ᐅ Insulating the upper floor ceiling. The ceiling construction consists of panels mounted on battens.

There is basically nothing against the use of installation panels

as long as the insulation setup beneath is correct.

Or what exactly do you mean by "installation panels"? I understand that to mean screed elements, chipboard, and OSB panels. So far, the discussion has been about the insulation setup rather than a new wearing surface on the floorboards?

Regards

I meant insulating boards, not OSB.

There are boards you can lay down that have very good insulating properties and are also walkable. Unfortunately, I can’t find the website right now.

Looking at it from a different perspective, the problem arises in the cavity between the beams, is that correct?

If I lift the old floorboards and install insulation between the beams and the straw-clay ceiling, would that be better?

And how should the vapor barrier / vapor retarder be installed?

Best regards

Maik

You probably mean Styrodur or PS20 polystyrene foam,

but in my opinion, these materials belong underneath a screed concrete layer on the ground floor or a concrete slab. Rigid foam does not allow moisture to pass through and would cause condensation problems in the beams again, so I think it is unsuitable for your construction.

"Looking at it differently, the problem arises now in the cavity between the beams, is that correct?"

I already answered that in my first post:

"Without insulation, the residual heat escapes through the floorboards and then through the uninsulated roof."

In addition, I would say that with the current uninsulated setup, no condensation problems should occur.

Now, since you directly asked me about the vapor barrier solution... I am personally completely against using plastic sheets and foils inside a house, especially in older buildings. But when I had to insulate an attic space in a new build, I used this method, while trying to minimize the risks:

My first solution was: gypsum plasterboards fixed on top of the beams (which was supposed to look “rustic” in the new build), then a vapor retarder stapled on top of everything, followed by battens screwed on top of the beams, and gypsum boards then fixed underneath, attached to the battens. Mineral wool was installed tightly and continuously over that, and done. The dew point was located inside the gypsum board, which can absorb some normal condensation moisture and release it again or simply allow it to pass through. This system did not trap any wood since the beams were below, and the battens were on top.

For concealed beams, I first stapled the vapor retarder tightly and with good overlaps from below to the beams, then installed battens from below followed by gypsum boards. Mineral wool was placed between the beams from above. Here, only the battens (impregnated) were below the vapor retarder. Therefore, moisture could not penetrate the beams. I have had no problems with either the first or second construction method over the past nine years. The attic is insulated on the sloping parts but is, of course, unheated.

Neither solution would probably work for you, since you certainly do not want to remove your panel ceiling on the clay plaster. If you also want to keep the floorboards, then I would recommend doing nothing. But since you are now considering removing the floorboards, here is my personal suggestion on how I would proceed in such an old building:

Unfortunately, remove the floorboards and inspect the beams: are they all in good condition? If you are already opening it up, this should be checked. Then check if there are any inserts already within the ceiling (so-called “plank boards”) containing clay, straw, rubble, or empty beer bottles. If yes, the demolition could already be stopped. If the material on the inserts is not wet or heavily contaminated, I would leave it in place. It also provides insulation and, due to its mass, reasonably good sound insulation if the floor is eventually finished. If there really is nothing between the floorboards and the ceiling, screw impregnated battens vertically to the sides of the beams as deep as possible, then install inserts made of gypsum board or thin tongue-and-groove wood panels on top of those. Pour loose-fill insulation over that, possibly with a fine mesh underneath to prevent leakage, but no plastic foil! Ideally, build the inserts so tightly that no gaps remain. Fill the loose insulation almost to the top of the beams (at least 20cm (8 inches) would be good in my opinion). Then leave the floor open, place walk boards on top, and that’s it.

If the attic remains unoccupied, moisture might rise unimpeded because it cannot be stored by insulation anymore and then move through the roof. In my layperson’s view, no vapor retarder would be necessary in that case. If the attic is later developed and heated, there is very little vapor pressure difference between two heated floors, so in my opinion, a vapor retarder would still not be needed. But please reinstall floorboards and avoid using toxic OSB or particle boards! Those can behave like vapor retarders and unnecessarily cause moisture to condense on the beams. Normal, diffusion-open coated floorboards, in my opinion, allow moisture to pass through. Avoid creating unnecessary barriers to convection.

Regarding the insulation material: I advise against any “bio insulation.” Most of these are impregnated with insecticides to prevent damage from bugs and other pests. Cellulose is also not without issues, as it is treated with toxic borates in my opinion. I trust expanded clay and untreated mineral perlite—so nothing biological.

Maybe you should discuss this with someone who can actually call themselves a “building expert” to get their opinion on this. I am also interested in objective criticism.

