Insulation under the floor slab - Is it sensible? Experiences

Here are the arguments why we decided on this. Mind you, we did NOT consider whether it would ever pay off; that was not our decisive factor.

- if you include the floor slab in the thermal envelope, thermal bridges are very unlikely to occur. We use the cellar as living space and hope to gain comfort through a pleasant, even floor temperature. Because especially at the edge near the exterior components, cold air currents are eliminated. For the insulation of our floor slab, the insulation layer was designed slightly larger than the floor slab, and we hope this prevents a thermal bridge at the junction between the floor slab and exterior walls.

- If there is ever water damage in the cellar (not so unlikely when I look at the water softener, but that's another topic), drying is easier if the insulation is outside.

My layman’s understanding simply tells me that if I don’t insulate under the floor slab but only inside, then some components are simply not insulated from below.

The only thing that concerned us for a long time was whether the better styrofoam down there can withstand such high weights at all and won’t eventually be compressed.

We’ll see….

From a thermal bridge perspective, clearly yes, insulating under the base slab has significant advantages. You will never get access to it again, so I am an absolute advocate for insulation underneath, better more than less.

There is no need to worry about the XPS compressing. It is an absolute standard to insulate under the base slab; I am not aware of a single case of any problem.

Mind you, we did NOT pay attention to whether it would ever pay off; that was not the decisive factor for us.

That is probably the crux of the matter. It will never really pay off, and you won't feel a difference in the house.

Let's put it this way: The better is always the enemy of the good. But it always has to be weighed financially as well.

On the topic of amortization: Insulation not only reduces energy costs but also increases the long-term value of a house. This applies even more to insulation under the floor slab since you never get access to it again and the requirements for insulation are always increasing.

It is always easy to say I can omit this or that part, it doesn't affect the living quality... but overall, living in a well-insulated house is noticeably more comfortable than in a poorly insulated house (assuming the right building services).

Currently, the floor slab has been offered with 10cm insulation underneath. I was just informed that omitting the insulation means a minimum price of 2150€ including tax. According to hearsay, 80 mm is sufficient for KFW 55. Now I just need to find out this afternoon to what extent the U-value of the floor slab deteriorates by omitting the insulation. Then I can theoretically calculate the impact on heating costs. As I said, I am only concerned with the financial benefit. The 100 mm or 80 mm Styrodur that has to be omitted must also be filled and compacted again with gravel or whatever. That also costs money, but not 2150€.

but overall, living in a well-insulated house is clearly more comfortable than in a poorly insulated house

... whereby insulation on the floor slab does not mean that the house is poorly insulated. And the second buyer does not care how well insulated it is or how much "creative accounting" was done for the KfW. What matters to them is the actual consumption.