Insulate the roof when the foot purlin rests on the ceiling joist

I am currently renovating my old house (about 100 years old). We have already installed a new roof covering with a modern underlay membrane, and now we want to renew the floor in the attic and properly insulate the roof/attic.

The problem is that the floor beams rest on ceiling joists (Picture 1). This is due to the roof design: the roof "bends" slightly (similar to a mansard roof, but concave, not convex).


The attic floor is an old beam ceiling construction, with clay on formwork boards. Basically, we see no reason not to keep it — the ceiling underneath is still in good condition, and clay is supposedly good sound insulation. Both the ground floor and the attic are inhabited and heated. We want to remove the old and quite worn floorboards and replace them with OSB boards (19mm or 25mm). Later, the OSB boards will be covered with laminate.

I want to install insulation between the rafters (20cm), and then 10cm of insulation under the rafters. On top of that, a vapor barrier foil and drywall panels.

However, through the gap between the wall and the floor beam, warm air will escape from the beam construction and directly enter the insulation, which of course is bad.



Can I seal the spot with mounting foam as shown below?



Mounting foam is elastic, airtight, and adheres well. If that’s not such a good idea, what else should I do?

Regards,

January

Construction foam is not airtight...unfortunately, I can't help you any more.

Hello

I would also recommend leaving the clay. If the surface is reasonably even, I would seal the vapor barrier directly on the clay surface with cartridge adhesive (e.g., Orcon F). This might be a bit more complicated around the ceiling beams... Then pull the OSB board over it.
You don’t need to worry about the warm air that penetrates into the insulation, since the insulation is well ventilated at the bottom (at least as shown in the sketches).

Regards, Nick