ᐅ Horizontal waterproofing of the foundation slab

Hello!

I’m interested in your opinion again.
In about one week, the screed will be poured (plumbers and electricians are currently busy). Now the following issue has come up:

- We don’t have a horizontal damp proof membrane on the slab (no basement) and the screed installer has pointed out that this apparently does not comply with the standards.

- We don’t have a horizontal damp proof membrane because the builder said it wasn’t necessary since we have a 40 cm (16 inches) capillary-breaking layer (Geocell foam glass gravel). This was also recorded in the contract protocol and the construction meeting minutes.

- The architect also confirmed the non-necessity with a building physics calculation, which is documented in the construction meeting minutes.

=> Our concern now is what, for example, the insurance company would say in case of a pipe leak. Could they refer to the standard and possibly reduce the payout in case of damage, even though the non-necessity was confirmed by our professionals?

Here is the structure again (as mentioned, no basement, so directly on the ground):

200 mm (8 inches) gravel
400 mm (16 inches) foam glass gravel (capillary-breaking)
200 mm (8 inches) concrete
110 mm (4.3 inches) EPS granulate
70 mm (2.75 inches) cement screed
10 mm (0.4 inches) tiles

What do you think? Should we definitely add a damp proof membrane?

Best regards
Jochen

Hello,

it is almost certain that it works without the waterproofing membrane on the foundation slab. Insurance companies will always refer to the recognized technical standards, so there could be an issue there. However, I find it hard to imagine such an insurance case. Pipe burst under the foundation slab? I don’t know...

Sometimes my imagination runs away with me.

At this point, only a spray-applied bitumen coating of about 3 mm (0.1 inch) would be possible, and this would be applied over all the pipes. Wouldn't that create some kind of seal against the floor construction?

Concrete slabs in direct contact with the ground, and above which rooms with primary use are planned (such as living spaces), must be sealed against rising moisture!

Capillary-breaking layers like gravel and foam glass gravel (the latter also providing excellent thermal insulation) do not prevent water vapor movement through the concrete. Concrete can be poured as watertight concrete, but this does not affect water vapor diffusion!

Water vapor always follows the physical principles of vapor pressure gradients. Under ground-contacting concrete slabs, we empirically expect high humidity levels. In the rooms above, there will always (!) be lower water vapor pressure.

Therefore, water vapor "moves" through the concrete into the living space, attempting to equalize the lower humidity indoors with the higher humidity below, in accordance with physical laws.

This means:
Regardless of whether gravel is placed under the slab or not, the indoor humidity will always be high, and mold growth will become a persistent and unwelcome issue in the building, with no effective way to prevent it in the future.

I am not one to "stoke fear"! I approach mold issues objectively. However, with the proposed slab construction without a vapor barrier, I clearly understand what is likely to happen for you shortly.

My recommendation:
Submit your concerns in writing and insist on a vapor barrier on the slab. Take photos of construction progress as evidence.

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Regards, KlaRa

Thank you for your detailed and clear explanation. It is likely more about the moisture issue rather than, for example, a possible pipe leak. In another forum, someone also raised the topic of radon exposure. However, we are in radon potential class 1 here.

The whole situation feels too risky for me, and I don’t want to take any chances here. In the end, we would be the ones dealing with the problem, and the impact could be quite significant compared to the cost. Therefore, we will apply a spray-on waterproofing membrane to the foundation slab and the installed pipes before placing the backfill. Better safe than sorry!

Thanks anyway for your input!

Best regards