Hello,
I want to insulate under the floorboards in my living room between and below the joists. There is a 6 foot deep viod beneath the room. I have been told I need a vapour barrier but am now confused as to where this should be - above or below the insulation. Grateful for the correct answer:-))
TIA
The VB always goes on the warm side of the insulation, so in this case the VB goes above the insulation.
NT
Opinions differ.
Personally what works in the roof should work in the floor.
1/. lift all floorboards 2/. fit suppports - nails, battens whatever, for e.g. Celotex. 3/. fit Celotex flush with joist tops 4/. use expanding foam in any gaps - seal tight. 5/. use celotex foil tape over all the joists 6/. refit the floor.
Thanks for that.
You can get close though... Hold the board under the the joists with one side of the board against one of the joists. Draw along the board with a pen against the other joist, & cut to the line.
NT
Yes, there are correct ones and incorrect ones :-) The correct opinion is that when a vapour barrier is required, it is on the warm side of the insulation.
Given that the OP has a 6' void under, mineral fibre (Rockwool) may be the more economic option. Netting or chicken wire below the joists with a good 200mm or more insulation between the joists. A vapour barrier is less critical to the floor, provided that the sub-floor void is well ventilated.
The trouble with rockwool,. is that its not windproof.
The greatest heat gains i ever for was boarding over Victorian floorboards with hardboard and sealing it. No insulation.
And it slumps.
Fine for lofts, not for floors.
IMHO of course.
But if the celotex is flush with the joist tops if ther is any spillage of water or wine or pipebursts in the room above it will seep betweent he floorboards and into the tiny gap between celotex and floorboards and rot them, whereas with an air gap there would be ventilation to evaporate it - but there would also be a bigger gap to be filled wqith water...
airpag or no airgap?
[g]
No, it will evaporate upwards fairly quickly.
And one assumes massive puddles would be mopped up.
When workng from underneath with celotex it will be impossible to use celotex foil tape over all the joists to make a vapour barrier, perhaps pinkgrip dryfix is less permeable than the normal expanding foam, and so there would be less condensation as warm wet air percolates through it downwards, and hence less rot in the joists?
How long has celotex/kingspan and foam been used to insulate floors, has there been enough years gone to remove some and see if it has led to rot due to condensation in the porous foams?
[g]
It's not "windproof" in a loft either.
One will always reduce heat loss by reducing the (unwanted) ventilation. That's as true to a floor as it is to windows, loft hatches, etc. After that, increase the insulation. Apart from the millimetre or so exposed to the outside surface, the speed of the air flow doesn't affect the emmissivity by any appreciable amount. What will affect the heat losses is gaps to the edges of the insulation; something that's more likely with loosely fitting rigid boards.
I would query the issue of 'sag'. The chicken wire would be more likely to sag, and even if it does, so what? You'll get a pocket of air between the top of the insulation and the floorboards, reducing the heat loss a bit more.
I was pointing out to the OP that the same return can be had for a lower outlay by using a less expensive insulation. It doesn't have to be mineral fibre quilt; solid batts, expanded polystyrene, even Polyfoam could achieve the same resistance for less money.
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