Filling wall cavities with foam: a €3 can and plenty of doubts

Filling wall cavities with foam: 3 euros a can and plenty of doubts

The air cavity in walls is, in theory, the cheapest insulation there is. In practice, in a large part of Spain's housing stock it is a two- or three-centimetre gap crossed by chases, holes and internal currents that behave like Swiss cheese. On that contradiction a DIY method has been built: injecting polyurethane foam into the gap to seal it all in one go. The current price of the product, around 3 euros per can at DIY chains, has turned what used to be an eccentricity into an option worth considering for anyone who wants to cut their bill without building work.

Why an empty wall cavity insulates less than it seems

The physical argument behind the technique is counterintuitive. Air has a very low thermal conductivity, around 0.024 W/m·K, but only when it is still. In a real cavity, the warm wall heats the air next to it, that air becomes lighter and rises; the air against the cold wall cools, becomes heavier and sinks. A closed circuit forms that carries heat from one side to the other without the air having to cross the whole gap. It is the same principle by which a wool jumper keeps you warm: wool does not insulate, it traps air.

Hence the conclusion defended by those in favour of filling: if the problem is circulation, any material that immobilises the air improves the gap's behaviour. Polyurethane foam does exactly that, and it also seals lateral leaks.

The argument against: rising damp and blown-out partition walls

Not everyone sees it the same way. The most repeated objection is that the foam can let moisture in through capillary action and create a problem where before there was only a high bill. Added to that is the risk that pressure injection will blow out the inner leaf of the wall, especially in narrow cavities and old hollow-brick construction. The method's own initial premise jokes about needing a staggering amount of foam, which gives an idea of the scale of the job.

There are also those who point out that the cavity is not always small. In some buildings it averages 8 centimetres, and there are even cases where a person can stand inside. In those wide gaps, blown-in rock wool or expanded polystyrene beads are put forward as a less invasive alternative to foam.

The calculation that decides it: 3 euros, 30 litres and a result you can't always feel

The economics are the most disputed part. A three-euro can yields, optimistically, about 45 litres; in confined spaces and with waste, the real figure drops to about 30 litres. That covers modest surfaces, so filling an entire home is not cheap, although it is no longer prohibitively expensive.

The problem is that the measured result does not always trinc. Some people filled the cavity of the smallest room in the flat and, by touch, could feel no difference between that wall and the ones still with an empty cavity. The conclusion from that experience was not to repeat it in the rest of the house. The full calculation, with litres per can, wall area and final price, is what separates an informed decision from an impulse buy.

Winter and summer: the same insulation for two problems

The seasonal doubt always comes up. If the insulation stops heat escaping in winter, won't it trap heat in summer? The physical answer is that heat always flows from hot to cold, so the same layer that slows the loss in January slows the gain in July. People who live in a well-insulated house notice it: the temperature does not drop below 15 ºC in winter and stays several degrees below the outside temperature in summer. In flats with only one insulated room, the thermal comfort improves in winter but the difference in summer is more debatable and rarely verified with meters.

---

The technique works on paper and has defenders with solid arguments. What remains unresolved is whether the energy savings justify the cost, the risk of damp and the possibility of blowing out a partition wall. The debate goes on.