Sealing on Styrodur insulation?

Hello everyone,

our shell builder installed the perimeter insulation (Styrodur) directly into the formwork (see photos), which caused the concrete and the Styrodur to bond.

Now we are about to plaster (outside) and I am hearing from different sources that this may cause problems regarding sealing, because normally the concrete is sealed with bitumen and only afterwards the Styrodur is applied. Bitumen on the outside of the Styrodur is said not to adhere.

Another plasterer thinks that sealing with sealing slurry (WeberSuperflex D2) would be the right way here.

Since I could not find anything about this online, I wonder if you might be able to give me some tips. Does anyone have experience with this?

How do, for example, people handle this when using precast formwork elements from Lohrelements?

Data: foundation slab with strip footings, no basement. No pressurized water. Masonry Ytong.

Best regards, Gerd

That is of course nonsense that the waterproofing does not stick to the Styrodur. If that were the case, you could not glue the Styrodur to the concrete with the waterproofing. I rather fear that bare concrete does not adhere to Styrodur, which could cause the panels to fall off. Probably something can still be done with insulation anchors. Nevertheless, capillary water could get between the insulation and the concrete. But this is only a layman’s assumption. If a sealing slurry is applied only on the outside, it must in any case be absolutely tight to the outside, including in the joints.

There are self-adhesive bitumen sheets that are used for this purpose. Just google "selbstklebende Dichtungsbahn Bodenplatte" and you will find various providers, with examples included.

Otherwise, I see the moisture issue as rather uncritical since your mason has also installed a barrier layer between the floor slab and the stone. Inside, a welding membrane or a Gefitas must be installed anyway since there is no basement. So nothing should happen there.

It seemed to me as if one really seals on the component itself and not on the insulation - although I am also an amateur.

With a basement, I would do that too, but since there isn't one. The slab is in contact with the ground anyway, so it probably doesn't matter if and where you put something there. The only important thing is that an internal barrier is installed, which must be connected to the barrier in the stone so that if there is moisture, it does not get into the insulation.

The sealing on the insulation is not permissible. DIN 18195 explicitly states that the sealing must be applied to the concrete base slab. The manufacturers of the sealants also make this very clear in their datasheets. Anyone who has ever conducted an adhesion test of a self-adhesive sealing membrane on, for example, Styrodur will find that the sealing membrane does not really hold. If this issue were examined by an expert in a report, the sealing would be rated as "deficient." According to the aforementioned DIN, the insulation is intended to protect the sealing and not the other way around. Another point is the statics. As a rule, the full width of the brick is used for load transfer. If the formwork of the insulation (Styrodur) is flush with the masonry brick, this area is missing for load transfer.