We had an extension built last year, about 12 sq metres floor space so not huge.
The floor level needs raising by 60mm to bring this up to the level of the threshold/step just in front of the folding doors - (in hindsight, more insulation should have been used under the slab to bring this height up.)
I have run central heating pipes around the perimeter of the room at low level, so the 60mm level will cover these.
The plan was to batten the floor using CLS timbers, first laying a polythene sheet down, putting loose fiberglass insulation between the batens, then fixing 18mm WBP ply on top. Then finally, laying the engineered oak (or solid oak) flooring down (floating).
I am concerned about condensation under the floor...is this a good/bad plan and if the latter, what other alternatives are there?!
NB: Slab is 14 months old, and is insulated and has a DPM too.
Get your local timber merchants to cut your timber to 40mm thickness then you can pack it and lay 40mm x 100mm timbers fixed to the floor then lay your 50mm drytherm fibreglass cavity insulation inbetween then fix 18mmT&G chipboard flooring. It is not a good idea to lay polythene on the slab,it will encourage condensation.
Should I use a particular type of wood for the batten, i.e. does it need to be treated or a certain thickness for strength? (I was planning on laying at 300mm ctrs.)
Ref insulation, the slab is already insulated and as celotex is £££ - I was going to compromise between the battens and use fiberglass batts. Other than the thermal performance, is there a reason not to do this?
you can use tanalized timber 75x50mm or 100x50mm but as your restricted to 60mm you need to reduce the thickness to 40mm,so have it cut for you at the timber merchants or use a circular saw to cut it. As for the insulation you cant get it at 40mm thick,so if you use 50mm drytherm it will squash to 40mm.Set the timber at 400mm centres and fix to the floor.