Currently the living room floor in my flat consists of 40mm wooden joists resting on a concrete slab and 18mm floorboards on the joists. There's a bit of fibreglass under the joists so the total from the concrete to the top of the floorboards is 61mm.
In order to soundproof I plan to remove the floorboards and joists, lay 20mm thick 50x50mm rubber pads like these at 400mm centres (total of 108 for my 12m2 room) https://www.mason-uk.co.uk/super-w-pad/ with two layers of 18mm OSB at right angles to each other and glued and screwed together.
That only comes to 56mm and I really need a bigger space to fit 25mm rockwool insulation between the pads, so I intend to use some cuts of 9mm OSB slightly larger than the pads to fit underneath them, making the sub-floor 65mm above the concrete. Although this is slightly higher than the existing floor, the adjacent rooms have a 15-19mm layer of ply and vinyl, making them 76-80mm in total, so I'll still have room to fit a further 11-15mm of ply/cork tiles/whatever on top in the living room. I may have to put a layer of 3mm levelling compound on the concrete slab if it's not flat enough though, which will leave me with 8-12mm to play with on top.
My question is whether there's any problem with using square-edge OSB for this or if I need to use T&G OSB or plywood (square-edge or T&G)? As the two layers will be joined, effectively creating a 36mm heavy and dense floor, I wouldn't have thought there was any need to use T&G as there might be when only using one layer (like the existing floorboards) and I fear using T&G just adds the risk of them starting to squeak over time as the wood expands and contracts and the joints starts to rub against each other.
I don't know if plywood is easier to treat than OSB to make it moisture resistant but I wouldn't have thought that was necessary in this case anyway. I note the plywood is somewhat lighter than the OSB, at 22.75kg vs 35kg, which will make it easier to move and work with.
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