Insulate the upper floor ceiling. The ceiling structure is panels on battens

There is actually nothing against using underlayment boards if the insulation structure underneath is right. Or what exactly do you mean by "underlayment boards"? I understand that to mean screed elements, chipboard, and OSB boards. So far it was about the insulation structure and not a new wearing layer on the floorboards? Regards

I meant insulation boards, not OSB.

There are such boards that can be laid out which have very good insulating properties and are also walkable.
Unfortunately, I can't find the website right now.

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

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

And what would the structure of vapor barrier/vapor retarder be?

Best regards

Maik

You probably mean Styrodur or PS20 Styrofoam,

but in my opinion something like that belongs under a screed concrete floor on the ground floor or a floor slab. Rigid foam does not allow moisture to pass through and would cause condensation problems in the timber framing again, so in my opinion it is unsuitable for your construction.

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

I already answered that in the 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 solution, no condensation problems should occur.

Yes, if you now directly ask me about the vapor barrier solution ... I myself am absolutely against bags and foils in the house, especially in old buildings. But in new construction I also had to insulate against my attic like this back then, whereby in my opinion I minimized the risk:

My first solution was: gypsum plasterboards from above onto the beams (which should be "rustic" in new buildings), then vapor barrier tacked over everything from above, then battens screwed from above onto the beams, gypsum boards then screwed from below through a stop on the battens. Then mineral wool applied seamlessly on top and done. The dew point was thus located in the gypsum boards, which absorb and release a little of the normal condensation moisture or simply let it through. The system enclosed no wood, because the beams were underneath, the battens on top.

With non-visible beams I first tacked the vapor barrier taut and well overlapped to the beams from below, then the battens from below and gypsum boards on top of them. From above then the mineral wool between the beams. Here only the battens (impregnated) were below the vapor barrier. Moisture could therefore not penetrate into the beams. For 9 years I have had no problems with either the first or second construction so far. The attic is insulated in the slopes but of course not heated.

For you both solutions probably do not come into question because you certainly do not want to tear down your paneled ceiling on clay plaster. If you also want to keep the floorboards, then I would do nothing. But you are now considering lifting the floorboards. Therefore my very personal suggestion for how I would proceed in such an old building:

Unfortunately remove the floorboards, check the beams, are they all okay? If you have the chance to open it up, you should check that at the same time. Then see whether there are already inserts in the ceiling (so-called "Schwartenbretter") with clay / straw / rubble / empty beer bottles on them. If yes, the demolition could be stopped. If the dirt on the inserts is not wet and too moldy, I would leave it in. It also provides insulation, thanks to its mass also decent sound insulation when the floor is later expanded. If there really is nothing between the floorboards and the ceiling, screw impregnated battens as deep as possible on the side of the beams (upright), then inserts made from gypsum boards or thin tongue-and-groove wood on top. Pour loose-fill insulation on it, possibly with trickle protection underneath, but no plastic foil! Best to build the inserts so cleanly that no gaps arise. Fill the loose fill up to nearly the top edge of the beams (at least 20 cm would be good in my opinion). Then leave the floor open, lay walk boards on top and that’s it.

If the floor remains unheated, moisture may rise that can no longer be stored by the insulation and pass unhindered upwards and penetrate the roof. In my amateurish opinion, no vapor barrier would be necessary in this case. If the floor is later developed, it will also be heated. Between two heated storeys gypsum creates little vapor pressure gradient so that in my opinion a vapor barrier is also not necessary. But please install floorboards and no toxic OSB or chipboard! Those things can act like vapor barriers and cause moisture to condense on the beams unnecessarily. Normal diffusion-open coated floorboards, in my opinion, let moisture pass. One should not place unnecessary obstacles in the way of convection.

Regarding the insulation material: I advise against any kind of "bio insulation". This stuff is usually impregnated with insecticides so that insects and other critters do not eat it. Cellulose is also not without problems in my opinion because it is treated with toxic borates. I trust expanded clay and untreated mineral perlite, so nothing biological.

Maybe you better discuss this beforehand with someone who can call themselves a "building expert" and see what they think of it. I am also interested in factual criticism.

Regards

Hello @Maiks,

if the existing construction works without condensation and the attic is not to be accessed, rolling out mineral fiber mats on the existing boarding is basically uncritical.

