Floor structure with uninsulated base plate

Hello,

I have a question about a floor construction for an uninsulated slab, see sketch in the attachment.
We want to implement the following construction, but have received partly different recommendations regarding individual details.

This mainly concerns which insulation material should be installed between the cross battens and whether a variable vapor barrier makes sense or not.
It primarily concerns the problematic area cold slab to insulation.

Does anyone have practical experience on how such a construction can be optimally executed?

Best regards
Jens

Hello!

I admittedly have no idea at all about this construction option and searched out of interest for "Kreuzlattung KVH" ...
The first search result in the well-known search engine already includes the recommendation for the subfloor construction, which does not provide for a vapor barrier between the insulation (cellulose in this case) and the floorboards.

With what argumentation was the vapor barrier recommended to you in between?
It probably wouldn't hurt either ....

Hello Jens. Your sketch is very informative regarding the existing layer structure. I assume (please check if my assumption is correct!!) that the specified component layer "Abdichtung" is really a waterproofing layer and not just a PE foil that should be considered as a vapor barrier. The thermal insulation of the floor structure takes precedence in your case. Impact sound insulation in the basement can be completely neglected, especially since it may not be a multi-family house, which is the topic here. The structure is - again: waterproofing present? - then relatively simple. Ground-contact floor slabs only dry properly after a very long time. And the structure should be installed on the concrete floor slab, not on screed. That means, regardless of an existing waterproofing below the floor slab, a functioning vapor barrier is required. This will be a PE foil with a thickness of ≥ 0.1mm, which is laid overlapping by at least 10cm at the edges and secured with good tape against slippage of the sheets and "gaps in the vapor barrier". As thermal insulation, I recommend a mineral-bound (mineral wool) insulation of about 10cm thickness, which is installed between the support beams (for the floorboards). Since solid wood floorboards are to be used as the top layer, the edge joints of about 10mm width must also later be able to release any moisture rising despite the vapor barrier to the indoor air. Admittedly, no one likes open edge joints of that width. But that’s not necessary, because here there is the option to either install ventilated wall moldings or standard moldings that are supported on the backside at intervals with small strips (these glued on the backside of the molding), so that the baseboards only adhere to the wall at points. With this last technique, the edge joint (at the solid wood floorboard level) is basically rotated by 90° and is now in the vertical plane. The decisive factor is that ventilation of the substructure can take place. -------------------- Regards and good luck: KlaRa

Hello KlaRa,

thank you for your detailed assessment.

The component sealing the top side of the base plate is supposed to be a "real" vapor barrier. Under the wooden stud walls there is already a masonry barrier sheet, to which the area-wide, overlapping vapor barrier e.g. Katja Sprint or Gefitas PE 3/300 should connect.

Is there an alternative to mineral wool? In the walls we have deliberately used wood fiber insulation, so we do not want to use mineral wool in the floor.

I see the most critical point at the transition of the cold top edge of the base plate and the insulation; are there still optimization possibilities to avoid condensate? Regarding the moisture-variable vapor retarder under the floorboards I have so far heard both pro and con opinions.

Hello Jens.
"The component sealing upper side of the slab shall be a 'real' vapor barrier."
I thought I had expressed myself unequivocally. A vapor barrier cannot be a sealing, and conversely, according to the definition, neither can a sealing be a vapor barrier.
Katja Sprint is a sealing membrane, which certainly fulfils the designation "sealing" in a specific sense. However, my experience here does not result from studying advertising brochures but rather from practice and from binding product data sheets.
Mineral wool is resistant to moisture, if it should occur to a relevant extent on the insulation.
With wood fibers, however, this is not the case. Rot and odors in that direction are then not excluded. Keep in mind: we are not dealing with stud wall construction here, but we are moving within the area of floor construction.
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"Under the wooden stud walls there is already a masonry barrier membrane..."
This can make sense for slabs on a sealing. However, masonry barrier membranes are intended to stop rising moisture. If a sealing exists under the slab, there is no rising moisture but possibly only rising residual moisture. And this is manageable in terms of magnitude with 150mm thick slabs.
And regarding condensation, there should be no problems. That would have to be calculated.
Film on top, then wood, and underneath again film is a guarantee for rot formation in the wooden construction. Wooden substructures must always be ventilated. Then any possible condensation (which I do not assume due to the surface temperature of the slab) is also prevented.
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"Moisture-variable vapor retarders" are unknown to me. Either a PE film has a sufficiently high sD value (at least 400m) or it does not.
Then it is not a vapor retarder but a material self-deception. In building physics, there is only a strict "either-or" here.
And a plank flooring must always be ventilated if rising (residual) moisture is expected.
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Floor constructions belong to the most complex structures in building engineering!
Whoever ventures into this topic must know the technical rules and also observe building physics.
If one does not adhere to this, one must not be surprised if long-term success does not occur.
Regards: KlaRa

So I wouldn't feel good about wood. What speaks against insulation and at the same time a support made of XPS?