly where you need it to go and the caulk should create a pretty good seal in the gap and at the end of the board. If you wait until the caulk has set up, you'll have some amount of blobbing there that will either need to be cut away, possibly opening up gaps where you didn't get it pushed into the opening very well, or simply leaving a very thin layer of caulk that may not last long, or it'll push the trim away from vertical and not look very nice for your finish carpentry.

Based on finally getting enough info to answer the question (that'll teach me to answer incomplete questions)...

In this case, I would pull/cut off any caulk that's in the least bit loose and replace it with new caulk. Make sure you get a good seal between the new and old caulk.

Actually, since part of that old caulk is against the smooth vinyl window and the rest is up against a piece of 2x4 that's going to be hidden, I'd scrape all of it off and reapply a single bead of brand new caulk. Failure of the old bead in one spot indicates that it may well be ready to fail in other areas and there's no sense in tearing the whole thing apart in another year to apply another patch.

  • Thanks for the input! You are correct in your perception of this photo being from the interior with the window on the left and wall on the right. I've added this clarification to the question. The gap is going to be too small for the nozzle of a can of spray foam, would I just 1/2 fill the gap between the window sill and drywall instead? So no need to remove the sill? I will have to hop on the roof and check the status of the exterior caulking. Apr 11, 2022 at 19:24
  • I'm not sure. That pic is so close in, it's really hard to get a reference. Another, wider view would help a lot.
    – FreeMan
    Apr 11, 2022 at 22:44
  • I've continued removing the sill on the right side and posted a wider photo. The sill/framing is very tight so I can't get to the failing caulk unless I remove the sill. That caulked gap is quite slim, hence the caulk not foam. But I could potentially cut back the drywall and try to fill between sill/framing rather than window/framing. Apr 12, 2022 at 14:20
  • @pirateforhire note updates
    – FreeMan
    Apr 12, 2022 at 14:37
  • Thanks for the input. I updated my photo again, as my area of concern isn't the area you circled, but the air seal between the window and the framing. I have no concerns about being able to caulk a small gap. That's definitely doable. Rather, my question is what is the best way to air seal a small gap between the window and the framing. My understanding is that your suggestion is not to re-seal around the window and the framing, and instead seal between the sill (which I have partially removed) and the framing instead. Is that correct? Apr 12, 2022 at 15:09

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