ᐅ Concrete slab – construction/insulation etc. – experiences, please!

Hello to all home builders so far,

We plan to build a single-family house of 150 sqm (footprint approx. 91 sqm), one and a half stories high with a 180 cm (71 inch) knee wall on a slab-on-grade foundation.

We have received several offers from builders (wooden prefabricated houses)... we have decided that the house should meet KfW 55 standard. Now, regarding the construction of the slab-on-grade foundation, one building specification looks like this:

- Topsoil stripped off and stored at the side, max 30 cm (12 inches), foundations excavated
- Installation of up to 30 cm (12 inches) fill material under the slab with compaction, 2.5 m (8 feet) of fill added around the perimeter
- External foundations made of concrete C20/25 with reinforcement: 2 pieces of 8 mm (0.3 inch) rebar
- Slab-on-grade made of reinforced concrete C20/25, 20 cm (8 inches) thick, reinforced with 2 layers of steel mesh O188 A, protective foil laid underneath the slab
- Foundation grounding installed
- Sewer pipes for wastewater underneath the slab up to the outer edge of the foundation
- Base wall plaster ends at the top edge of the slab, front side of the slab insulated with rigid polystyrene foam board

That was offered at a relatively reasonable price. However, and we are aware of this, a few items are still missing, such as empty conduits, utility connections, etc. According to the builder, all this is possible at a low additional cost.
I am skeptical though... Another builder who constructs to KfW 40 standard says the slab-on-grade should be at least 25 to 30 cm (10 to 12 inches) thick, foundations made of steel fiber concrete, a layer of gravel under the slab, and Styrodur (extruded polystyrene) insulation under the slab.

Can anyone share insights or experiences on this? What additional costs can be expected in relation to the slab, for example, graveling the driveway for crane access — and what additional costs might come up?

Thanks in advance for your input.

Good evening,

The base bid (BB) is the framework on which a quotation is based. An additional agreement or an options list specifies the extra costs. In the case of KfW 55, the floor insulation should be listed and priced there.

Therefore, always consider the BB in connection with a personal offer.

Regards, Bauexperte

Bauexperte schrieb:
The insulation under the slab should be itemized and priced for KfW 55.

Regards, Bauexperte

KfW 55 can also be achieved with insulation on the concrete slab.

oleda222 schrieb:
KfW55 can also be achieved with insulation on the ground slab.

To my knowledge, this creates thermal bridges at the junctions with the rising walls, which should be insulated or separated, for example, using capping bricks.

Does anyone have experience with foam glass gravel or foam glass panels?

There are weak points in insulation beneath the vapor barrier as well, but there are effective solutions for both cases. I cannot assess whether one solution is better than the other or if it depends on the contractor responsible for which variant they can implement more effectively.

And as is often the case, the magic word is: overall package

It already shows here that there are many ways to achieve the goal, and whether insulation is installed below, on top, or within the foundation slab is not that important at first, as long as the overall target – KfW 55 – is met. The wall construction also plays a role in this. By the way: A foundation slab that is 20 cm (8 inches) thick insulates just as poorly as one that is 30 cm (12 inches) thick, which is almost not at all. It is much more about the structural engineering and bedding as to whether 20 cm (8 inches) are sufficient or not.

I don’t know which conduits you want to install in the foundation slab, but you have probably already noticed that the connection lines for water, electricity, TV, telephone, etc. are missing.

Good luck

K1300S