I'm renovating the windows of my 1936 house. Doing the first window and I notice there's a big gap between the window frame and the wall and I was wondering the best way to fill it in so I can caulk it.

I bought a 1/2 inch backer rod, and that seems the right size, HOWEVER, the hole is really deep and gets a lot wider about 1/2 inch in. This means the backer rod keeps falling through the gap when I try to install it.

Is there something else I can use to fill the gap or should I put something behind the backer rod to stop it falling through?

Any advice here is very much appreciated!

Here's a picture of the gap: gap next to window

And here's a picture of the gap with a backer rod 'falling through' the gap where it moves around freely:

falling through gap

  • 1
    Would something like this be appropriate? homedepot.com/p/… Feb 26, 2017 at 21:43
  • Looks like to me some new brick molding plus the sealing is the best option
    – Deven
    Aug 31, 2022 at 18:41

3 Answers 3


If I were you I'd try this

  1. Fill the gap with expanding foam. Give it a full day to cure (it expands so doing it too soon will potentially leave it uneven) and cut any bubbles off so it's flush
  2. Buy some really good outdoor (oil based) caulk. The local big box sells something called Quad but if that's not available you can probably find something similar in grade. Caulk over the joint (1/2 inch is about the limit so be liberal). That will seal the window and protect the foam.
  • 2
    If you are not practiced with foam a good idea is to tape off the window frame and brick ( anywhere you do not want foam ) because it will expand and possibly get on those surfaces and it is a b$#%# to get off. USE MINIMALLY expanding foam made for doors and windows.
    – Alaska Man
    Feb 27, 2017 at 1:31
  • Ok, awesome. I have some QUAD. Will Caulk work on top of the foam or do I need something between the two? Feb 27, 2017 at 15:21
  • @Alaskaman Good tip! just regular painters tape? Feb 27, 2017 at 15:24
  • I would put the caulk over the foam. Be sure to cover it all, as the foam will degrade some if it's exposed to the elements
    – Machavity
    Feb 27, 2017 at 15:34
  • FYI I ended up putting some window specific expanding foam in the gap, then in some cases putting some wood trim over the gap (big gaps), in other cases caulking over Jun 11, 2020 at 15:16

I like the idea of using foam to fill the gaps, it will insulate the void and provide a support for the backer rod. However, everything depends on getting the foam at the correct depth: too shallow and the backer rod will still fall back into the void...too much fill and the backer rod AND caulking will not fit.

So, be sure to mark the depth required and practice with the foam on a sample board to see how much you can fill, how much the foam will continue to expand and where it stops for the correct depth.

A word of caution, if the window is old, the frame could tend to twist and move out of alignment when the foam is applied to one-half of the window frame. How sturdy is the window frame?

Also, a backer rod is required, you can't just caulk back to the foam. A backer rod is actually a releasing agent so the caulking will stick to the two materials on the sides (in your case, brick and wood frame) and not to the material behind the caulking. DO NOT OVER FILL. It's not how much caulking you can get in the joint, but rather the proper depth. Most people fill too deep. Most manufacturers recommend a ratio of 3:1. That is to say, 3 wide and 1 deep. This allows the caulking to expand and contract and still stay adhered to the sides. So, if the joint is 3/4" wide, do not fill any deeper than 1/4".

By the way, to me, caulking and sealant are different. Latex caulking is paintable, but dries out over time. Silicone sealant is more flexible (better for locations with movement due to thermal issues, etc.) but can't be painted...as easily.) I doubt if there's a lot of movement between these two materials.

  • Question - can I not just Caulk on top of the foam? Does it not adhere to it? Feb 27, 2017 at 15:21
  • I wouldn't use a latex caulk outside. Yes, they tend to be water resistant after curing, but I've had some where you wipe it with a dark cloth months after curing and you get residue. Latex works best where you only get occasional moisture. Silicone works better, but be sure it's outdoor (some are vague on outdoor applications). Too often people skimp and just use bathroom caulk because it's cheaper. That's why I recommend oil-based outdoor. No guessing if it's outdoor.
    – Machavity
    Feb 27, 2017 at 15:48
  • Yes, I prefer silicon sealant for exterior use too. However, it's difficult to paint. It will adhere to everything, including the foam. So, test it first if you don't use the backer rod.
    – Lee Sam
    Feb 27, 2017 at 18:20
  • Why not painted urethane caulk?
    – Matthew
    Jun 3, 2018 at 15:38
  • @Matthew Urethane (or polyurethane) caulk will break down (deteriorate) over time with UV exposure. Silicone will not. Urethane caulk “can” be painted, but it needs to cure for a week or two first.
    – Lee Sam
    Jun 3, 2018 at 15:49

The Gap I have is three times that size and in the winter the the room gets really cold no insulation around the window so I'm filling the open area with insulation then a log flat strip of backer rod and then with cement caulking

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