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Expanding foam around windows: why gap filler is the wrong can

Updated: 23 October 2026

A new uPVC window is in, and the gap around the reveal looks like a job for the can you used under the sink. Expanding foam around windows is where that habit bows the frame and blocks the drainage. None of the twelve amazon.co.uk cans in the [expanding foam comparison](/hub/fillers-adhesives-and-sprays/expanding-foam) is titled low-expansion or window-and-door. They are gap filler. This page is that limit. The shake-and-fill method for an ordinary hole stays on [how to use expanding foam](/hub/fillers-adhesives-and-sprays/expanding-foam/sub/expanding-foam-how-to-use). Prices are the 19 October 2026 shelf figures. We used maker pages and those listings, not a rigged frame. A qualifying purchase can earn a commission at no extra cost to you.

A new uPVC window is in, and the gap around the reveal looks like a job for the can you used under the sink. Expanding foam around windows is where that habit bows the frame and blocks the drainage.

Our top picks

Soudal Soudal Soudafoam Gap Filler 750 ml

Best overall

Soudal Soudal Soudafoam Gap Filler 750 ml
8.7/10about £7.19
View offer
Gorilla Filler Gorilla Filler 500 ml

Rank 2

Gorilla Filler Gorilla Filler 500 ml
9.2/10about £6.75
View offer
Gorilla Gorilla Heavy Duty expanding foam 353 ml

Rank 3

Gorilla Gorilla Heavy Duty expanding foam 353 ml
7.4/10about £9.93
View offer
On this page
  1. 1.Why gap filler can bow a frame
  2. 2.Gap filler is not Window & Door foam
  3. 3.Drainage paths on a uPVC frame
  4. 4.Sunlight, and when to stop

Why gap filler can bow a frame

Standard polyurethane gap filler is built to swell. Penosil’s UK note says straw foam should not be used on joints that might warp. A light bead of the wrong foam is still the wrong foam. Filling only halfway, the straw rule for a pipe chase, does not turn a high-expansion can into a frame can. A 10 mm to 15 mm gap down a uPVC jamb, with the frame screwed but not packed at every point, can take a push from foam that keeps expanding. The sash scrapes, or the weather seal no longer meets. Timber and door frames move too. Gun control does not fix the chemistry. The Soudal 750 ml at £7.19 is easier to dose, and Soudal says fill Gun Gap Filler to about 65 per cent, with high 3D expansion for general gaps. The screw changes the dose. It does not change the foam into the window formula. If you already bought a gap-filler can for this frame, leave it for a masonry hole.

Gap filler is not Window & Door foam

Soudal’s UK range splits Gap Filler, the cavity can, from Window & Door foam for mechanically fixed frames. That window sheet says the foam is not UV-resistant. The window SKU is not in this comparison. The champagne twin at £12.99 lists Pipe, Window, Wood, and Not Water Resistant, with no low-expansion figure. The gun cans at £7.19, £28.99, £47.99 and £52.44 are titled gap filler or gun grade. The Ferris twin at £15.99 includes Window and is still a straw gap filler. Gorilla Heavy Duty at £9.93 lists Window, with a gap claim up to 2.5 cm and no low-expansion claim. Gorilla Filler at £6.75 does not claim a frame-safe expansion rate. A surface field is not a test. The 2.5 cm line is a maximum crack size, not a promise that a jamb stays straight. If you still need a can for a brick hole, use the expanding foam filler buying guide.
Soudal Soudal Soudafoam Gap Filler 750 ml

Testsieger

Soudal Soudal Soudafoam Gap Filler 750 ml
8.7/10about £7.19
View offer

Drainage paths on a uPVC frame

uPVC frames drain through weep holes. A bead that covers those slots, or swells into the chamber, holds water in the profile. You see it later as mist in the glazing or as water at the wrong place in the rain. Find the drainage path and keep it clear. Low-expansion foam, used only where the maker says the joint may be foamed, is still kept off the weeps. Gap filler from this table does not get that job.

Sunlight, and when to stop

Sunlight is the next limit, including on a correct frame foam. Soudal’s window-and-door sheet says the foam is not UV-resistant. Gorilla Heavy Duty says outdoor cured foam must be painted or coated or sunlight may discolour it. Trim only when that can allows it, then cover the bead. Stop and call the installer when the window is still under warranty, when the frame is already bowed, or when you cannot see whether the fixings and packers are doing the structural work. Foam is not a substitute for the brackets. If the opening is a fire door or a compartment line, ordinary foam is not the seal. Use these cans on masonry cracks and pipe chases. Keep them off the jamb.

Common questions

Can I use ordinary expanding foam around a uPVC window?
No. The cans in this comparison are high-expansion gap filler. Post-expansion can bow a uPVC or timber frame, and straw foam is the worse of the two for joints that might warp. Buy a foam the maker labels for windows and doors.
What is low-expansion foam?
It is a frame formula that swells less than standard gap filler, so a fixed window or door frame is less likely to move. You still use a light bead and you still keep drainage open. Low expansion has to be on the product, not assumed because you squeezed gently.
Is Soudal Gap Filler the window foam?
No. Soudal sells Window & Door as a separate product for mechanically fixed frames. Every Soudal ASIN in this set is Gap Filler or a handheld gap-filler twin. The single gun can at £7.19 is the cavity can, not the frame can.
The listing says window. Is that enough?
No. Surface recommendation includes Window on the champagne twin, the Ferris twin and the grey 353 ml Gorilla can. None of those listings gives a low-expansion figure. An attribute is not a test.
Will foam stop the window draining?
It will if it covers the weep holes or swells into the drainage chamber. Find those slots before you foam anything, and do not bridge them. Water that cannot leave the frame shows up in the glazing or at the sill.
Can I paint the foam on an outside frame?
Cured outdoor foam needs a coating. Soudal’s window-and-door foam is not UV-resistant, and the Gorilla Heavy Duty listing says sunlight may discolour uncoated foam. Paint or cover the bead after it has cured to that can’s clock. Painting does not undo a bowed frame.

Related pages

Gun foam or straw foam

Best expanding foam

Expanding foam

Expanding foam filler guide: straw, gun or neither

How to use expanding foam without emptying the can

Sources

Public UK sources used for this comparison. amazon.co.uk prices, stars and review counts were taken from the listings on 19 October 2026.

  1. HSE isocyanates (retrieved 19 October 2026)
  2. Retained Regulation (EU) 2020/1149 (retrieved 19 October 2026)
  3. HSENI diisocyanates (retrieved 19 October 2026)
  4. Fire safety: Approved Document B (retrieved 19 October 2026)
  5. Hertfordshire Building Control PU-foam note (retrieved 19 October 2026)
  6. Soudal Soudafoam Gun Gap Filler (retrieved 19 October 2026)
  7. Soudal Soudafoam range (retrieved 19 October 2026)
  8. Penosil UK straw foams and gun foams (retrieved 19 October 2026)
  9. Polycell Expanding Foam (retrieved 19 October 2026)