DIY Doctor

Suspended Timber Floor Insulation Advice

Postby Windinherhair » Mon Oct 29, 2018 4:31 pm

Hi folks.
Our livingroom floor is carpeted but, when you move the pouffe, the floor at that part is really cold. We have decent crawl space and access under the floor.
It's very well ventilated (bleedin drafty) with 6 or 7 air bricks.
Can we put up any box standard insulation, maybe a Rockwool type product, and hold it in place with netting? Or do we have to consider damp membranes n whatnot?
We're in a semi-detached with about 40 square metres of ground floor. There are "walls" down there, but access through gaps...a bit like potholing!
Any advice would be much appreciated.
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Simply Build It

Postby DocMartin » Sun Nov 11, 2018 1:34 am

You can put up rockwool in between the joists and hold it in place with netting or you can cut the solid type insulation - Cellotex/Kingspan etc and polyurethane foam it into place.

I don't believe there is any requirement for anything else. The space below your timber floors should be ventilated by air coming through airbricks in the walls. If it were not ventilated, moisture from the ground could build up in that space and cause rot to occur in your joists/floorboards.

However, your wooden floors are not sources of moisture so putting insulation under them doesn't cause a moisture problem for them. Nor will they have condensation forming on them under certain circumstances as they are warm surfaces.
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Postby Windinherhair » Tue Nov 13, 2018 9:26 am

Thank you Doc.
I've been looking at getting the pir foil insulation boards.
Can anyone comment on the best quality brand? Kingspan, Celotex, etc? Are they really just the same?
Probably looking at around 75mm thickness. The joists are 90mm deep, but I want it to fit between the tops of the dividing walls and the underside of the floorboards, without fouling too much on the cables that run through.
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Postby stoneyboy » Sat Jun 27, 2020 10:22 pm

Hi windinherhair
Your plan will work but you need to fill the full depth of the joists. Rather than using netting it's easier to use nylon string zigzagged under the insulation fixed using a staple gun. DPM is not needed.
Regards S
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