My bedroom wall gets wet every winter and the paint peels off. I'm thinking about renovating but I'm not sure if I should use bituminous membrane, polystyrene, or that polyurethane spray I saw at Leroy Merlin. Which one is cheaper and actually works?
Asphalt membrane is really the cheapest and works well for this kind of problem, especially if the moisture is coming from the outside of the wall. Polystyrene also solves it, but it's usually more expensive and needs more careful application. That polyurethane spray you saw does work, but it ends up being way more expensive in the end and depending on what's causing the moisture (whether it's actual infiltration or just air condensation), maybe you don't need to spend that much. Before anything else, figure out if water is actually getting in through the wall or if it's just moisture from the air condensing, because the solution changes quite a bit.
The problem usually isn't just choosing the material, but figuring out where the moisture is actually coming from. If the wall only gets wet in winter and the paint peels, it could be condensation (humid air from the room hitting the cold wall) or moisture rising from the ground. That changes the solution quite a bit.
If it's condensation, it's not worth spending on bituminous membrane or polystyrene - you need ventilation (open the window, get a dehumidifier) and quality paint. Now, if the moisture is coming from outside or the ground, then insulation actually helps. Bituminous membrane works well if you apply it from inside, but it's more effective if you can put the barrier on the outside (if the wall has external access). Spray polyurethane costs more and needs a professional, but it adheres better to uneven surfaces. Polystyrene is in between and requires framing.
Before you spend money, take some photos and see exactly where the water shows up - whether it's just in a bottom corner or across the whole wall, if there are mold stains, stuff like that. Because if it's just condensation, the cheapest solution is really just ventilating the room well and letting the wall dry with a heat gun or a portable heater for a few weeks.
Before you pick any of these materials, it's worth figuring out whether the moisture is actually coming from outside or if it's just condensation. The difference is huge: if the wall only gets wet in winter and the paint peels, it might be that warm, humid air from the room hits the cold wall and condenses. In that case, no interior insulation will fix it - what you really need is better ventilation, to dry out the room or raise the wall temperature.
Now, if you confirm it's moisture coming from outside (wall always wet, signs of water infiltration, etc), then insulation makes sense. Asphalt membrane is genuinely the cheapest and most practical, especially if you apply it behind the render. Polystyrene also adheres well to walls but it requires you to plaster over it afterwards, which adds to the work. As for that polyurethane spray, it works well for sealing small cracks and specific spots, but it's not the best solution for an entire wall - and the price per square meter gets quite a bit higher.
The ideal thing would really be to check where the water's coming from: feel the wall inside and out, see if the render is degraded, if there are clogged gutters or drainage problems around the house. Sometimes the problem sorts itself with something as simple as fixing the water tank or cleaning the surrounding area. If it's genuinely deep infiltration, asphalt membrane is the most cost-effective way to go.
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