Polyurethane foam in false ceilings: insulates, doesn't repel

Polyurethane foam between false ceilings: insulates the home, doesn't scare mice away

Filling the gap between the false ceiling and the reed mat of the attic with polyurethane foam doesn't drive rodents out, but it does dampen the cold and street noise. No one disputes that. What's in question is whether that filling becomes a heated cushion for rats to breed and keep scurrying right above the room. The doubt is raised by the owner of an old village house, with an attic you can only enter with your arms raised, among cobwebs and bugs, and a reed ceiling already gnawed by rodents. His plan: put cans of foam between the two surfaces, without filling all the way up, to cut off their runs and also insulate.

Does polyurethane foam work to insulate a false ceiling?

Using expanding foam as cavity insulation isn't a workshop whim. It's commonly used on facades and for sticking tiles on roofs, they recall in the exchange itself, so the problem isn't technical suitability but the material's behavior inside a closed cavity. Foam also has an overlooked side effect: when it expands, it fixes loose wood, according to one participant.

The acoustic nuance deserves its own section. Some warn that foam can act as an acoustic bridge: if someone steps on the attic, the footsteps will transmit more easily to the room below. And some argue the opposite, that an air cavity insulates more than the foam itself. In a narrow gap, rock wool doesn't fit, so a pure cavity isn't a real option.

Why mice still get in even if you fill everything

Here comes the reality check. Rodents don't need to eat wood to settle in: if they enter a house, it's because there's food within reach or they're looking for shelter. They go through roof gaps, holes in walls, the edge of the floor. From inside, the owner confirms the obvious: there are inaccessible holes even when you go up to the attic, plus others that can't be seen, hidden between the adobe of the wall and the false ceiling.

Sealing everything is impossible. One participant's conclusion is uncomfortable: no material prevents them from entering if they want to. Foam doesn't kill or repel them, and some warn it's soft, easy to gnaw, and a good thermal insulator for breeding. Others flatly disagree with that antiestéticar and claim mice neither eat foam nor nest in it. There's no data on this, only conflicting experience.

The affected person's track record doesn't inspire confidence in gentle methods either. He put down four kinds of poison and they only tried a little of one. A Radarcan-type repellent didn't work. The cats, when he had them, changed nothing upstairs. And someone who lives with field rodents—voles, shrews—insists they're different species from house mice.

Plaster with glass: the traditional remedy that foam doesn't replace

Against modern insulation appears the age-old recipe: plaster mixed with small pieces of bottle glass or broken crockery, applied liquid and with the help of a flexible tube to reach corners. The logic is devastatingly simple: the rodent that gnaws the plaster hits the glass and damages its teeth, so it abandons the spot.

The thread leaves a lesson in vocabulary and craft for anyone with an old ceiling. What the thread calls sticks is reed matting, held on the battens with a braided esparto rope called tomiza and nailed with black wide-head nails known as chinches. Over that reed matting, the ceiling is plastered with coarse then fine plaster. Details that confirm the gap where foam is now to be injected is, in reality, a construction solution with centuries of history.

How much it costs and how can foam is applied

On the practical side, according to one thread participant, there are offers of two large cans of polyurethane foam for 7.50 euros at a well-known DIY chain. To apply it, the can is always held upside down and it's advisable to moisten the surface beforehand with a water sprayer for proper expansion. A widespread trick is to insert a straw into the nozzle and glue it to direct the stream.

For those who want to go further, there's a foam designed for spraying without a compressor or special protection due to its low isocyanate content, marketed under the brand Penosil EasySpray. The debate itself warns that not just any polyurethane will do: it must be specific for spraying.

The DIY pulse: neither professionals nor nonsense

The conversation drifts into a squabble over who can do these works. The stance of hiring professionals is rejected outright by the person who lives in the house, and in exchange, recycling and makeshift repairs are championed: reusing old materials, accepting the shoddy result as part of the thrill of not spending. The confrontation surfaces when someone assumes a woman can't put up certain ceilings alone. The response is dry: there are men who know less.

What almost everyone admits is that access is the real bottleneck. The entrance is tiny, you have to enter with your arms raised, unable to bring in wide boards, with a clip-on lamp and a power strip as the only lighting. No noble material goes in there easily.

---

With these ingredients, it's reasonable to expect foam to improve thermal insulation and dampen some noise, without coming close to solving the underlying problem. What's repeated in the thread points to combining sealing what can be sealed—because not everything can—with patience and poison where possible, and the certainty that the mouse will return every year. Anyone expecting absolute silence shouldn't buy cans.