ᐅ White tank waterproofing – insulation on the interior or exterior?

Hello,

There is an ongoing heated discussion here about whether it’s better to have the builder install 12cm (5 inches) of foam insulation made from petrochemicals under the basement floor, or if we should do the equivalent insulation ourselves inside, directly beneath the screed. The latter might allow for more self-performance.

The house is not located in an area with hydrostatic pressure.

What are your preferences?

The energy consultant is fine with either option. However, wouldn’t interior insulation risk creating a thermal bridge at the wall? The standard approach tends to be placing the house on expanded polystyrene.

The exterior walls will be insulated with 12cm (5 inches) of expanded polystyrene.

Best regards!

Inside, you lose ceiling height or have to build higher.

Off the top of my head, I can’t think of a way to do this without thermal bridging. Wouldn’t you have to extend below the slab anyway? Otherwise, a significant thermal bridge will form at the outer corners.

Why can’t you contribute your own work to the insulation under the slab?

Of course, both on the outside and the inside. On the outside, as suggested, at least 12 cm XPS (5 inches). Since it is structurally important and cannot be revised later, I would personally avoid using EL there, at least for warranty reasons. On the inside, you can install the waterproofing membrane and, for example, 5 to 10 cm (2 to 4 inches) of foil-faced PIR insulation yourself.

This assumes that underfloor heating is planned in the basement. If there is no underfloor heating (or, by the way, even if you plan a concrete slab with thermal mass activation), then you can only insulate from the outside, but in that case, I would definitely consider going thicker than 12 cm (5 inches).