Filling a gap under a window sill

[xeny]

In my spare room (which is currently in use as a wfh office) I have noticed that the window sill has a gap under it, around 3mm high, and the full width of the window.

The house is of about 1970s vintage with cavity walls and the gap extends all the way back into the cavity, but no further - there is a significant draft that I'd like to get resolved before the temperature falls this autumn.

I'm pretty sure the sill was disturbed when double glazing was fitted many years ago, and I've not noticed until I've had to sit next to if for the past few months.

What's my best choice of material to fill the gap? Some kind of expanding foam, plain filler, or something else?

I'd like to get a reasonable depth of material into the gap, to provide a degree of insulation, and presumably it needs some flexibility to cope with differential movement between the wood of the sill and the brick of the wall.

[staffordian]

My instinct would be to try expanding foam, though the nozzle might not be slim enough to fit in the gap.

It will probably expand outwards too far, so I'd cut that away once set with a Stanley knife or similar.

I'd try to cut it back so that it sits slightly recessed relative to the wall, and finish it with a bead of silicone sealant , which should take up any movement.

I'll be interested to hear other suggestions.

[swill453]

3mm doesn't seem very much, and relative to the area of a wall I wouldn't have thought it really needs any depth of insulation. I'd have thought anything that seals and stops the drafts would do the job.

Something like Sikaflex comes in a variety of colours https://www.screwfix.com/c/sealants-adh ... brand=sika

Scott.

[sg31]

The cill is merely a finishing detail of no structural significance other than closing the cavity. I'd be tempted to use a decorators caulk to fill the gap. Alternatively some form of paintable mastic.

If you are painting the wall don't use silicone sealant as it won't take paint easily.

[kempiejon]

Gripfill would be my weapon of choice, I'd advise against the expanding foam unless you have a large space like a couple of inches to fill, I tried to fill a 1/4 inch gap round a poorly fitted window and it went too many places I didn't want nor need it, was too expandy. Gripfill is a bit like putty in tube you pipe in with a gun, it's cheap, you can squeeze it in and it will expand just a little to snuggly fill the gaps. You can smooth off whilst still wet and paint the next day.

[bungeejumper]

+1 to the Sikaflex proposal. I used it earlier this year on a deep concrete floor crack that might (for all I knew) have compromised the underlying damp course, and it was straightforward and effective.

It goes on like a mastic (using a sealant gun such as https://www.toolstation.com/prodec-stan ... gun/p52047), but it skins over quickly and hardens in a couple of days. That's plenty of time to get it fettled into its final shape.

I'm another one who's made the mistake of using an expanding filler on a small or shallow gap. I cursed its tumescent tendencies for days. Won't make that mistake again!

BJ

[richfool]

For a small crack like that, pretty much any decorator's caulk or filler would work, particularly if you are likely to be painting over it. (It can be smoothed off with a wet finger and painted).

[AsleepInYorkshire]

In my spare room (which is currently in use as a wfh office) I have noticed that the window sill has a gap under it, around 3mm high, and the full width of the window.

The house is of about 1970s vintage with cavity walls and the gap extends all the way back into the cavity, but no further - there is a significant draft that I'd like to get resolved before the temperature falls this autumn.

I'm pretty sure the sill was disturbed when double glazing was fitted many years ago, and I've not noticed until I've had to sit next to if for the past few months.

What's my best choice of material to fill the gap? Some kind of expanding foam, plain filler, or something else?

I'd like to get a reasonable depth of material into the gap, to provide a degree of insulation, and presumably it needs some flexibility to cope with differential movement between the wood of the sill and the brick of the wall.

Please watch this video (be patient please)

https://www.bing.com/videos/search?q=ex ... ORM=VDRVRV

When you've sealed the gap any residual foam can be cut back with a sharp knife.

Then I would suggest that you consider planting something like this under the cill to conceal the foam. Glue or pin tack.

https://www.wickes.co.uk/Wickes-Pine-Gl ... m/p/121231

AiY

[xeny]

Thanks to all for the suggestions.

I'm concerned that the foam may overexpand, but I'm not convinced that simple sealant will penetrate as far as I'd like. The wall is north facing and pretty exposed, so even inside the cavity there seems to be quite a bit of air movement.

Therefore on a democratic basis, I've ordered a can of expanding foam and a can of silkaflex, and will experiment a little. After all, how hard can it be?

[bungeejumper]

This hoary old story about expanding foam is still doing the rounds. Just don't try this at home, kids. **

https://www.songofthepaddle.co.uk/funny ... t2879.html

BJ

**No, seriously, don't try it at home. People have had horrific skin burns from misadventures with the stuff. Trying to make their own surfboards, and suchlike. Yikes.