Replacing suspended wooden floor with insulated solid floor

2 Dec 2013
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United Kingdom
Dear experts,

I live in a ground-floor flat in an old building. The subfloor seems to be a 20th-century concrete slab, and there is a suspended wooden floor over it. In half of the flat this is very shallow - the total depth from the top of the floorboards to the concrete is only about 90mm - and elsewhere it's about 160mm. The joists are tiny and must be supported off the slab mid-span as there is no bounce. There is next to no ventilation under the floor. The ceiling height is only about 2300 so increasing the floor level is not attractive.

I'm considering options for insulation, and one option may be to replace it with a solid floor - i.e. insulation boards over the concrete, then screed.

Does anyone have any thoughts about the pros and cons of doing this? Where I have only 90mm the thickness of the screed becomes problematic, i.e. I could have 50mm of insulation and 40mm of screed, which isn't a lot of either! I will have the usual selection of pipes and cables to duct somehow. Any alternatives?

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I think a 50mm celotex or similar board between the joist is probably the easiest option and will probably be more thermally efficient than what you are planning. It also gives you the option for services under the floor, such as speaker or network cables. Just worth checking however.. is this a leasehold flat? You should check your lease as its unlikely that under the floor is part of your lease.
I own the freehold. (I'm in Scotland, flats are different!)
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