ᐅ Insulation for Screed in KfW 55 Standard – No Perimeter Insulation?

Hello everyone,

I am currently looking into the insulation of the slab or screed.

At the moment, for our KfW 55 standard, no perimeter insulation is planned below the slab.

We have a quote for this, but I find it quite expensive—about 3,000 euros for 100mm (4 inches) XPS with a thermal conductivity of 0.040 W/mK.

Therefore, I am considering investing more in the insulation above the slab if that turns out to be less costly.

So far, the planned build-up is as follows:

65mm (2.5 inches) floating cement screed
30mm (1.2 inches) insulation panel with a thermal conductivity of 0.040 W/mK
70mm (2.8 inches) insulation with a thermal conductivity of 0.035 W/mK

I was thinking about replacing the 70mm (2.8 inches) insulation with a PUR foil-faced board with a thermal conductivity of 0.023 W/mK.

Do you think this makes sense, and what might the cost be?

Do you have any other suggestions?

Thanks

How large is the area? Is the €3000 net or gross?

To be honest, it doesn't seem too expensive at first glance. The material cost for the slab is probably around €15-20 per square meter (≈1.4-1.9 per square foot), plus labor. You know the area yourself, so try calculating.

EPS on top is certainly cheaper as a material than XPS underneath. But the difference won’t be huge. PUR is considerably more expensive, but requires less insulation thickness. This saves on overall build-up height.

Why do you want to insulate more than planned?

The floor area is approximately 75 m² (800 ft²).

I have been considering whether it makes sense to build “a bit” beyond the basic requirements, especially with a view to the future. In principle, I am satisfied with the KfW 55 standard.

As an option, I can upgrade the collar beam insulation from 240 mm WLG 35 to 240 mm WLG 32 for around 900 euros.

I have also thought about the base slab. There is the option of 100 mm XPS perimeter insulation with a WLG rating for about 3,000 euros.

Now I have considered improving the screed insulation as a “middle ground.”

Does this make sense?

Thank you very much.

In my opinion, that doesn’t make any sense. It’s better to aim for the next KfW level right away. This increases the value of the house and provides you with a bit more financial support.

I find the plan very good. To save costs, you could install the insulation yourself.

With the underfloor heating, you will have higher heat losses to the ground compared to radiators because it operates at a higher temperature. This could lower the supply temperature, increase efficiency, and thus save energy.

Especially if you install the PUR layer yourself, which should not take more than a day. Maybe it will be worthwhile then.