XPS panels basement insulation - no bitumen layer

Hello everyone,

we are building with individual contracts and have awarded the shell construction to a very good regional shell constructor. We did not have a soil survey done before the start of construction because the municipality had already drilled on our property, and it was found that we have clay and silt.

We are building a KFW 55 house with a basement, 2 of the basement rooms have daylight due to a slight slope and are intended to become living basements with underfloor heating, etc.

For the insulation of the concrete basement, XPS boards from the company Austrotherm were used. The whole process was supervised by a construction supervisor.

Now more and more people are building in the region (in other construction areas), and we noticed that on most houses a black coating is applied before the insulation boards are attached, and then the boards are glued on. In our case, this is not so; the boards were fastened with dowels without this layer.

The construction supervisor and the shell constructor both say independently that since we do not have pressing water, we do not need this coating.

Am I worrying unnecessarily, or should I have all the boards removed (can they be reused or do I have to use completely new ones?) at my own expense and have the coating applied afterwards?

Backfilling has not yet taken place.

A point bonding has been contractually agreed, it is not specified there exactly whether a bitumen layer or similar is applied in between. However, the manufacturer of the boards requires the coating on its website.

How was this executed in your case?

With the soil conditions, you probably have temporarily accumulating seepage water...

And what would that mean? I need the coating?

I found this with other manufacturers.... but it doesn’t say anything about it either

However, the manufacturer of the panels prescribes the coating on his website.

If this is stated like that in "official" product datasheets, then the processing company must adhere to it. Period! That makes the discussion about the load case "exiting seepage water" unnecessary – which you would have to prove in case of doubt. If you have a white tank, I would, however, see that as uncritical, but you have not mentioned anything about that.

(...) For the insulation of the concrete basement ... n (...) Now more and more people are building in the region (in other construction areas), and we have noticed that on most houses a black coating is applied before the insulation boards ...

It may be that the other houses simply have a different basement wall structure, namely a masonry basement wall, instead of a concrete ("white tank") basement. The WU concrete is impermeable to liquid water but not to water vapor. According to the "shot glass formula" (an S-glass amount of water per day per square meter), some water is allowed to diffuse through WU concrete. This is then "ventilated away." This can and should be prevented by an additional bitumen coating. However, this is then the suspenders and belt method. It is not necessary but does no harm—especially in the case of unplanned cracks and defects—provided it is a planned white-tank concept.

From personal experience:

Only that in our case the basement will be below ground level. One is the DIN-compliant solution (bitumen thick coating), the other is the practical solution. We had the choice with or without (and also on a slope). The variant with it would have cost us €3000 net more. We decided against it because the shell builder, architect, and construction supervisor are of the opinion that the slabs do not require this additional measure due to the pressure of the soil and watertight concrete basement.