I recently bought this house, so know very little of the history. However, in prepping for painting, I found that there is a substantial gap between my window sill trim and drywall. You can see the thickness in the photo. It runs for about 6ft, with the thickness varying a bit across the run.

I am looking for suggestions for the best way to patch it. My first thought was to use some spray foam, cut that flat, tape it, then use joint compound. But I am not at all confident that is the right approach, so very much appreciate your thoughts!


  • What about window casing? Depends on your aesthetic, of course, but I think it's pretty common to have a piece of trim/casing running horizontally underneath the sill to cover up that gap. Oct 13, 2021 at 14:34
  • Thanks for the thought, Susie. We are really trying to keep it as minimal as possible. But perhaps that is the only/best solution. I wonder if something like this would work, that is so small it is unlikely to be noticed homedepot.com/p/…
    – evt
    Oct 13, 2021 at 14:44
  • a backer rod (weird name for a foam snake) sold in paint sections would help insulate before addressing the surface.
    – dandavis
    Oct 14, 2021 at 5:41

2 Answers 2


The "right" way to cover the gap is with a small piece of trim. This can be a pretty simple thing to do because we can take a few shortcuts. I would recommend some 1/2"x3/4" shoe molding since you want something simple.

Shoe molding

There's also a 1/2x3/4 rectangular moulding available if you don't want any curves:

s4s moulding

Both of these are "standard" profiles that should be sold anywhere that has moulding. You mentioned using a small cove piece, and that's ok as well, but you have to keep in mind what the cut edge is going to look like.


The cove has a little back cut on the inside corner so that will leave a hole on the end. The way a trim carpenter would handle that is to make a small return on the end of the molding, but if you aren't up to that, the hole needs to be filled with caulk which could look messy. The other moulding options won't have that issue.

Once you decide on the moulding and cut it to size (small handsaw is fine), then you can glue it in place with some adhesive caulk. Normally it would be brad nailed into place, but gluing it in place is fine.

  • Great detailed answer. Thank you.
    – evt
    Oct 13, 2021 at 16:46

Easiest would probably be a paintable caulk. Very forgiving, flexes with seasonal change, and depending on your colors, you wouldn't even need to paint it (white on white, for example).

Otherwise, as someone mentioned a piece of trim could cover it as well, but that's a lot more involved.

  • Thanks, Jamie! I had thought caulk as well, but that gap seemed too large. At the largest point, it is about 1/2in. I assume it isn't possible there?
    – evt
    Oct 13, 2021 at 15:00
  • 1
    Most caulk manufacturers recommend no more than filling a 1/4 inch gap. I've had success with greater than that. You could possibly fill it partially, let that completely cure, and then fill the rest, or use some spackle or joint compound to fill it some, and then when that's dry, caulk over the rest for a smooth finish. If it were me, I'd just run the caulk and see how it goes. The great thing is underneath the sill like that you'll never really see it. The whole application is just for cosmetics, so the 1/4 thing isn't nearly as important as for an application that needs a great seal.
    – Jamie M
    Oct 13, 2021 at 15:04

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