Laminate in the basement: Lay PE foil despite vapor barrier?

Hello,

we are planning to lay laminate flooring in the basement. It is often said that a foil should be placed under the laminate in the basement.
Does this also apply if a vapor barrier has already been applied to the raw concrete?
And if so, why is a foil only required under the laminate in the basement, but not on the ground floor and upper floors?

Thanks!

Hello questioner. Laminate flooring is not suitable for the case that the corresponding basement room has already been assigned a subordinate use (thus not as living space) in its planning. A vapor retarder on the raw concrete slab does not protect the layers above (this also includes the cement screed) from slow dampening, because firstly a considerable residual amount of water always remains in a ground-contact concrete slab and secondly a vapor retarder cannot prevent a water vapor flow (there is no such thing as a "vapor barrier"; if anything, it is a seal or a vapor retarder). While a floor slab can release its residual moisture only "upwards," this is different with inhabited floor ceilings. There, the residual moisture of a concrete ceiling migrates upwards as well as downwards, depending on the vapor pressure gradient. Another note: In a pure basement room (thus not in a living space) there is no thermal insulation (and why should there be). If one were to lay laminate flooring on cold concrete there and use the room regularly, for example as an office, the dew point would be undershot on or possibly also under the laminate flooring elements. Air humidity can condense there as water. And because carrier boards for laminate flooring elements are usually made of moisture-sensitive wood-based materials, deformations and swelling cannot be excluded. However, there is an alternative for installation: laminate flooring elements suitable for installation in bathrooms. These carrier boards are moisture-resistant and do not deform (so easily). Good luck: KlaRa

Hello KlaRa,

thank you very much for the detailed answer.
Now, in our case, the basement is insulated on the outside and a 6cm styrofoam insulation (plus 2cm impact sound insulation) is laid on the vapor barrier. The entire basement has underfloor heating. One room is regularly used as an office.
In addition, a controlled residential ventilation system is installed.

Does the following still apply: foil under the laminate?

Hello questioner.
With the current additional information, it becomes clear that it is a living space located in the basement with a floating screed.
The basic rule here is: Yes, PE foil is necessary as a vapor barrier!
Regards: KlaRa

if I may join in here:

in our basement, a sealing membrane with bitumen (A*ujet) was laid on the floor slab made of WU concrete, followed by polystyrene insulation and screed.

Is a PE foil still needed here or can the screed be covered directly with elastic coverings such as linoleum or rubber?

WU concrete, that is concrete made to be "waterproof," is always permeable to water vapor! In physics and building physics, we differentiate between the different states of matter. Thus, WU concrete allows water to penetrate the cross-section only to a limited extent under a water column of up to 5 bar (I think it is about 5 cm). The water vapor pressure, on the other hand, "marches" through the structure regardless of the w/c ratio, always following the lower partial pressure. In the case of a slab on the ground, the lower pressure is always in the space above it.
This means: YES, a PE foil should also be installed as a vapor barrier here on the screed. This still corresponds to the current state of the art as well as to the majority of the installation instructions from manufacturers of floating laminate floors (unless these have been approved by the manufacturer for wet rooms).
Regards: KlaRa