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Expanding foam buying guide

Updated: 23 October 2026

You have a gap, a pipe hole, or a draught, and the amazon.co.uk shelf in this comparison runs from about £5.82 to about £15.99. An expanding foam buying guide has to decide the valve and the fire limit before the price, because a cheap straw can and a gun-grade can are not the same tool. The [insulating spray comparison](/hub/diy-supplies/insulating-spray) holds the seven cards and the prices captured on 19 October 2026. Peter F. checked the listings against UK review texts and did not run a lab test. A purchase through our links can earn a commission at no extra cost to you.

You have a gap, a pipe hole, or a draught, and the amazon.co.uk shelf in this comparison runs from about £5.82 to about £15.99.

Our top picks

Soudal Soudal Soudafoam Gap Filler

Best overall

Soudal Soudal Soudafoam Gap Filler
9.2/10about £7.19
View offer
Gorilla Filler Gorilla Filler 500 ml

Best value

Gorilla Filler Gorilla Filler 500 ml
8.6/10about £6.75
View offer
Gorilla Gorilla Heavy Duty 353 ml

Alternative

Gorilla Gorilla Heavy Duty 353 ml
8.4/10about £9.93
View offer
On this page
  1. 1.Straw can or gun thread
  2. 2.What B3 does and does not mean
  3. 3.Volume, gap width, and daylight
  4. 4.What this can is not

Straw can or gun thread

Five of the seven cans are straw cans: Paint Factory, Gorilla Filler, Gorilla Heavy Duty, the Gorilla Weatherproof two-pack, and Ferris. Gun grade is printed on only two: Soudal Soudafoam Gap Filler, 750 ml, and Bond-It Fill 'n' Fix, model BDFIXG750. Neither box includes the gun. UK reviews often say the straw nozzle blocks overnight. That is tolerable for one hole today and a poor plan for a week of gaps. A gun thread lets you meter the bead and come back if you leave the can on the gun. The contrast is on the gun foam versus straw foam page. If you do not already own a gun, add that cost before you treat the Soudal can as cheaper per millilitre.

What B3 does and does not mean

Bond-It's listing states DIN 4102 Class B3, and the same line says this is a foam rating for products with no fire-resistant additives. B3 on that scale means the material is readily flammable. It is not a UK approval for a fire-resisting joint, and it is not a pass against Approved Document B. None of the other six listings prints a fire class. Price does not repair the class. Bond-It at about £9.31 is dearer than Soudal at about £7.19 and still states B3. The title's 40-times expansion is a claim UK reviews dispute. Who should skip it is on the best expanding foam page.
Soudal Soudal Soudafoam Gap Filler

Testsieger

Soudal Soudal Soudafoam Gap Filler
9.2/10about £7.19
View offer

Volume, gap width, and daylight

Volume runs from 353 ml to 750 ml. Gorilla Heavy Duty prints tack-free in 8 to 12 minutes and trimming in 45 minutes. Do not paste them onto the Gorilla Filler 500 ml can, which says the cured foam can be cut, painted and sanded and does not give a clock. Soudal's listing says full cure in one day. Bond-It says 24 hours. Paint Factory's title says quick setting and gives no time. Gorilla Heavy Duty says outdoor foam must be painted or coated because sunlight may discolour it. Soudal's sheet says cured foam is not UV-resistant. Waterproof and weatherproof are not a licence to leave a bead bare. The Gorilla two-pack brand field can read JKG Unknown, and Ferris can read FERRIS.

What this can is not

These aerosols are one-component polyurethane gap fillers. They are not an electrical coating, and they are not a cavity barrier. Approved Document B, requirement B3, is about unseen fire spread in cavities. A B3 filler is not that barrier. A household straw can is still a sensitiser: gloves, goggles, ventilation, no flames, and do not burn cured foam off the timber. None of these seven cans is a tested low-pressure window foam. If the opening is structural or a fire door, that is a fitter's job.

Common questions

Is B3 foam a fire stop?
No. Bond-It's own line says DIN 4102 Class B3 is a rating for foam with no fire-resistant additives. Hertfordshire Building Control says a foam fire stop is only the joint that was tested, often with mineral wool behind it, to BS EN 1366-3 or 1366-4. The other cans here do not print a fire class. Do not use any of them to fire-stop a service penetration.
How much foam is in a 500 ml can versus 750 ml?
The cans hold 353 ml, 500 ml, or 750 ml of liquid. That is not the volume of cured foam. Soudal's UK gun gap-filler sheet quotes about 28 litres from 750 ml and 0.037 W/m·K. The amazon.co.uk listing does not repeat it, and the sheet is not a result for the straw cans. Bond-It's 40-times line is a title claim reviewers disputed. Fill the hole you can see, short of flush.
Is this the foam people spray on loft rafters?
No. Loft and roof spray foam is a different product, usually a sprayed two-component system on the rafters. The RICS consumer guide covers that work, including listed-building consent and valuation paperwork. Commons Library briefing CBP-10658 covers mortgages after roof spray foam. The cans here are handheld aerosols for gaps, pipes, and skirtings. Do not buy one to coat a roof slope.
Why is the Ferris twin pack about £15.99 when a single Gorilla Filler can is about £6.75?
Ferris is two 500 ml straw cans, brand field FERRIS, at about £15.99. Gorilla Filler is one 500 ml can at about £6.75 with 4.3 from 7,701 reviews. If you have one hole, the twin packs make you pay for a second can the nozzle may never dispense.
What gap width is actually stated?
Only Gorilla Heavy Duty states one: gaps and cracks up to 2.5 cm, on a 353 ml can at about £9.93. The other six listings do not give a millimetre limit in the captured fields. A wider void is not automatically a job for the 750 ml gun cans. Those cans still expand after they leave the valve, and a frame joint has its own rules.

Related pages

How to use insulating spray

Insulating spray on windows and doors

Insulating spray

Best expanding foam UK: one gap, a gun, or neither

Gun foam vs straw foam: valve, reuse, and one hole

Sources

Public UK sources and the amazon.co.uk listings compared on this page. Prices stored 19 October 2026. Updated 22 October 2026.

  1. HSE: Isocyanates (retrieved October 2026)
  2. HSE: Isocyanate spraying (retrieved October 2026)
  3. RICS: Spray foam insulation consumer guide (retrieved October 2026)
  4. Commons Library CBP-10658 (retrieved October 2026)
  5. Hertfordshire Building Control: PU foam (retrieved October 2026)
  6. Soudal UK: Soudafoam Gun Gap Filler TDS (retrieved October 2026)
  7. Polycell: Expanding foam (retrieved October 2026)
  8. Soudal UK: Filling gaps and holes (retrieved October 2026)