Vapor barrier under parquet with underfloor heating?

[Deltaman]

Hello

I am renovating an extension of approximately 50m2 with a basement constructed around 1982, featuring a lightweight concrete floor structure as shown in the attached image. I have removed the foam, chipboard, and plastic film on the top and instead plan to frame a floor with glass wool between the joists and lay grooved chipboard with water-based underfloor heating, finishing with Bjelin's 9mm Woodura wood parquet.

The manufacturer Bjelin (and all other manufacturers for that matter) says that I should install a vapor barrier between the underfloor heating and the parquet (see the attached installation instructions). I would resolve this with foam featuring an integrated vapor barrier (e.g., Kährs Tuplex).

The problem is that my constructor says the vapor barrier is completely unnecessary and could even be a risk construction that traps moisture within the structure!?

The basement is heated but will likely always be a few degrees cooler than on the ground floor, especially in the summer without additional heating. I know enough to say that a vapor barrier should always be on the warm side, but if it's placed right under the parquet, and the underfloor heating beneath the vapor barrier is warmer than the room above, then what is the warm side?

I am currently quite puzzled, so all suggestions, thoughts, ideas, knowledge, etc., regarding the necessity and placement of the vapor barrier are welcome!

 

[O]

The manufacturer Bjelins (and all other manufacturers for that matter) then says I should install a vapor barrier

Written instruction from the manufacturer.

my engineer says that the vapor barrier is completely unnecessary and can even be a risky construction

Someone else "says that."

Which of these do you choose to follow?
Choose for yourself, "someone on a forum" (me) might say

 

[Deltaman]

Written instruction from the manufacturer.

Someone else "says that".

Which one do you choose to follow?
Choose for yourself, "someone on a forum" (me) might say

Of course, you normally always follow a manufacturer's instructions BUT their only interest is to protect their product so it doesn't get damaged. A certified designer works with and designs moisture-proof structures for entire buildings, and when there's a contradiction, you seek knowledge from other sources. So grateful for responses from members with knowledge on the subject.

 

[Jansson69]

Of course, one typically always follows a manufacturer's instructions, BUT their only interest is to protect their product from damage. A certified designer deals with and designs moisture-proof constructions in entire buildings, and when there's a contradiction, one seeks knowledge from other sources. So I appreciate responses from members with expertise on the subject.

We install and have done so for surely twenty years, always using age-resistant plastic foil on the water-based underfloor heating if laying wooden floors.. You have to ask yourself who will bear the costs if something goes wrong, does your designer take responsibility for the floor if condensation water forms? You should consider that it's not just one manufacturer describing this but almost all, and I would trust them more than a single designer..