ᐅ Concrete slab – key factors to consider

Hello everyone,

We are considering building a timber-framed house and have already had initial meetings with several specialists in this type of construction. Over the weekend, we tried to compare the construction specifications from the different providers, and the differences become apparent starting with the foundation slab...

Two of the quotes include an energy-efficient foundation slab with underfloor heating. According to online research, this is referred to as a so-called Swedish slab, which is supposed to be good in terms of energy efficiency.

Another quote is a bit more detailed but, from what I understand, does not seem to describe an energy-efficient foundation slab, or does it?

"The foundations are made of concrete of the required strength class according to structural requirements, assuming a soil bearing capacity of 0.20 N/mm² and soil classes 3 to 4, in accordance with DIN standards. A continuous foundation of galvanized strip steel according to VDE regulations is installed. The base slab is made of waterproof reinforced concrete or waterproof fiber-reinforced concrete according to DIN 18195. Alternatively, for an additional charge, a horizontal insulation layer in the form of an overlapping and welded bituminous waterproofing can be installed."

How do these two variants affect everyday living later on? Does it take longer for the heating system to warm up, or is the energy transferred into the ground simply higher?

What is the current state of the art, and can anyone say how much more an energy-efficient foundation slab typically costs?

Thanks and best regards

These are all nice marketing claims.

Even a standard concrete slab becomes an "energy-saving slab" by adding insulation underneath and/or on top. Energetically, they are all the same. They all have the same slow-responding underfloor heating. And whether it takes 4 hours or 4.5 hours for the temperature to change slightly doesn’t really matter in the end. Only conventional radiators (with high flow temperatures) heat up quickly.

Christian144 schrieb:

Foundation made of galvanized strip steel

I would really like to see that. (I assume this actually refers to the foundation earthing conductor, but this only shows how much the descriptions are focused more on marketing than on technical details.)

On the other hand

Christian144 schrieb:

Alternatively, for an additional cost, a horizontal insulation in the form of an overlapping and welded bituminous waterproofing layer can be installed.

For me, this is not just a "nice to have" but rather a "must-have". If the slab contractor doesn’t do it, someone else will have to install it.

Hello and thanks in advance,

@ K1300S -> The construction description mentions a foundation earth electrode, which seems to be a typo or autocorrect error ;.)

-> Could you briefly explain what is meant by the "...horizontal insulation in the form of an overlapping and welded bituminous waterproofing membrane"?

When grounding, be sure to involve the electrician right from the start, as they need to document and confirm the correct installation.

Christian144 schrieb:

-> Could you briefly explain what is meant by the "...horizontal insulation in the form of an overlapping and welded bituminous waterproofing layer"?

This means that a thick membrane (bituminous sheet) is laid on top of the concreted foundation slab, with the individual sheets welded together so that no water can rise from below the slab into the walls. I would never build without this, unless it’s a treehouse or a structure on stilts.