Updated
Gaps Around A New Window Frame
The window looks fine but there are gaps, cracked filler or bare foam between the frame and the wall.
Find a window fitter
What's happening?
A window is not fitted tight into its opening. It is set into a slightly larger hole, packed square, mechanically fixed, and then the perimeter joint is sealed and finished on both faces. That joint is doing three separate jobs: keeping weather out on the outside, keeping warm moist room air out of the reveal on the inside, and closing off the wall cavity so the frame is not sitting against an open cavity. Visible gaps, or expanding foam left proud and unpainted, mean the joint was never completed. The window itself may be perfectly good — the failure is at the perimeter, which is exactly where the draughts, the damp reveals and the cold patches come from.
What usually causes it?
- Perimeter joint never sealed on the internal face, so room air reaches the reveal and condenses in it
- Expanding foam used as the finish rather than as packing, left exposed to sunlight where it degrades and crumbles
- No cavity closer fitted, leaving the frame against an open cavity that channels cold air and water
- Frame packed on soft or missing packers, so it has moved slightly after fixing and opened the joint
- External sealant applied over dust or in the wet, so it never bonded and has peeled from one edge
- Opening cut oversize during removal of the old frame, leaving a gap too wide for sealant to bridge properly
- Cill end caps or cill-to-reveal joints left open, so water gets behind the frame at the two worst corners
Is it dangerous?
Not a physical hazard, but a route to two problems that are. An unsealed reveal cools and wets, and damp reveals are one of the commonest places mould appears after new windows go in. An open cavity beside a frame lets water track inward, which shows up as damp on the internal wall beside the window and is easily mistaken for something the wall is doing on its own.
Can I fix it myself?
Trimming back and painting exposed foam, and renewing failed external mastic on a joint of sensible width, is reasonable DIY. Two things are worth leaving alone: do not inject more expanding foam into the gap to fill it, which pushes frames out of square and traps water in the reveal, and do not simply seal over the inside if the outside joint is what is failing — that traps water in the wall instead of keeping it out.
When should you call a window fitter?
Call the installer first if the windows are recent, because this is defective completion of their own work, and where they are scheme-registered the scheme monitors registrants. Call a window fitter if the installer will not return, if damp or mould has appeared at the reveals since the windows were fitted, or if you can see straight into the wall cavity beside the frame.
What will the window fitter actually do?
- Open up and inspect the perimeter joint to find whether the frame is packed and fixed correctly behind it
- Fit cavity closers where the cavity was left open beside the frame
- Repack and re-fix the frame square where it has moved, before sealing anything
- Complete the joint properly on both faces — a sealed internal air barrier and a weather seal externally
- Close the cill ends and the cill-to-reveal junctions, which are where water gets behind a frame first
- Make good the internal reveal finish and confirm the sash still closes evenly onto its seal
Frequently asked questions
We’re happy to answer all your questions.
Need a window fitter?
Compare local window fitters on Tradehand — coverage areas, accreditations and phone numbers, no middleman.