Best regards

Hello @Maiks,

If the current construction is free of condensation and the attic is not intended to be used as a living space, rolling out mineral wool mats over the existing floorboards is basically unproblematic.

The concern that this could shift the dew point further inward within the collar beam ceiling is unfounded...

There are two critical areas for the transport of water vapor (from the warm indoor air):

Diffusion:
Water vapor molecules penetrate solid building materials and condensation can occur if one layer in the thermal envelope is less permeable to vapor than a layer further inside.

This does not seem to be the case here, as no moisture accumulation has occurred so far(?).

Insulation applied directly onto the floorboards (without first applying a vapor control layer) does not worsen the diffusion behavior of the current construction.

The required thickness of the insulation must be calculated according to the energy-saving regulations since it also depends on the U-value of the existing top ceiling.

It is advisable not to save here and to work with at least two layers with staggered joints!

Convection:
If indoor air can freely and unintentionally exchange with outdoor air through uncontrolled airflow, the risk is significantly higher than with diffusion.

A pressure-difference based leakage test (a simplified blower door test) is recommended in any case.

In conclusion:
Remote assessments should always be regarded with caution!
On-site consultation is definitely preferable.

Best regards,
Klaus-Peter Stieler

Hello

Thanks already for the replies.

Did I understand correctly, @KPS, that if there is no moisture between the straw-clay ceiling and the floorboards, it is safe to add insulation on top?

There is no visible moisture between the floorboards and the straw-clay ceiling (I opened a few spots from above).

I got some advice today.

According to the advice, the construction would be as follows:

1. Remove the floorboards
2. Install 180mm (7 inches) PAVAFLEX insulation between the joists
3. Place a PAVAFLEX airtight membrane on top
4. Then either reinstall the old floorboards or leave it open

What do you think about this?

Best regards

Maik

Hello Maik,

As far as I know, mineral wool insulation should definitely be installed with a vapor retarder, because even a 1% moisture increase causes the insulation performance to drop significantly (by almost 50%). Additionally, this insulation material does not handle moisture exposure well. ISOVER also warns: “Make sure the ceiling structure has sufficient diffusion resistance to avoid condensation and mold growth. Especially with wooden beam ceilings, the installation of a vapor retarder film is often necessary.”

Therefore, I cannot consider insulation installed directly on the floorboards—especially without a vapor retarder—as risk-free. The dew point inside the wooden structure would only be irrelevant if the ceiling underneath (panels on clay plaster) is completely airtight. In winter, due to large temperature differences—particularly in rooms below with kitchens or bathrooms producing water vapor—a strong vapor pressure gradient can develop. Warm air then leaks through the smallest gaps and condenses on the vapor retarder or directly on the mineral wool (if no retarder is used). The condensation drips into the wooden structure. Since the fibers lose their insulating properties quickly when exposed to moisture and have little moisture storage capacity, this would, in my opinion, be an undesirable solution.

Furthermore, the floor would practically become unusable, and any future renovation would require insulating the cavity anyway to avoid losing height and having to build new load-bearing floor structures.

There are design examples with mineral wool and wood fiber insulation boards placed ON TOP of the floorboards. However, these require a great deal of effort. Both a vapor retarder AND an airtight membrane are mandatory! However, airtightness should ideally be achieved at the ceiling below, not above the floor structure, because condensation cannot be ruled out otherwise.

Therefore, my non-binding recommendation remains to use cavity insulation between the joists with moisture-regulating insulation.

It seems you have now also received this advice from another source (a professional?). PAVAFLEX is a wood fiber insulation board that the manufacturer promotes for breathability and moisture storage capacity. PAVATEX recommends insulating the cavity WITHOUT a vapor retarder.

According to the design mentioned above, the insulation and joists are only covered with an overlapping, well-sealed airtight membrane, then the usable floor layer is installed—initially this can be just the “walking planks.” However, the airtight membrane must not be damaged in the process.

If the manufacturer’s claims are true, this insulation should be superior to the loose-fill options I suggested, as it can be installed more densely while still providing good moisture regulation. It would, in my opinion, be important to clarify whether and how this insulation has been chemically treated (e.g., to prevent fungal attack).

I suggest inviting a PAVATEX representative to your house for on-site advice! The consultation should be thorough enough that you fully understand the proposed solution and feel confident about it. Warranty claims for insulation work rarely occur anyway, because even when installed by a professional, damage cases typically blame the customer: heating too much or too little and/or ventilating too much or too little.

Best regards

PS: Of course, there is currently no moisture between the ceiling and floorboards because, as I tried to explain yesterday, there are no layers present yet that would allow condensation to form.