ᐅ KfW 40 / 40 Plus Insulation for the Foundation Slab

Hello everyone,
we are planning a new build without a basement and want to insulate the concrete slab optimally. The house will likely be constructed with 42.5 cm (17 inches) aerated concrete. It is especially important to me that the insulation is well protected against thermal bridges.
Do you have any suggestions, experiences, or expertise on this topic? Which materials and methods would you recommend to effectively insulate the concrete slab?

I have read a lot online but haven’t become any wiser, as there is very little information available about KFW 40 and thermal bridges at the same time.

Our contractor’s recommendation is to insulate only above the slab.

Thank you in advance for your help!

m0LN4rius schrieb:

It is particularly important to me that the insulation effectively prevents thermal bridges.

That applies to every new build. Whether you can improve it slightly and save €1.25 in heating costs per year through perfect optimization is another matter.

m0LN4rius schrieb:

Recommendation from our contractors: insulation only on the upper side.

Then build it like that. Alternatively, your architect or planner should propose a solution that perfectly fits your individual construction project.

m0LN4rius schrieb:

Since we are building near railway tracks and a level crossing, we have repeatedly emphasized that we want to use bricks for sound insulation and durability instead of aerated concrete. This request was completely ignored, and instead the response focused on „You will get a KfW-40 house,“ referring to aerated concrete 42.5.

m0LN4rius schrieb:

We are planning a new build without a basement and want to optimally insulate the foundation slab. The house is expected to be built with 42.5cm (17 inches) aerated concrete.

Do you now agree with your possible general contractor's disregard of your preference, or where does the shift toward their proposed wall construction come from?

m0LN4rius schrieb:

The main issue is that this is our only building option, as the plot is also being brokered through this licensee.

How economically reasonable the wish to build without a basement is cannot be assessed without knowing the plot. As I understood, if the cancellation deadline is met, you would have to look for a plot again first.

m0LN4rius schrieb:

It is especially important to me that the insulation is well protected against thermal bridges.
Do you have suggestions, experience, or expertise on this topic? Which materials and methods do you recommend for effective insulation of the foundation slab?
I have read extensively online but have not become any wiser, as there is very little information combining KfW 40 and thermal bridge issues.
Our contractors recommend insulation only on top.

Which contractors specifically, and why do they recommend insulating only on top?
If the exterior walls are insulated only above the foundation slab, they are essentially not insulated underneath. This approach, from my perspective, does not come close to the EH40 standard you aim for any more than using aerated concrete in 36.5cm (14 inch) thickness would.

If you continue to look for your own plot, will the plan still be for a semi-detached house?

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We have a basement on a slope and 20cm (8 inches) high-compressive XPS insulation with KFW40 standard under the slab. We are also using 42.5cm (17 inches) masonry units. According to many opinions, insulation above the slab is no longer considered state of the art.

paddi schrieb:

We have a basement on a slope and 20cm XPS high-compressive insulation under the slab for KFW40. Also using 42.5cm (16.7 inches) thick masonry units. According to many opinions, insulation above the slab is no longer up to date.

This has nothing to do with opinion or trends. The walls rest on the slab, and there is no insulation between the slab and the walls. If there is no insulation under the slab, temperatures from the interior and the ground exchange through the slab acting as a conductor, making the slab a pathway for cold from the ground into the masonry. This heat transfer through the masonry is essentially the same as having a wall with lower thermal insulation. Therefore, my statement: "Masonry with 42.5cm (16.7 inches) thickness without insulation under the slab will no longer be adequate compared to 36.5cm (14.4 inches) masonry with insulation under the slab." It’s like leaving a bicycle next to a fence with only a lock around the frame instead of properly securing it to the fence—it can be easily stolen. Once the cold penetrates through the slab into the wall, insulation laid as a blanket on the floor only benefits the interior space, but no longer the walls. Partial insulation is nonsense. And here, KFW40 standard is supposed to be aimed for.

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