KFW 55 - Insulation under the floor slab

What do you think could happen? The insulation under the slab isn't that thick ... even if it gets crushed, the house won't fall over, right (-:

I don't think so, but I feel better when my house stands on solid ground.

Just use better insulation on the floor slab.

Our offer also included 10 cm WLG 035 under and on top of the floor slab. The heat demand calculation showed that this was not necessary for our Energy Saving Ordinance 16 compliance and planned with 14 cm WLG 035 insulation on the floor slab -> U-value of 0.24. This saved us almost €3,500, since now there were a total of 6 cm less insulation and no basement wall block was needed. However, this resulted in us having a 22 cm floor buildup and "only" 2.53 m ceiling height - but that's standard anyway.

The bottom line: The front doors were installed 1 cm too low and the screed company was cautious, which cost us another 0.5 cm, and the heating installers insisted on their Rolljet with only WLG 045. Now 2 cm of insulation were missing, which caused the U-value to go up, and I didn't want to argue with the heating company to deviate from their usual procedure. So we were faced with the problem of what to do? We talked again with the screed company, who went from 7.0 cm back to 6.5 cm and we took an aluminum foil laminated 10 cm insulation WLG 023. That cost us about €800 more than the normal WLG 035. But now we have a U-value between 0.18 and 0.19 instead of the calculated 0.24 and still saved €3,500.00 - 800.00 = €2,700.00 compared to the variant with insulation under the floor slab.

But it only works if you are flexible with the buildup height and probably not building turnkey. If you also do the insulation yourself (Anyone can really do that, but it takes some time. However, the result will be significantly better, because in my opinion a company simply doesn't have the time for the care that you put into doing it yourself), you save a big chunk of money overall.

we also did a test calculation with a WLG023 insulation and that would have made a negligibly small difference for us (I think 0.5 kWh/m²a) Did it really make that much of a difference for you? You built without a basement? Which company is your insulation from? With an 800€ price difference, you seemed to have gotten away very cheaply...

We have 12cm sub-floor insulation Styrodur / XPS. The advantage is that there are no thermal bridges, since the insulation on the slab is not under the walls (exterior and interior) but only between them. Additionally, you then have a dry slab (Styrodur = waterproof), because the sealing is practically under the slab instead of on top of it (bitumen membrane). The slab is thus inside the thermal envelope, and you don’t have to be an expert to imagine that a dry warm slab under the house is better than a cold and wet one. Yes, up to the bitumen membrane the slab is wet despite waterproof concrete, otherwise you wouldn’t need this sealing.

But in the end, it’s a matter of personal preference.

Concrete couldn’t care less whether it is cold and wet or warm and dry. With Styrofoam, I wouldn’t be so sure after 50 or more years. But you’re right, in the end two philosophies collide there. However, this only applies to houses with basements. For houses without basements, there is no way around perimeter insulation under the slab. Since our entrance area is located between the house and the garage, perimeter insulation is installed here, but as mentioned, not in the basement.

0.05 is about right. I played it through again. It is 0.044 improvement, since 14 cm compared to 13 cm (3 cm but with the worse Rolljet WLG045) are related. With 14 cm vs. 14 cm it would be 0.058. I calculated that with ubakus using the 3-week trial version. We have no basement.

It is the Iko Enertherm PIR Aluminum WLS 023.

We installed the insulation ourselves. The price was 220 sqm x 5 cm thickness at a total of 8.16 €/sqm. Through the company it probably would not have been much more expensive (The company’s material price for WLG 035 was not much higher than the material price when I got quotes).

The additional cost of €800.00 should pay off in about 18 or 24 years.

0.044 W/m²K x 84 heating degree hours x 100 m² = 369.6 kWh x 0.065 €/kWh = €24.00 savings/year (difference to the heat demand calculation in the building application) = 33 years payback.

0.072 W/m²K x 84 heating degree hours x 100 m² = 604.8 kWh x 0.065 €/kWh = €39.31 savings/year (difference to the actual situation) = 20 years payback.

If gas prices rise, the payback period naturally shortens.

Since I’m not even 30 years old and plan to live there for more than 20 years, it was worth it for me – especially since the additional cost is covered by the own work which was not planned yet in the financing.

We still have 10 cm insulation of the base – so the floor slab is insulated on the sides and from above...