ᐅ Floor structure and subfloor construction in the attic with loose fill?

Thanks for the information.

I recently had a floor installer on site to get his opinion.

He gave me the following two suggestions/recommendations:
Option 1: Wet screed

• Mineral wool between wooden joists
• 16 mm OSB
• 10 mm wood fiberboard (as decoupling layer, installed floating)
• 40 mm wet screed
• approx. 8 mm flooring (e.g., click vinyl)

• Total build-up height: approx. 74 mm (3 inches)

Advantages:
+ cheapest option

Disadvantages:
- high build-up height

Option 2: Dry screed (2-layer)

• Mineral wool between wooden joists
• 10 mm wood fiberboard (as decoupling layer, installed floating)
• 2 x 18 mm gypsum fiberboard (Knaufprio) (screwed together, loosely laid on wood fiberboard)
• approx. 8 mm flooring (e.g., click vinyl)

• Total build-up height: approx. 52 mm (2 inches)

Advantages:
+ low build-up height
+ just as stable as wet screed
+ lighter than wet screed

Disadvantages:
- very expensive (2-3 times the cost of wet screed)

His opinion on vinyl flooring with cork:

• becoming very popular and widely offered
• he is skeptical because of lack of long-term experience (only available for about 5 years) and (still) no standardized regulations (e.g., allowable deflection limits, etc.)

What do you think about all this?
The problem is that due to the high joists, we only have a limited build-up height of about 5 cm (2 inches). If we exceed that, we would have to accept a small step down to the adjacent room...

Hello Markus.
Wet screed is absolutely not an option given the current situation!!
Installing a wet screed on a wooden substructure means introducing significant (!) moisture into the building, which is exactly what you want to avoid in construction.
Simply put: this will not work well. And it will not work out.
Option 2 sounds reasonable and will work.
Be assured (speaking from experience): the money you might save with option 1, you will have to spend at least four times over on renovation. I'm definitely not trying to scare you.
But the problems you will bring into the apartment with that moisture are going to be significant.
Aside from the load-bearing capacity of the supporting joists, which must first be checked by a structural engineer.
A calcium sulfate screed will load the surface with about 74kg/m² (15.2 lbs/ft²), a cement screed with about 95kg/m² (19.6 lbs/ft²).
For most wooden beam ceilings, this is too much—especially considering that an additional 200kg/m² (41.1 lbs/ft²) is typically factored in for residential construction.
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The choice you make is up to you, as is the responsibility.
Focusing only on the expected savings has financially ruined many well-intentioned (or naive) homeowners. And I know what I’m talking about!
Addendum:
If a floor installer refers to normative deflection requirements regarding an LVT (luxury vinyl tile) floor, it usually means they are trying to appear knowledgeable to a non-technical person, without actually having the expertise.
This aspect itself is completely irrelevant for a top covering!
What is normatively regulated applies to PVC planks.
Multilayer modular coverings, as the term goes, are covered in DIN EN 16511.
Just something to consider regarding competence.
Best regards, KlaRa

Hi KlaRa,

Thank you very much for your prompt response and your explanation, especially about the wet screed. That does worry me a bit since the man seemed quite knowledgeable.

Anyway, it looks like option 1 is off the table. Do you know approximately what load I would have per square meter with the two 18mm (3/4 inch) gypsum fiberboards (Knaufprio)?

And what is your personal opinion on the increasingly popular vinyl flooring with cork underlay as impact sound insulation? Would you recommend it without reservations, or do you see any significant drawbacks?

