Posted By: djhRockwool and netting shouldn't have caused damp, so I think something else must have been causing/supplying the dampness. So the first order of business is to find out what caused the problem. Did you or can you now ask these 'damp specialists' how they thought the rockwool and netting was causing damp, and what they recommend instead?

But if they both recommend PIR than I wouldn't put much trust in them, to be honest.

One of the main things to look at is ventilation. How are the rooms ventilated?

It may be your house has fairly high humidity (get a cheap T&H meter to check. Well worth a few pounds!) and the under-floor insulation meant that the underside of the insulation was the coldest place so condensation formed there.

Until you know what happened and why (or can take a pretty good guess at it), there's no point planning new insulation.

Posted By: tonyI am one of the few that like to see a vapour barrier on the warm side of the insulation for floors. For me this doubles as an air tightness layer something missing from all old suspended wooden floors.

Nice to be able to work from underneath. Well done sorting out the ventilation.

Posted By: Peter_in_HungaryAs I recall most of the previous posts that I have seen here when insulating suspended floors recommend a VCL under the floor boards, as Tony says on the warm side of the insulation. Moisture from the the house air will only be a problem when it condenses out due to a drop in temp., on the warm side of the insulation there will be no temp. drop therefore no condensation.

Using foil-backed PIR board would have its problems because it is difficult to get a good fit between the timbers and people usually foam the sides to get a good fit. Whilst the foil will provide a VCL for the insulation the joists won't have a VCL and the foam is only partially vapour closed (AFAIK) so there may still be a problem at the joists with condensation.

I suspect the original problem of damp in the insulation was in part due to no VCL under the floor boards. (Which IMO both the original installers and subsequent specialists should have known)

One solution may be to treat the under floor like a warm roof, that is to put the insulation outboard of the timbers so if you have the space put the insulation under the joists which would keep all the timber warm and condensation free. A VCL could then be put across the bottom of the joists on the warm side of the insulation where it belongs.

Another solution would be to put the VCL under the floor boards and around the joists then fill between the joists and a good amount under the joists to keep the bottom of the joists warm

One problem with board insulation is that mice love it and can destroy it in v. short order unless it is protected.

Posted By: DannySheffieldIt's a 3 bed 1930s semi, it has two air bricks on each long side and one on the short side, all now clear.

Posted By: Mike1

Posted By: DannySheffieldIt's a 3 bed 1930s semi, it has two air bricks on each long side and one on the short side, all now clear.

That's unlikely to be adequate.

Building Regs Part C require 2 opposing walls to have ventilation of either 1,500mm²/m run of external wall, or 500mm²/m² of floor area (whichever is greater).

A 1930's semis measures maybe 5m wide, so would require 7,500mm² of ventilation. A new brick-sized terracotta airbrick will give roughly 1,250mm², so that would be 6 air-bricks, not 1 or 2 (or 3 airbricks, if they are double height).

You'd also need to make sure that the flow from one side of the building to the other isn't blocked by intermediate walls.