ᐅ OSB boards or tongue-and-groove planking for the floor in a convertible attic?

• We are building a bungalow with an expandable attic space featuring a 38-degree roof pitch.
• The intermediate ceiling already has a vapor barrier and insulation installed.
• The roof beams are approximately 24cm (9.5 inches) high and spaced 50cm (20 inches) apart, with insulation in between.
• The mineral wool insulation is about 16cm (6.3 inches) thick.

The roof was originally intended to remain unheated (cold roof). However, we are also considering adding insulation next summer. We have read about mold issues with OSB boards and cold roofs because, unlike tongue-and-groove boards, OSB is vapor-tight.

• Are 22mm (7/8 inch) or 25mm (1 inch) OSB boards or 24mm (15/16 inch) tongue-and-groove boards better suited for the subfloor?
• Is a counter batten system useful to allow ventilation between the beams, or should the OSB boards be laid directly on the beams?
• Should the boards be fixed with screws or installed as a floating floor?

Tongue-and-groove boards. Make sure to leave some ventilation space for the insulation. Nail them in. No mold at all. By the way, 38 degrees Celsius (100°F) is quite like a witch’s house. Lower it to 30-33 degrees Celsius (86-91°F). karsten

Nordlys schrieb:
Tongue and groove boards. Make sure to allow some air space for insulation. Nail them.

Is tongue and groove also the right choice if we insulate the roof and install underfloor heating and laminate flooring?

I don’t think so. Something would have to be done with the dry screed, but I’m not sure.

Our carpenter had the same concerns. He also thought that a wooden beam ceiling combined with underfloor heating could be problematic because the wood expands and contracts with heat. However, when we asked our heating engineer, he said this is not an issue. Nordlys has already mentioned the right keyword: dry screed.

Nordlys schrieb:
Tongue and groove boarding.

What thickness was installed?