It’d be “ok” but not optimal. You’re trying to remove any trace of a route for the heat to come through via conduction or convection from the warm space below to the cold space above. Ultimately the wood layer, assuming it’s totally covered, won’t do much to your insulation (may indeed help by reducing air movement) but if you leave any bits uncovered it’ll pass the heat around itself quite well until it finds the uncovered area to lose it and begins. Depends on your returns to input effort requirement I guess.
Insulating materials are cheaper in the UK (where they are government subsidised) than in France but they aren’t the same quality. Isover glass fibre stuff is in a textile cover to reduce irritation and inhalation. It’s still foul as soon as you cut it though so I’ve started using recycled polyester which has an A+ health rating.
The thicker the better. The passive house standard in Sweden is 500mm in the roof but Denmark is milder so aim for 350 – 400 mm. Roll the layers in different directions to cut losses along the joins.
When I did my loft a couple of years ago I put down 200mm between the joists, then ran 4″x”2 batons across the joists with another role of 200mm between them with flooring over the top of the lot. I now have loads of storage space and a nice warm house.
We used the B&Q offer when it was on a couple of years ago. It was well worth the work and the cost. Our house is now able to be heated just by the wood burner now.
If you are stuffing your lofts full of fluff, remember to leave ventilation gaps between the rafters at the eaves. Otherwise you may get condensation in the roof space, which will make your insulation damp and less effective (and eventually all your timber rotten too).
