ᐅ Concrete slab with strip foundation and frost protection apron

Hello friends of house building,

Until now, I always assumed that you insulate a slab foundation using formwork. Simply placing a shell of XPS insulation and then pouring the slab. Of course, that’s a very simplified explanation.

Our construction manager now suggests a frost skirt. This means that the masonry and the slab will be insulated with x cm of insulation material, and the strip footing will receive 5 cm (2 inches) of insulation. The area below the slab will not be insulated because the strip footing with the frost skirt makes it unnecessary, AND insulation of walls, roof, etc., provides more benefit than insulating the slab. It could be insulated, but it would add more cost.

What do you think? We do need to keep an eye on costs, and if it is more than just a nice-to-have, we have no problem with it, but I must admit that we have little information (and no experience at all) on this.

So long...

Hmm, the typical construction method in the northern regions involves reinforced concrete strip footings, about 80cm (31 inches) deep, with a steel core, and a reinforced concrete slab poured between them. The load-bearing walls are then built on top of these footings. After the walls are completed, a layer of bituminous membrane is applied to the concrete floor using a torch-on method as a moisture barrier from below.

Next, the electrician installs the wiring, and the plumber lays the water pipes. Then insulation is added, usually a type of foam similar to polystyrene (Styrofoam), but more stable and walkable. On top of this, the plumber installs the underfloor heating pipes. Finally, a screed layer is poured over everything to complete the heated floor. This is the standard practice for foundation construction in Schleswig-Holstein (SH).

Is this how it should be done? That would be fine and sufficient. It's not necessary to place insulation directly on the ground. Karsten

Nordlys schrieb:
a reinforced concrete slab in between. The load-bearing walls on the strip foundations.

Honestly, I can’t imagine that this method is still used anywhere nowadays. Wouldn't the walls create significant thermal bridges? Or did I misunderstand you?

In principle, insulation below the concrete slab using XPS shouldn’t cost much more than achieving the same insulation effect later with EPS under the screed.
What I find quite interesting in this context is the use of foam glass gravel. Maybe look it up online for more information.

But no. Here in SH, almost all houses are built exactly like this and not otherwise. It works well. We don’t freeze.

Really no insulation on any part in contact with the ground?

Of course, the occupant won’t freeze because of that, but from a building physics perspective, it’s still self-destructive... and all of that without any real necessity. What is supposed to be the "advantage" of this construction method?

Sure. Over the past three years, I have seen numerous concrete slabs being constructed. Twice, a prefabricated floor panel was used, but otherwise, it was always done as I described, regardless of the general contractor.