Material for insulating between ground floor joists

[realslyshady]

Hi folks. Me again.

We're planning to insulate between the joists on the ground floor — once the boards were numbered, mapped and lifted (need to do this anyway to readjust some of the joists and replace others) I was going to staple garden netting to the joists at the right depth, then lay rockwool or similar to floor level, then relay the floor, using reclaimed boards to patch the many, many gaps it has at the moment. A joiner was round today to give an estimate on some other work, and he felt that mineral wool was a poor choice — he reckoned I should use Kingspan or similar, with expanding foam to fill gaps. My research had suggested this was a bad idea, but now I'm unsure. Any advice? Thank you...!

 

[MikeG]

Mineral wool is half as effective an insulator as PIR (Celotex, Kingspan etc), but if you have unlimited depth it is far easier to fit, and far easier to get right. Celotex usually produces real issues with workmanship. Of course, Celotex is pretty much impervious to the passage of moisture (people like to use the word "breath", erroneously, in this context), and in contrast mineral wool/ fibreglass is vapour-open. For some reason (it's fashion, but don't tell anyone) fibreglass gets a bad press in green circles, and amongst the less knowledgeable in the field of renovation. It's a great product, really user friendly these days, cheap, and utilises a recycled material. So, if you have enough depth of joist to be able to use a sensible amount of mineral wool, then go for it. I suggest putting battens across the underside of your joists to reduce the sagging that you'll get even with using a net, and aim for having the underside of the floorboards in full contact with the top of the insulation.

 

[realslyshady]

Thanks very much for this — it's what I first thought, and good to get back on track. The joists are big — 9"x3" — so there's comfortably room for 200mm of insulation. I was originally thinking about that fleece quilt, but it's way out of our budget. Good advice about additional battening beneath the joists too. Thank you!