Can you insulate under floorboards from underneath?

[ixos]

We would like to insulate under the floorboards in our living room and sand the boards down. There is a void underneath the floor - is it possible to install insulation from underneath? We are trying to avoid pulling all the boards up and doing it from above as the boards will get trashed.

Yes, provided you are willing and able to get into the space and it is big enough to work in. You will need to take up enough boards to make an access hatch.

If you aren't willing to go down there yourself, don't expect anyone else to do it.

We just insulated under our floorboards in our front living room by pulling down the basement ceiling underneath and inserting thick Celotex sheets between joists for sound and heat proofing. The basement was being converted anyway. We will then glue thin pine strips in cracks between floorboards and professionally sand and stain with hardwax oil.

Done this in other rooms and works very well.

Thanks both. I think there is enough room to work in without it being too horrible. It didn't phase the central heating people. Does the celotex stay in place when inserted between the joists or do you have to secure it in some way?

The celotex needs cutting to size and it stays in place with friction as it is a solid foam with a plastic backing. Its not like loft insulation. It is fairly solid, does not bend much but comes in various thicknesses.

if you have an unheated void below, especially a ventilated one, it will be draughty and in that case I think mineral wool fibre is better. It can be stuffed tightly into irregular gaps, unlike the rigid foam slabs which need to be cut to a tight fit. The edges of the room are especially prone to draughts, but they come up through the gaps between boards..

mineral wool is also cheaper. The varieties made by Knauf and treated with Ecose (coloured brown and also sold under own-brands) does not shed irritant dust and fibres like the older yellow fibre. It is usually retained with garden netting stapled to the joists.

If you are working under a floor, clear away all rubbish and dirt, clean out the airbricks, insulate all pipes and run any cables that you may want in the next 20 years or so.

Brilliant. Thank you!