The house that I am building has a suspended timber floor (in some places!) which was intended to have 200mm of mineral wool insulation between the I-joists. The plan was that there would be pieces of 12.5mm fibreboard sitting on the top edges of the bottom flange of the I-joists with the insulation resting on top.

Due to a mistake on my part, the I-joists are only 195mm deep and so the distance from the top of the joist to the top of the fibreboard will only be about 135mm. I could probably compress 200mm of insulation in there but i don't think that it would have the same effect! As far as I can see, the best way round the situation would be to treat the joists as traditional solid joists and insulate down to the bottom of the joist.

My question is whether there is any benefit in still using the fibreboard under the joists or whether a combination of netting and battens (perpendicular to the joists) would have a better effect. I am intending to put a plastic vapour barrier on top of the joists under the chipboard floor. The crawl space will be draughty (we're in Shetland) but should be pretty dry.

If I go down the netting route, has anybody got any recommendations on what type of netting to use? Most things here get attacked by the salt air so I'm not sure that chicken / rabbit wire would last. Does strawberry netting last forever?

Thanks for any input.
Andrew

The fibreboard provides a wind barrier which prevents "wind-washing" & thermal bypass type effects reducing the effectiveness of the insulation. It is also very "breathable" so it lets out any water vapour without causing condensation.

You could replace it with breather membrane supported by battens perpendicular to the joists. Or you could run the breather membrane over the top of & perpendicular to the joists, down both side faces of each joist & across the bottom of each joist bay, so that it supports the insulation without needing battens. If you do this then make sure the membrane is stapled tight to the side faces of each joist to avoid providing a thermal bypass route. You should still fit the polyethylene air/vapour barrier on top of the insulation to protect the warm side.

David

You could use 100mm EPS beneath the joists.

Would wood wool boards work in this situation (bit of alliteration there). They have some insulation value, are breathable are weather resistant and structurally stable. I think 25mm is the slimmest board, up to 50mm.

comment edited - I hadn't read previous post properly

Yes there was - it gave a link to good pictures and advice - I think the link went to cambridge ecohomes or something like that.

That link appears to be dead. Update:

https://cambridgecarbonfootprint.org/wp-content/uploads/2021/09/DIY-Underfloor-Insulation-2-Methods.pdf

Here's the folding technique to get the insulation under the joists from that pdf. I assume an air barrier would be a better option than the netting used here as it would stop wind washing.

Hopefully a condensing boiler with a cold casing, insulate the pipes

Thanks, I'm also keen on going underneath joists. I saw in other threads that EPS boards underneath were recommended, seems like I'll be ok with either option.

I'll be doing it from underneath in this case so I could install a VCL, but it would have to wrap around each joist and I wondered if that might cause problems and I'd be better off leaving it out.

I'll need to investigate what the ventilation requirements of the boiler are. Edit: it's a room sealed combi, so I don't think I'll be missing out on much heat by insulating above it.