Base plate - construction/insulation etc. - experiences please!

Hello to all previous builders,

we want to build a single-family house 150 sqm (footprint approx. 91 sqm), 1 1/2 stories with 180 Drempel on a slab-on-grade foundation.

We have received some offers from [BU] (prefabricated wooden houses) ... we have decided that the house should have [KFW 55] standard. Now the question of the slab-on-grade construction, a building description looks for example like this

- Topsoil removed & stored on the side max 30 cm, foundations are excavated
- Installation of max. 30 cm fill under the slab with compaction, 2.5 m fill all around
- Exterior foundation concrete C 20/25 - steel reinforcements 2 pcs. 8 mm rebar
- Slab made of reinforced concrete C20/25, thickness 20 cm, reinforcement steel mesh 2 x O188 A, protective foil laid below the slab
- Foundation earth conductor installed
- Sewer pipes for wastewater below the slab up to the outer edge of the foundation
- Plinth - wall plaster ends at the top edge of the slab, front side of the slab is provided with polystyrene rigid foam board

That was it at a relatively reasonable price. However, and we know this, a few points are still missing like empty conduits, house connections, etc. according to the [BU] all possible at a low extra cost. I do not trust it, however ... another [BU] building [Kfw 40] says that the slab should be at least 25 - 30 cm thick, foundations made of steel fiber concrete, gravel layer under the slab, insulation Styrodur under the slab.

Who can tell me something about this or share their experiences? What additional costs come with the slab, e.g. graveling of the driveway for the crane - what extra costs can be expected?

Many thanks in advance for your assessments.

Good evening,

the BB is the framework on which an offer is based. A so-called supplementary agreement or options list regulates the additional costs. In the case of KfW 55, the subfloor insulation must be listed and priced therein.

So always consider the BB in connection with a personal offer

Rhenish greetings

The sub-slab insulation should be listed and priced for KfW 55.

Rhenish greetings

KfW55 can also be achieved with insulation on the base slab.

KfW55 can also be achieved with insulation on the floor slab.

However, this creates thermal bridges in the area of the rising walls, which should be insulated / separated, for example by means of skirting stones.

Does anyone possibly have experience with glass foam gravel or glass foam boards?

Even with insulation under BP, there are weak points; for both, there are functional solutions. I cannot assess whether one solution is better than the other or whether it depends on the executing contractor which variant they can implement better.

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

Here it already becomes clear that many roads lead to Rome, and whether the insulation is placed under, on, or within the floor slab is not that important at first, as long as the overall goal - KfW 55 - is achieved. The wall structure also plays a role in this. By the way: a floor slab that is 20 cm thick insulates just as well as one that is 30 cm thick, namely almost not at all. It depends much more on the structural design and the bedding whether 20 cm is sufficient or not.

I don't know which empty conduits you want to lay in the floor slab, but you have probably already noticed that the connection lines for water, electricity, TV, telephone, ... are missing.

Best regards

K1300S