Hello Markus,
The load-bearing capacity of 2x18 mm (approximately 3/4 inch) gypsum fiberboards (Knaufprio) per m² (per square meter) is not only a matter of the dry screed system (this term is commonly used for all dry screed assemblies) but primarily depends on the spacing of the supporting joists. And as it turns out: in a country where even cucumber sizes are standardized, there is no single regulation specifying joist spacing based on live loads.
Only a regulation from Austria was helpful in this regard until about eight years ago, but it has since been withdrawn.
One can generally assume that 2 x 18 mm (3/4 inch) OSB boards or BRIO18, with these elements not screwed together and a joist spacing of less than 50 cm (20 inches), can reliably support a load of 2 kN/m² (41.7 psf).
The so-called impact sound insulation between the boards should not be too thick; otherwise, the entire surface will behave ductilely when walked on and be subjected to bending stresses. Personally, I would proceed as follows:
On the first layer of elements (which can be screwed onto the joists),
one layer of "Regupol(R) Sound 47," with a sheet of paper as a separation layer on top, followed by the second layer of Brio or a similar product.
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(Quote) "And what do you personally think of the increasingly popular vinyl with cork underneath as impact sound insulation?"
Answer:
LVTs (luxury vinyl tiles) have seen the greatest increase among flooring types since well before 2017.
They are available for adhesion or fixing to a subfloor, with click systems (which I assume is what you mean?), and as loose-laid versions with a heavy backing.
The cork on the backside cannot really serve as a "serious impact sound insulation." Even if attached below the support panel, it does not help improve walking comfort. It is more of an “anti-squeak” layer when the elements are installed directly on a cement screed and there are still sand grains on the surface—nothing more!
It offers little benefit but is not harmful. That is how I would put it….
I don’t see any disadvantages!
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Good luck with your purchase and installation: KlaRa

Hi KlaRa,

First of all, thank you very much for your detailed response. It’s great to have people like you who are willing to share their knowledge with non-experts and also have the patience to do so.

I still don’t fully understand exactly how you would proceed. Could you please describe the individual layers step by step in detail for a layperson, including precise names of the products?

The recommendation I received was:

• Place mineral wool between wooden joists (200 mm (8 inches) high)
• 10 mm (0.4 inch) wood fiberboard (as decoupling layer, laid floating on the joists)
• 2x18 mm (0.7 inch) gypsum fiberboards (Brio 18) (fastened together with screws, loosely laid on the wood fiberboard)
• Approximately 8 mm (0.3 inch) floor covering (e.g. click vinyl) on top of the gypsum fiberboards

How exactly would you change this now? For example:
- Do you mean the "Regupol(R) Sound 47" as a substitute for the 10 mm (0.4 inch) wood fiberboard? And would the Regupol then simply be laid loosely on the joists?
- What kind of paper do you place as a separation layer between which materials?
- What is the first layer you would screw directly onto the joists?

Wishing you a nice Sunday,
Markus

Hello Markus,
Before I sign off for the weekend, here’s a quick update:
Installing mineral wool between wooden joists (200 mm high) can be done. However, impact sound insulation improves with increasing mass. Mineral wool doesn’t add much in this regard; a perlite loose fill would be better. But the choice depends on how much weight the rafters below can support. You don’t want the whole thing to fall into the room after the ceiling joists underneath give way.

10 mm wood fiberboard (used as decoupling, laid floating on the joists): how is that supposed to work? How can you protect this layer against unintentional shifts over the coming years?
You can nail thicker fleece strips (for example, 30 mm thick) onto the joists, making sure the nail heads are well covered (they should not protrude). That would work.
On top of that, lay double-layer OSB panels floating, screwed together and glued tongue-and-groove.
This would create decoupling above the fleece strips (on the joists).
Or
It doesn’t matter whether you use OSB or two layers of 18 mm gypsum fiberboards (Brio 18).

(Quote) “How exactly would you change this now? For example:
- Do you mean Regupol® Sound 47 as a replacement for the 10 mm wood fiberboard? And would the Regupol then just be laid loose on the joists?”
Answer:
Nail fleece strips on the joist layer,
place one layer of OSB or Brio on top,
then lay the Regupol mat floating over that,
and on top of that, the second layer of Brio or OSB, glued tongue-and-groove.

(Quote) “What kind of paper would you use as a separating layer between what?”
Answer:
It doesn’t matter. Newspaper layers or wrapping paper, possibly also crepe paper (sometimes used as a slip layer, historically also under wet screeds).
The main point is that sliding can occur between layers so that no internal stresses develop, which could eventually lead to warping of the finished floor surface.
The mineral wool in the cavities, the fleece strips on the wooden joists, and the Regupol mat together will provide a good amount of sound insulation. Technically, the values of the individual layers should not simply be added, but it works effectively in practice.
Finally, a reminder:
You will never achieve the same level of sound insulation with timber construction as with solid construction.
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Signing off: KlaRa