Insulating a First Floor


6 Sep 2019
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United Kingdom
Hi all,

I have a first floor room in the house that forms a "bridge" over a car port, between my house and the neighbour's house. The floor appears to be a normal suspended floor, with t&g floorboards and the underside (ie the car port ceiling) being boarded with some kind of asbestos board.

There is no underfloor ventilation, and the direction of the joists means no cross ventilation will occur even if I did fit ventilator grilles at each end.

Am I correct that if I lift the boards and lay insulation ontop of the car port ceiling that condensation between the top of the insulation and underside of the floorboards is not going to be a problem? My reason for this assumption is that the underside of the floorboards and the top of the insulation will both be warm. My only concern is that the joists will act as cold bridges and there may be pontential for condensation to form on them?


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My understanding (in this example I am using kingspan) is that you would fit the kingspan flush with the top of the joists, using something to maintain a gap on the cold side.
Fill it with insulation. You would be better off with 80 - 100mm foil backed insulation on the underside of the floor boards for heat, and the rest rock wool for sound.

There will be a lot of draughts and thus heat lots through gaps around joists and the ceiling, so do what you can to seal those.
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Thanks for the replies. I always understood a cold unventilated void would be more vulnerable to condensation. I am guessing that as water vapour is lighter than air, that moisture laden air is less likely to drop into the floor void?

I did think that stuffing the void would be best to avoid condensation, however as there will then be >100mm of insulation, any ring circuit cables in the floor in 2.5mm cable will have their capacity reduced below 20A so will need replacing with 4mm cable or the MCB reduced to 20/25A

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