Insulating under floors


5 Apr 2010
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United Kingdom
Our house features a floor construction that includes a fairly level concrete slab across which 3"x3" joists are laid (shimmed as required to achieve levelling) and then with floorboards nailed down across these joists.

Over the years, a lot of the shimms have worked loose and the floor creaks and groan as you walk across it. Also, a draft blows through the gaps between the floorboards on windy days making the house impossible to heat.

If I remove the floorbaords, re-shim the joists and lay new flooring would there be a problem with also adding insulation between the joists (essentially insulating the new boards from the cold concrete below) or should I leave the channels between the joists open for under floor ventilation?

If I do add insulation, what type would be best?


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It does not sound as though you have a traditional suspended floor, rather you have something with little space beneath....?

If so, wy not solid fill the entire void with celotex (foam board) insulation?
Yup, exactly. Not a suspended floor, just use light weight joists supported across the floor on shims (basically little piles of thin ply). There is no more than 4" of space between the concrete and the bottom of the floorboards at any point in the house.

As you say, something like celotex seems like an obvious solution but I'm just wondering whether there might be some reason (ventilation perhaps) why I might need to maintain the shallow voids under the floor after all.


The theory is that if you take up all the air space with celotex and say, expanding foam or mastic then you leave no room for moisture rich air to occupy the space and deposit any condensation.

Also a vcl may be useful below the floor boards.
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Got you, makes sense. So, lay the insulation, then the vcl then the boards on top of that?



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