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MovedGoalPosts · 28-02-2012, 15:41

Even in modern conventional house construction the void under a timber floor would be ventilated to reduce the risks of rot from excess moisture. Remember that in an external environment, unless you have the ground fully sealed with an impervious barrier to the walls there will be moisture both from the ground but also the general side entries. Even the timber flooring will not be imprevious. Thus damp can arise, including condensation moisture. Any plastic sheet under an external paving concrete slab is laid simply to aid the construction process and rarely would it be of sufficient grade and fully lapped that it could act as a reliable damp proof membrane. I can't therefore recommend you fill this void with any insulation materials.

As stated above expanding foam of the types available in aerosol cans will be very costly. It isn't intended to fill anything more than small voids. It also doesn't resist weather and damp very well and could actually entrap moisture making things worse. I doubt you'd get even coverage and if you overfill you risk distortion of timbers.

If you must fill the void then I'd suggest the polystyrene beads that have been used as retro fill by blown injection, rather than rockwool or other fibres. Most of these insulants are more intended from cavity walls or to lay over ceilings, so in the underfloor location may not perform well and again could actually cause problems with mositre entrapment. You certainly won't get a guarantee, and for the small area involved the installation costs could be prohibitive.

The best option would be an overlay to the current timber floor. Lay a vapoud check plastic sheet, place rigid foam boards over that (Wickes now sell the polyurethane type boards, as well as other foams for use with loftsapce boards that should work), and you can if needed place some timbers between these insulations, ideally crosswise to the current joists so the loads are spread, before you lay new timber flooring, perhaps of moisture resistant 19mm chipboard, or if you are being extravagant plywood.. The cheap job, which would simply reduce draughts would be a simple overlay of thin 3-6mm sheet plywood, to the existing flooring to block up any gaps.

28-02-2012, 15:41	#12
MovedGoalPosts Inactive Join Date: Jun 2003 Location: 127.0.0.1 Age: 59 Posts: 15,868	Re: Expanding Foam as Insulation Even in modern conventional house construction the void under a timber floor would be ventilated to reduce the risks of rot from excess moisture. Remember that in an external environment, unless you have the ground fully sealed with an impervious barrier to the walls there will be moisture both from the ground but also the general side entries. Even the timber flooring will not be imprevious. Thus damp can arise, including condensation moisture. Any plastic sheet under an external paving concrete slab is laid simply to aid the construction process and rarely would it be of sufficient grade and fully lapped that it could act as a reliable damp proof membrane. I can't therefore recommend you fill this void with any insulation materials. As stated above expanding foam of the types available in aerosol cans will be very costly. It isn't intended to fill anything more than small voids. It also doesn't resist weather and damp very well and could actually entrap moisture making things worse. I doubt you'd get even coverage and if you overfill you risk distortion of timbers. If you must fill the void then I'd suggest the polystyrene beads that have been used as retro fill by blown injection, rather than rockwool or other fibres. Most of these insulants are more intended from cavity walls or to lay over ceilings, so in the underfloor location may not perform well and again could actually cause problems with mositre entrapment. You certainly won't get a guarantee, and for the small area involved the installation costs could be prohibitive. The best option would be an overlay to the current timber floor. Lay a vapoud check plastic sheet, place rigid foam boards over that (Wickes now sell the polyurethane type boards, as well as other foams for use with loftsapce boards that should work), and you can if needed place some timbers between these insulations, ideally crosswise to the current joists so the loads are spread, before you lay new timber flooring, perhaps of moisture resistant 19mm chipboard, or if you are being extravagant plywood.. The cheap job, which would simply reduce draughts would be a simple overlay of thin 3-6mm sheet plywood, to the existing flooring to block up any gaps.