Expanding foam is one of the most useful products on any site — it fills, seals and insulates gaps in seconds. But used badly it bows door frames, overflows joints and sets rock-hard in the wrong place. This trade guide explains the different types of expanding foam, what each is for, and how to apply it for a clean, professional result.
What is expanding foam?
Expanding foam is a polyurethane (PU) product that expands as it leaves the can, then cures into a rigid, lightweight foam. It bonds to most building materials and is used to fill, seal and insulate gaps that would be awkward to close any other way. Once cured it can be cut, sanded, plastered or painted over. You will find the full range in our polyurethane foams section.
Types of expanding foam
Not all expanding foam is the same. Choosing the right type for the job is what separates a tidy fill from a messy one.
Gun-grade vs hand-held (straw) foam
Hand-held foam comes with a disposable straw applicator — cheap, convenient and fine for the occasional gap, but harder to control and wasteful once opened. Gun-grade foam is applied with a reusable foam applicator gun, giving far more control over the bead, less waste, and the ability to stop and start a can over several days. For anyone using foam regularly, gun-grade is the professional choice.
Fire-rated expanding foam
Where foam is used around fire-stopping penetrations — pipes and cables passing through walls and floors — a fire-rated product is essential and often a building-regulations requirement. Our B1 Fire Resistant Expanding Gun Grade PU Foam is designed for exactly these applications, offering fire resistance alongside the usual sealing and insulating performance.
Low-expansion (window and door) foam
Standard foam expands with real force, which can bow a window or door frame if it is packed into the reveal. Low-expansion foam is formulated to expand gently, making it the safe choice for fitting and sealing around frames without distorting them.
What is expanding foam used for?
Expanding foam earns its place because it does several jobs at once — filling, sealing, bonding and insulating. Common uses include:
- Filling and sealing gaps around pipes, cables and ducting where they pass through walls.
- Sealing and insulating around window and door frames (with low-expansion foam).
- Filling large voids and cavities that would be impractical to fill with sealant.
- Adding insulation and reducing draughts and noise through gaps.
- Fire-stopping service penetrations, using a fire-rated foam.
How to use expanding foam
A clean finish comes down to preparation and patience. The basic method is:
- Protect surrounding surfaces and wear gloves — uncured foam is extremely difficult to remove from skin and clothing.
- Clean the gap and lightly dampen it with water; moisture helps PU foam cure properly.
- Shake the can well, then fit it to the gun or straw applicator.
- Fill the gap only around halfway — the foam will expand to fill the rest, and overfilling is the most common mistake.
- Leave it to cure fully (this can take several hours depending on conditions) before cutting off excess with a sharp knife.
- Cover or paint the cured foam — PU foam degrades in sunlight, so it should not be left exposed outdoors.
Gun-grade foam and looking after your foam gun
If you use gun-grade foam, the gun is an investment worth protecting. Clean it with a dedicated expanding foam gun cleaner whenever you remove a can and before foam has a chance to cure inside it. A well-maintained gun gives consistent control for years; a neglected one clogs solid and has to be replaced.
Expanding foam vs sealant — which do you need?
Expanding foam and sealant solve different problems. Foam is for filling and insulating larger gaps and voids; a flexible sealant is for sealing narrower joints that need to stay watertight and move with the building. For smaller joints, around baths, windows and trim, a cartridge sealant is the right tool — see our guide to what mastic is and how to choose it.
How long does expanding foam take to dry?
Expanding foam usually becomes touch-dry within about 10 to 20 minutes, but it should not be cut or trimmed until it has fully cured, which can take several hours depending on the bead size, temperature and humidity. Larger beads and cold, dry conditions slow curing down, because PU foam relies on moisture to cure. Always check the product datasheet and let the foam cure fully before cutting it back or building over it.
Can you paint over expanding foam?
Yes — once the foam has fully cured and the excess has been trimmed flush, it can be filled, plastered or painted over. This matters for more than looks: cured PU foam degrades in sunlight, so any foam used externally must be covered, rendered or painted to protect it. Never leave expanding foam exposed to UV outdoors.
Common expanding foam mistakes to avoid
- Overfilling the gap — the foam expands significantly, so filling around halfway is usually enough.
- Using standard high-expansion foam around window and door frames, which can bow them; use low-expansion foam instead.
- Leaving cured foam exposed to sunlight outdoors, where UV breaks it down.
- Forgetting to dampen the gap first — a little moisture helps the foam cure properly.
- Not cleaning the applicator gun between cans, which lets foam set solid inside it.
Shop expanding foam at The Mastic Shop
We stock trade-quality expanding foam and accessories. Browse the polyurethane foams range, pick up B1 fire-rated gun-grade foam, or keep your applicator in top condition with expanding foam gun cleaner.
