We're building a house on a strip foundation with a plinth that sticks out 40 cm. I've heard different opinions: some say you definitely need to insulate it with polystyrene foam, while others think it's unnecessary. What actually helps prevent freezing and dampness?
Don't listen to people who say insulating the foundation is unnecessary. A foundation that sticks out 40 cm is a cold bridge through which the entire first floor freezes, plus moisture accumulates there and mold can appear. You need to insulate, but the main thing is to do it right: expanded polystyrene (or mineral wool) must be laid with waterproofing underneath and a concrete apron on top so that meltwater doesn't seep under the insulation. Without this, even the most expensive insulation will absorb moisture and lose its properties.
A protruding socle that's 40 cm high is indeed a vulnerable spot, but the real question isn't whether to insulate it or not, but how to do it properly. If the house is occupied and heated, the first-floor floors will be cold without insulation, and condensation will accumulate under the house. Polystyrene foam works here, but it's important to understand: it needs to be protected from direct moisture and UV exposure. Just gluing it on and leaving it is wrong, because it absorbs moisture and loses its properties. You need waterproofing on top and a protective finish (plaster, cladding, or at least vinyl panels).
Here's the practical side: before insulating, make sure the foundation drains water properly. If meltwater accumulates nearby or groundwater sits close to the surface, the polystyrene foam won't help much - sort out the drainage first. A good approach: drill drainage holes around the perimeter in the socle at about 50 cm depth (every 1.5-2 meters) - this way moisture won't get trapped under the insulation and will just drain through the holes.
If your climate isn't particularly harsh and the socle is relatively dry, you can get by without full polystyrene insulation. Instead, at least lay a thin layer of foam (50 mm) between the waterproofing and the finish - it's the minimum that will already reduce freezing. The main thing is don't leave the socle bare and don't ignore drainage, otherwise it's guaranteed to be damp and cold.
You do need to insulate it - the question is just about doing it right. With a foundation that protrudes 40 cm, cold will get into the first-floor floor no matter what - that's physics, not opinion. But here's the key: you need to insulate on the outside completely, including the underground part of the foundation for at least 60 - 80 cm deep. If you only insulate the visible part above ground, cold will just find a way around through the underground base, and you'll get minimal effect.
Polystyrene or mineral wool - both work, but for different reasons. Polystyrene is good because it doesn't absorb water and doesn't lose its properties in contact with soil; mineral wool is cheaper but needs a vapor barrier. If you're trying to save money, you can use mineral wool just for the part above ground and polystyrene for the underground part - that way you're easy on the budget and still get good protection.
Here's what people often miss: before you insulate, you absolutely have to do drainage around the perimeter and install a foundation skirt. If moisture builds up right at the foundation, no polystyrene will save you - you'll get dampness through half the house. A foundation skirt about 1.5 meters wide, sloped away from the house, solves half your moisture problems. And wait for the foundation to fully settle and dry before you start insulating - otherwise you might end up with shifting later that cracks and damages the insulation layer.
The key thing is not to rely on foam insulation alone and forget about waterproofing and ventilation! If the foundation protrudes 40 cm and freezes through, moisture from the soil gets in along with the cold, and foam without proper moisture barrier underneath will accumulate condensation and rot. You definitely need to insulate, but do it as a package: first quality waterproofing around the entire perimeter, then insulation, and make sure you plan out a drainage system and vents in the foundation so moisture doesn't get trapped.
Will you have a basement under your house or just empty space between the ground and the floor? Because if it's just a strip foundation without a basement, then insulating the outside is really critical - the cold will go straight into the first-floor slab. But there's a detail people often miss: the insulation needs to be protected from groundwater moisture not just from below (waterproofing underneath it), but from above too - a thin screed or solid finishing will prevent the polystyrene from getting waterlogged. Just gluing foam to bare concrete and hoping it lasts 20 years is naive.
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