DIY Doctor

suspended Wood Floor Over Insulated Slab Query

Postby Sarahden » Mon Jan 04, 2016 5:04 pm

We have built an extension and had the base built for us ~ the usual layers of hardcore, sand, DPC, rigid insulation, more DPC and poured concrete. Because we want a 'traditional' timber floor, we allowed for joists and floorboards.
At last inspection BCO said we needed air-bricks to ventilate under the floor. I pointed out that the floor was insulated and waterproofed etc (which he had forgotten) so he then said we had to insulate between the joists so there was no gap under the floor. This means a hefty extra expense and more insulation than the floor really needs. Why not a space under the floorboards?
If it is to do with internal moisture 'collecting' and condensing in the space, would a VCL under the floorboards suffice?
I don't usually question a BCO, but his response was a bit vague as if he wasn't quite sure. I do appreciate it's not the standard way to do a floor ~ being a mix of two types of construction!
Any guidance gratefully received!
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Postby B4Less » Mon Jan 11, 2021 12:08 pm

Hi, This is due to movement of air. If air cannot move around and have somewhere to go you will get a build up of moisture. So in this case because you raised the floor you could need some sort of air flow from 1 side to the other. I'm not sure if juts filling the void with insulation is the answer but would help. If possible put in a small vent from 1 side and the other and leave notches in the floor joists fixed to the floor to allow air move from 1 side to the other. That should fix the problem.
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