Properly sealing the basement floor slab. Alternatives?

Hi,
after the water damage in our basement is now being fixed, meaning the damp screed, the underfloor heating, and the insulation will be removed, the basement floor slab is now to be sealed as a precaution.

A few facts:
House built in 1999
Almost 100 m² basement
Underfloor heating in the basement (total approx. 17 cm floor construction so far and also planned again)
Clayey soil
Load case according to DIN 18195 - rising seepage water
Drainage all around the house
Floor slab 20cm (not WU concrete)
Below that 15cm blinding layer / sand / gravel
Basement blocks outside 36.5cm Poroton, outside with thick coating and perimeter insulation
Above the first row of blocks R500 bitumen membrane
Below the first row of blocks flexible sealing slurry
Max. groundwater level allegedly 50cm below top edge of floor slab

The basement is not used as a potato cellar.
Underfloor heating should be reinstalled. Basement use for the time being as a party and guest cellar. Possibly as a granny flat or for the son in a few years.

I have now obtained a few offers from roofers, masons, and so on, and everyone wants to seal the floor slab differently somehow.
Maybe you can exclude one or the other method or recommend one to me directly.

It should be (if possible) cost-effective. I have sorted the (previous) offers from cheap to expensive.

Offer 1:
Single-layer Knauf Katja Sprint sealing membranes with overlap on the floor slab
Edge area to the masonry glued to floor slab with bitumen adhesive
Edge area at the first row of blocks with flexible sealing slurry and then pulled up with Katja Sprint
Corners cold-bonded with Knauf connection strips.
On top 2x 5cm polystyrene, foil, underfloor heating, and 6cm cement screed

Offer 2:
4mm flexible sealing slurry on the floor slab
Brought up to the first row of blocks
On top 2x 5cm polystyrene, foil, underfloor heating, and 6cm cement screed

Offer 3:
Single-layer V60 S4 (+AL?) single-layer on the floor slab, welded with overlap
Brought up to the first row of blocks
Edge area additionally sealed beforehand with sealing slurry
On top 2x 5cm polystyrene, foil, underfloor heating, and 6cm cement screed

Offer 4:
Cold bitumen primer on the floor slab
On top single-layer PYE 200 S5 talcum-coated, welded overlap
Brought up to the first row of blocks
On top 2x 5cm polystyrene, foil, underfloor heating, and 6cm cement screed

Offer 5:
Hot bitumen on the floor slab
Foam glass (exact thickness and foam glass type not yet specified)
Foam glass glued with hot bitumen up to the edge area
Covered with a V60 S4
On top 1x ??cm polystyrene, foil, underfloor heating, and 6cm cement screed

Regarding offer 5, the roofer wanted to get back to me. He had not processed it that way yet but would recommend it to me. He wanted to get back regarding exact planning, execution, and price. Due to the high material price, he was already sure that it would be the most expensive offer.

Regarding possible formation of condensation water at the edge area of the insulation and thus the formation of a condensation pond under the insulation, a mason considered laying 5cm polystyrene on the floor slab, then Knauf Katja Sprint, and again 5cm polystyrene on top. A precise execution could not be given to me here either. He said he would pass this on to a roofer friend who would contact me.

I hope you give your (constructive) input.
Many thanks in advance.

Hello,

I hope you give your (constructive) two cents on this

My constructive two cents are: bring in someone who knows about it very quickly and under no circumstances assign the work on your own. If you have no clue what is right and wrong – and since we have no exact knowledge of the building substance, soil conditions, etc., we certainly will not give any recommendation here – an expert should explain and recommend the appropriate measures to you. Saving at the wrong end with such sensitive work can become quite expensive.

Kind regards

Hello,
My constructive two cents are: quickly bring in someone who knows what they’re doing and under no circumstances assign the work on your own. If you have no clue what is right or wrong – and since we here will certainly not give any recommendation without exact knowledge of the building fabric, soil conditions, etc. – an expert should explain and recommend the right measures to you. Saving at the wrong end with such sensitive work can get pretty expensive.

Kind regards

Hi Bauexperte,
maybe you missed it, but I HAVE brought in specialists.
Everyone apparently has their favorite method of how it should be done, or among the roofers there was quite a bit of overlap.
I was shown various alternatives, which I will repeat here and hope that someone can tell me which I should rather choose and which rather not.

Details about the building fabric and soil conditions are also in the text by the way. If there are further questions, just write.

And please critique or improve the offers...

Hello,

maybe you overlooked it, but I DID bring in experts. Everyone apparently has their favorite method of how it’s done, or with the roofers it overlapped quite well.

I did read that you spoke with trade-related experts and that you also have their offers. What I meant with my "two cents" was simply that you need someone who can explain to you the meaningfulness of the proposed measures based on your house and its parameters. Only when you are sure which advantages and disadvantages you are taking on with the respective alternatives can you make the optimal decision.



For example, I do not believe that it is enough to have a better sealed base plate alone. With rising seepage water, a waterproof masonry is always advisable; perimeter insulation and bitumen thick coating alone are not sufficient here. And I would certainly aim for a different solution instead of a drainage – with which you pull in all the water from the nearby surroundings.



Ask a building expert you trust – someone who is familiar with the problems of older houses – and discuss the various possibilities and measures with him. If you rely solely on the statements of the craftsmen – who surely submit their offers to the best of their knowledge and belief – you will, in my opinion, soon have water in the basement again despite extensive renovation measures.



Kind regards