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

There are 2 problem areas for the transport of water vapor (from the warm indoor air):

Diffusion:
Water vapor molecules penetrate solid building materials and condensation can then occur if a building material layer in the thermal envelope is more diffusion-tight than one located further inside.

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

Insulation applied to the boarding (without first placing foil on the boarding) does not worsen the diffusion behavior of the current construction.

The required insulation thickness must be calculated according to the Energy Saving Ordinance, as it also depends on the U-value of the existing top ceiling.

One should not save here and work at least double-layered with overlapping joints!

Convection:
If indoor air can exchange with outdoor air unintentionally and unhindered through free flow, there is a significantly higher risk potential than with diffusion.

A differential pressure supported leakage search (as a simplified blower door test) is advisable in any case.

In conclusion:
Remote diagnoses should always be treated with caution!
An on-site consultation is definitely better.

Best regards
Klaus-Peter Stieler

Hello

Thanks already for the answers.

Did I understand correctly @KPS that if there is no moisture between the straw clay ceiling and the floorboards, you can safely put insulation on top?

No moisture is visible between the floorboards and the straw clay ceiling (I opened some spots from above).

I got some advice today.

The construction would be like this according to the advice:

1. Remove floorboards
2. 180mm PAVAFLEX insulation between the beams
3. PAVAFLEX airtight membrane on top
4. Then either the old floorboards or leave open

What do you think about that?

Best regards

Maik

Hello Maik,

To my knowledge, mineral wool should definitely be provided with a vapor barrier because the insulation performance decreases significantly (by almost 50%) with just 1% moisture ingress. Additionally, this insulation material does not handle existing moisture well. ISOVER also warns: "Ensure a sufficient diffusion resistance of the ceiling construction to avoid condensation and thus mold growth. Especially with wooden beam ceilings, the installation of a vapor barrier foil is often necessary."

Therefore, I cannot consider an overlay insulation—especially without a vapor barrier—as risk-free. The dew point in the wooden construction would only be "irrelevant" if the ceiling below (panels on clay plaster) is absolutely airtight. If, in winter, very high vapor pressure differences arise due to large temperature differences—especially with kitchens and bathrooms below that produce water vapor—then the warmer air "whistles" through the smallest cracks and condenses on the vapor barrier or directly on the mineral fiber (if there is no barrier). The condensate drips into the wooden structure. Since the fiber’s properties deteriorate very quickly and significantly when exposed to moisture and it has almost no storage capacity, this would not be a desirable solution in my opinion.

In addition, the floor becomes practically unwalkable, and in a later renovation, the cavity would be insulated anyway to avoid losing height and having to build new load-bearing structures for the floor.

There are construction examples with mineral wool and wood fiber insulation boards also ON TOP of the decking. However, this requires a high level of effort. Both a vapor barrier AND airtight membrane are prescribed! However, the airtightness should preferably already be achieved by the room ceiling and not only above the ceiling construction, because then condensation formation cannot be ruled out in my opinion.

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

Ultimately, it seems you have now also received this recommendation from another source (expert?). PAVAFLEX is a wood fiber insulation board advertised by the manufacturer as breathable and moisture-storing. PAVATEX recommends the construction of insulation in the cavity WITHOUT a vapor barrier.

The insulation and beams are only well overlapped and bonded with an airtight membrane according to the above-mentioned construction, covered on top by the wearing layer, which can initially consist of the "walking boards." However, the airtight membrane must not be damaged by this.

If the manufacturer’s statements are indeed true, this insulation material should even be superior to the loose-fill insulation I proposed because it can be installed even more tightly and should also ensure such good moisture regulation. In my opinion, it would still be important to clarify whether/how this insulation was chemically treated (e.g., against fungal infestation).

You should best get the PAVATEX representative to your house and have them advise you on-site! The consultation should be so good that you understand the proposed solution and also feel confident about it. Warranty claims for insulation measures rarely occur because even when installed by a specialist company, the customer is usually blamed in the case of damage: too much/too little heating and/or too much/too little ventilation.

Best regards

PS: Now, of course, there is no moisture between the ceiling and the boards because, as I tried to explain yesterday, no layers exist so far that would cause condensate to form.