DIY Doctor

Insulation under floorboards

Postby jlc » Sun Aug 02, 2009 3:00 pm


I'm in the process of re-decorating a small back bedroom and am replacing some damaged floorboards in the process, prior to having a carpet put in

The thing is that below this bedroom is the bathroom - it's a Victorian house with downstairs bathroom - so I'm wondering about putting some insulation underneath the floorboards.

I've read that if I want to use fibre insulation it should be suspended in netting that is attached to the joists. Why is this? Does it need netting or can I just push it along between the joists? There's board nailed to the underside of the joists so the insulation would just rest on this. Would that be OK?

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Postby Jeffx » Tue Sep 22, 2009 12:43 pm

Looking at doing this in all rooms as renovating the whole house.
I can see the use for a net on the ground floor as there is space of 3' under the joists, in the 1st floor there is ceiling so not much point in a net there I think.

My question would be is what insulation?
The joists are only 4.5", and you need 2" for ventilation, so only about 50-60mm of insulation possible.
Everything on sale seems to be 150, 200mm unless you get aluminium blankets at 5x the price, or start cutting up cavity wall blocks.

I have seen some Eco recycled plastic 100mm which comes in two 50mm but seems difficult to cut to fit the joist gaps.

Any recommendations anyone?
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Postby Perry525 » Wed Sep 23, 2009 7:07 pm

Insulation is a very good idea, especially the ground floor where you have a suspended wooden floor.
The problem with ground floors is that builders fitted air bricks that enable the cold winters air to move under the floor, and last winter the air temperature outside was minus 9.7 C.
Fill the space between the joists with closed cell insulation like polystyrene (you can get it from Wicks) cut it to a tight friction fit and it will stay there and make an incredible difference to your feet. Make sure you leave no holes.
Then get up in the loft and do the same between the joists up there. Polystyrene is good in lofts not only is it good insulation but you can stand things on it without it degrading and it performance will last for years.
I find cutting polystyrene with a sharp knife is best, a saw makes such a mess.
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