When to Use Expanding Foam and When Not To
What expanding foam actually is
Expanding foam is a polyurethane that goes on like a liquid and then swells up and cures into a firm, dense barrier. It fills gaps that no filler or caulk will bridge, and once it's cured it blocks airflow and keeps moisture out. It's one of the more useful things on the shelf and it's also one of the easiest to get wrong, so it's worth knowing both halves.
Where it earns its place
1. Awkward gaps that caulk can't bridge
Think of the gap around an old vent you've taken out, or the space behind a badly cut architrave. Anywhere the gap is too wide or too uneven for a bead of filler, foam will take up the space and stay put.
2. Where pipes or cables come through a wall
That gap where the garden tap's pipe pokes through the brickwork, or the hole behind a dryer duct. Those openings let in draughts and insects, and water too once the weather comes from the wrong direction, and foam closes them off properly.
One caution on this, because it's the mistake people make. Foam on its own is not a rodent barrier. Rats and mice will chew straight through cured foam, so if you're sealing a hole they've been using, pack it with steel wool or fine metal mesh first and foam over the top of that.
3. Shed and ceiling joins
Backyard sheds leak air through every seam. Running foam along the joins keeps things drier and stops wasps and spiders setting up in the gaps, which is a decent afternoon's work for the price of a can.
Where it goes wrong
Use the right grade around doors and windows
This is the big one. Standard high-expansion foam keeps pushing as it cures, and if you pack it into the gap around a door or window frame it will bow the frame inwards and leave you with a door that won't close. Use a low-expansion foam, usually sold as a window and door grade, anywhere near a frame. It's a different can and it's worth reading the label for.
Keep it away from heat
Cured polyurethane foam is combustible. Keep it well away from flues, heater outlets, downlights and anything that runs hot, and don't use it as a fire stop, because that's a different product entirely.
It isn't structural and it isn't UV stable
Foam seals, it doesn't support. Don't ask it to hold weight or to patch a structural crack. And if it's going anywhere outside it needs painting or covering once it's trimmed, because sunlight breaks it down and it'll go crumbly within a season or two.
Tips from the tin, and a couple from me
- Only fill about half the gap. It expands a long way and it will find the rest on its own.
- Wear gloves and eye protection. Uncured foam is an irritant and it does not come off skin without taking some skin with it.
- Work in a ventilated space and read the safety directions on the can before you start.
- Trim the excess once it's fully cured, with a sharp knife or a small saw.
- Don't use it around light switches or anywhere it can get into electrical fittings.
Shop related products
- GSA Acrylic Gap Filler 450gm. For the narrow gaps where foam is too much.
- Soudal T-Rex Crystal Clear Sealant and Adhesive. Where you want a flexible seal rather than a filled void.
- Standard Utility Knife with 4 Spare Blades. For trimming the foam back once it's cured.
- Ugly Fish Tradie Safety Glasses. Eye protection for anything that sprays.
Did you know we stock expanding foam in the shop in 500ml and 750ml, along with work gloves, and it isn't on the website. Give us a ring on (08) 8536 4333 to check what's on the shelf before you come down.
Where to go next
If the job you're actually chasing is a draught rather than a hole, start here instead:
There's a range of building materials on the website too.
I like a job where you can see the result the same afternoon, and this is one of them, as long as you resist the urge to fill the whole gap in one go.
Cheers,
Candeece
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