How to Use PU Foam Spray: A Practical Guide to Expanding Foam

PU foam spray (also called expanding foam, polyurethane foam or gap filler) is the aerosol can you see on almost every site in Kenya. It fills the gap between a window or door frame and the wall, packs around pipes and cables where they pass through, and closes voids that mortar or packing would take far longer to fill. This guide covers the one-component aerosol foam we stock and how to get a clean, solid result from every can.

What PU foam spray is

A can of PU foam holds a one-component polyurethane that is ready to use. When you press the valve, it comes out as a wet foam that expands and then cures by reacting with moisture in the air and on the surface. Once cured it becomes a light, rigid-but-slightly-flexible foam that sticks to concrete, block, brick, wood, metal and most plastics.

It is not the same thing as the two-part spray insulation that is pumped onto roofs and walls by specialist rigs. The aerosol can is for filling, fixing and sealing gaps, not for insulating a whole roof.

Straw foam or gun foam?

• Straw (hand-held) foam has a plastic tube on the valve. It needs no tools, which makes it the usual choice for occasional jobs and small gaps. It tends to expand more after it leaves the can, so it is easier to overfill.
• Gun foam screws onto a foam applicator gun. The gun gives much finer control of the bead, lets you stop and start without wasting foam, and suits contractors fitting many frames in a day.

Can size matters too. A 750 ml can is the standard site size; larger 900 g cans give more foam per can for bigger jobs.

Where it works best

• Fixing and filling around window and door frames before trims and sealant go on.
• Closing gaps around pipes, conduits and cables where they pass through walls and slabs.
• Filling voids behind skirting, around water tanks and in cavities that are hard to reach.
• Reducing draughts, dust and noise coming through gaps around frames.

It is not a structural adhesive, and on an exterior joint it is not the weather seal on its own. Outside, the foam fills the gap and a sealant finishes the joint.

How to apply PU foam spray, step by step

1. Prepare the gap. Remove dust, loose mortar and oil. Foam bonds to clean, sound surfaces.
2. Wet the surfaces lightly. PU foam cures with moisture, so a light mist of water on the substrate helps it cure fully and evenly, especially in dry weather. Mist, do not soak.
3. Shake the can well. Shake hard for about 20 to 30 seconds so the contents mix properly. Shake again if you stop for a while.
4. Hold the can upside down (valve pointing down) unless the label says otherwise, and fit the straw or gun.
5. Fill only part of the gap. The foam keeps expanding after it leaves the can. As a rule of thumb, fill about a third to half of the depth and let it grow into the rest. Overfilling wastes foam and can push a frame out of square.
6. Work in layers on deep or wide gaps. Apply one layer, mist it lightly with water, let it rise, then apply the next.
7. Leave it to cure. Most foams are touch-dry within minutes and can usually be cut after about an hour, with full cure in around 24 hours. Times are longer in cool or very humid weather, so follow the can label.
8. Trim flush. Once cured, cut the excess off with a sharp blade or saw.

Protect the foam outdoors

Cured PU foam does not like direct sun. Left exposed, it yellows, turns brittle and breaks down over time. On any exterior joint, cover it with sealant, plaster or paint once it has been trimmed. Around a window frame, the usual system is foam to fill the gap and a neutral weatherseal silicone over the outside joint.

Common mistakes to avoid

• Not shaking the can enough, which gives runny, poorly expanding foam.
• Filling the gap to the top, so the foam bulges out and has to be cut back.
• Applying to dusty or bone-dry surfaces in hot, dry weather without misting.
• Cutting the foam before it has cured right through, which leaves a soft core.
• Leaving foam uncovered on an outside wall.

Safety and clean-up

Wear gloves and eye protection (our site PPE checklist lists the basics): uncured foam sticks hard to skin, clothes and tools. Work with ventilation and keep the can away from heat and flames, since it is pressurised and flammable. Wipe off uncured foam straight away with a foam cleaner or acetone; once cured it can only be removed mechanically. Store cans upright in a cool place out of direct sun, and read the safety data sheet for the specific product.

PU foams we stock

We stock straw-type and site-size PU foams including Hauser Basic 750 ml, Sista PU Foam Universal 750 ml from Henkel, Akfix 805 Multi-Purpose PU Foam, Proson 606, Knicker PU Foam 750 ml and IG Star foams in 700 g and 900 g cans. See the full PU foam range, with piece and carton prices.

For the outside joint, see our silicone sealants. Not sure which foam suits your job? Visit the trade counter on Old Mombasa Road, Nairobi, or WhatsApp us and we will talk it through.