Suspended Timber Floor Underfloor Insulation

7 Dec 2010
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United Kingdom
I have recently insulated underneath our suspended timber ground floor, using Celotex boards in between the joists. There is an undercroft beneath the ground floor with almost standing room in places, so plenty of ventillation from underneath and all air bricks were left free when fitting the insulation. Following advice given, I fitted 80mm boards from underneath so that they are flush with the bottom of the 100mm joists, thereby leaving a 20mm air gap in between Celotex boards and floor boards. This is apparently to allow any water spillages from above to run off and/or evaporate thereby preventing any 'dry rot' occurring. A 10cm was left between the Celotex boards and the external walls, and the space filled with rockwool, again to allow some air to ingress and circulate in the space between the Celotex and the floor boards.

But now there is a strong draft coming through some gaps in between the floor boards and the air in the 20mm air space is freezing cold like the outside air. What have I done wrong. Should I push all the Celotex boards right up against the floor boards, or do I need to close off any potential gaps on the edges of the Celotex boards, and divert the air flow from the air bricks away from the air space created?

thanks for any authoritative advice, i've been hearing all sorts of conflicting opinions on this
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The insulation should be hard up against the underside of the floor boards and any gaps should be sealed with expanding foam, the insulation should also be held in position with nails or battens to avoid the insulation falling out. Whoever advised you to do it flush with the bottom was giving incorrect advice:

Remember to fill all the tiny gaps that you can't get the normal insulation in ie the gap between the final joist and the wall etc etc.
Many thanks for your prompt response and confirming what I also thought might be the case. The person providing the advice was ademant that the Celotex pushed up flush against the floor boards created a vapour barrier for any spillages from above the floor seeping in between the boards, which may lead to 'dry rot' in the floor boards and wall rendering. With the Celotex boards pushed up against the floor boards, is there anywhere for spilled water to go or dry out, i.e. is there sufficient air penetrating through the Celotex boards and/or from above the floor boards to allow moisture to evaporate?
Please note that our suspended timber floor is just that, the original floor boards with gaps filled with DraughtEx expanding rubber strips, no carpet or other laminate flooring above that. So I guess there is certainly a risk for water spillages to seap through the floor boards. Does this make a difference to your advice that the Celotex boards should be pushed up hard against the floor boards from underneath?
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Any water that is spilled onto the floor is going to find its way onto the top of the insulation wherever the insulation is placed if the gaps between the boards are not completely sealed (which is unlikely unless the boards are newish, continuously glued and T&G). As we all do in such circumstances, if you spill a drink or whatever you mop it up PDQ! Even if some water does get down there it will dry out soon enough as with the insulation fixed up to the underside of the boards) it is now (comparatively) warm. If it were a big spill it is possible some water will be absorbed into the insulation but I really do think you're worrying about nothing! wink:
A Structural Engineer and a Building Inspector both told me to use mineral wool beneath the floor up against the floorboards to allow spillages to drain.

Good luck.

You do not get dry-rot from one spilt cup of tea or even a basin of water.

You need a constant supply -like a leaking gutter/water-pipe - to keep the area wet for months
Thanks everyone for the advice, I'm going ahead and rectifying the problem by pushing the boards up against the floor boards... any spillages can then be managed appropriately as and when they occur. Cheers!

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