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

Hi

I wanted to insulate the ceiling of the upper floor since the attic is not developed.

The ceiling structure consists of panels (on battens) – straw clay plaster – cavity – floorboards.

At first, I thought of filling the cavity between the floorboards and clay plaster, but according to a company, the clay plaster’s fastening is not stable enough.

Now I am considering adding insulation on top of the floorboards. How thick should the insulation be at minimum, and what is the best construction method? What about a vapor barrier?

Best regards

Maik

Hello,

maybe I’m misunderstanding, but is there any reason not to fill the cavities with stone wool or another insulation material? If it is properly wedged in, the already low weight will rest against the wood frame, and OSB boards or plywood can be installed on top.

Yes, that can certainly be done that way, but I didn’t want to remove all the floorboards.

Maik

Hello

Now that the insulation is finally being installed (which I didn’t manage to do last year), I wanted to ask again about the best way to proceed.

I was thinking of laying 16mm (5/8 inch) insulation on top of the existing floorboards.

Is this an acceptable approach, or are there other things I should consider?

Best regards

Maik

Hello Maik, in my opinion, not entirely without risk

what you are planning!

"The ceiling structure is panels (on battens) – straw-clay plaster – cavity – floorboards."
"At first, I thought the cavity between the floorboards and the clay plaster would be filled, but according to one company, the clay plaster’s attachment is not stable enough."
"I now wanted to apply the material directly on top of the floorboards."

The build-up seems critical to me mainly because the ceiling of a heated living space borders on an unheated floor below. The company may well be correct in their statement, but the cavity can also contain so-called “inserts” that serve as insulation carriers. The ceiling structure beneath these inserts is then fully decoupled and not additionally loaded. Very lightweight loose fill materials could also be used so that inserts like plasterboard on battens, rather than thin tongue-and-groove boards, can serve this purpose.

However, since you want to save yourself the work of removing the floorboards, you basically want to just roll out “mats” or similar insulation on top. In my opinion, this would shift the dew point into the floorboards: warm air rises from the living area, passes through the panels, is partly stored by the thickness of the clay plaster; what isn’t stored continues to move upward and accumulates in the cavity (convection). Without insulation, the residual heat escapes through the floorboards and then the uninsulated roof. With insulation, condensation would form below the insulation, likely mainly on the underside of the floorboards in the cavity. In my view, this leads to moisture build-up and possibly damage to the entire timber structure.

Using a vapor retarder is recommended by the manufacturer when using mineral wool. Placing the foil on the floorboards would, in my opinion, only protect the mineral wool from moisture but not the timber structure itself. A vapor retarder is installed on the warm side, so for ceilings it’s located below the insulation. The “on-floorboard insulation” approach is, in my opinion, unsuitable due to the critical construction. Especially in winter, when 18-20°C (64-68°F) warm air from below enters the cavity and cools slowly to around 15-20°C (59-68°F), a significant amount of condensation is likely to form. The insulation on top slows the cooling but does not prevent it.

In my opinion, all viable solutions require removing the floorboards and properly insulating the cavities. For this, mineral wool systems with vapor retarder foils below and around the joists, or various loose fill solutions, are appropriate. As far as I know, if the loose fill—such as perlite or similar—is sufficiently thick, a vapor retarder may be omitted because the water vapor rising by convection causes some condensation within the insulation, which it can safely store and later release. There are sophisticated software tools and engineers who can calculate these effects accurately.

I am not a building physicist but have dealt with these issues in theory and practice for years, so I hope my explanations are not too far off.

Best regards

That sounds logical to me regarding the moisture, even as a layperson.

I just find it strange that some companies sell special underlayment panels to lay down on the floor structure. Of course, everyone wants to make money. But I don’t want my ceiling to rot, develop mold, or collapse.

Can anyone else share their experience with renovating and insulating an older building?

Best regards

Maik