How to Use Expanding Foam for Draught Sealing

Why Expanding Foam Deserves a Place in Your Toolkit

If you have ever knelt by a skirting board on a February evening, feeling a cold draught curling around your ankles, you already understand why draught sealing matters. Expanding foam — properly called polyurethane foam — is one of the most effective materials for closing up the awkward gaps that sealant alone cannot fill. It squirts out as a sticky liquid, then reacts with moisture in the air and grows to several times its original volume, setting into a rigid, insulating plug that bonds to brick, timber, metal and masonry.

The catch is in that word "grows". Used carelessly, expanding foam can bow window frames, crack plaster, lift floorboards and leave you scraping hardened yellow blobs off a doorstep for a fortnight. Used well, it is a tidy, professional-looking fix that cuts draughts, reduces heat loss and quietens the noise coming through service penetrations. This guide walks you through doing it properly.

Choosing the Right Foam for the Job

Not all cans are equal, and reaching for the wrong one is the most common mistake. There are three broad types you will find on UK builders' merchant shelves:

• Standard gap-filling foam — high expansion, general purpose, ideal for deep voids in masonry and around joist ends. It grips hard and fills big spaces, but it pushes.
• Low-expansion / window and door foam — designed specifically for use around frames where the substrate is fragile. It grows far less, so it will not distort a uPVC frame or crack a brittle render.
• Fire-rated foam — pink or red in colour, tested for fire resistance. Required around penetrations through fire-rated walls and ceilings, such as pipework passing between flats.

There are also gun-grade and straw-grade cans. Gun-grade foam needs a reusable applicator gun but gives far better control, which matters when you are working around a window reveal. Straw-grade cans are cheaper and fine for one-off jobs, but the flow is harder to regulate. If you have more than a couple of gaps to fill, buy the gun.

Preparation: The Step Everyone Skips

Foam bonds to what it touches, so anything loose, greasy or dusty will simply fall away later. Take ten minutes before you start:

• Remove loose debris, old mortar dust and crumbling filler with a brush or vacuum.
• Dampen the surfaces with a light mist of water from a spray bottle. Foam cures using moisture, and a dampened gap gives a better bond and a more predictable expansion.
• Mask off surrounding surfaces with low-tack tape and a sheet of cardboard or plastic. Overspray is very difficult to remove once cured.
• Check the temperature. Most foams work best between 5°C and 30°C. Cans stored in a cold shed will dribble rather than flow, so bring them indoors overnight.
• Shake the can vigorously for at least twenty seconds — you will hear the mixing ball rattle. Shake again every few minutes while working.

Techniques for Pipes, Window Frames and Awkward Gaps

Around pipework penetrating a wall — overflow pipes, condensate drains, boiler flues and cable conduits — work in stages rather than one big blast. Apply a bead no thicker than about 30mm, wait for it to rise and skin over, then top it up if needed. Foam expands roughly two to three times its dispensed volume in a confined gap, and considerably more in an open one.

For window and door frames, always use low-expansion foam and apply it in the gap between the frame and the reveal, not over the face of the frame. Keep the nozzle moving steadily along the joint so the foam forms an even ribbon rather than a series of blobs. Fill only about half the depth of the gap and let the foam do the rest of the work. Any more and you risk bowing a slim frame or cracking the plaster bead inside.

Deep voids, such as the space around a soil pipe entering a cavity wall, benefit from being filled in layers. Squirt a layer, let it cure for twenty minutes, then add the next. This prevents the core of the foam from staying wet and uncured, which is a common cause of shrinkage and failure later.

Once cured — typically within an hour at room temperature, though cooler conditions slow this considerably — trim the excess with a serrated knife or old bread saw, cutting flush with the surrounding surface. Never trim while the foam is still tacky; you will simply smear it.

Finishing, Protecting and Common Mistakes

Expanding foam is not UV stable. Left exposed to sunlight it will yellow, chalk and crumble within a season, so always cover it. Cut it back flush, then overcoat with a suitable filler, mortar, or a frame sealant. This also gives you a neat, paintable finish.

The mistakes we see most often are worth naming plainly:

• Overfilling. More foam does not mean better sealing. Fill partially and let expansion finish the job.
• Using standard foam around frames. It will push and distort. Choose low-expansion every time.
• Ignoring fire regulations. Where a gap forms part of a fire compartment, only fire-rated foam is acceptable.
• Foaming over wet timber. Trapped moisture leads to rot. Deal with the source of damp first.
• Skipping the sealant layer. Foam is an insulator, not a weather seal. On external gaps, finish with a good-quality external frame sealant over the cured foam.

Used with a little patience, expanding foam turns a draughty, awkward gap into a tight, insulated joint in minutes. Shake the can, damp the surfaces, go steady, and finish the job properly — your heating bill will quietly thank you.