Hi - I've been looking into insulating the suspended timber floor of my Victorian semi and was hoping for some final advice/comments before I crack on with it! I've read plenty of posts on the subject but haven't quite come across what I'm looking to do as it as it covers both a crawl space and basement garage.

The space is circa 3.7m wide x 7.5m long with joists c225mm deep running width ways. The back half is over a crawl space which I can access from below, the front half being a basement garage converted some years ago. For various reasons I'm not looking to remove the flooring above so my proposal is to insulate from below:

Crawlspace> Breather membrane stapled to underside of floorboards/sides of floor joists (to both protect the insulation and to aid in airtightness for the flooring above)
140mm of Knauf Earthwool (speaking to Knauf will provide U value of circa .22) and will give me space for services running within joist structure.
Supported by netting pulled stapled to side of joist then pulled across and stapled again
Crawlspace floor loosely covered by non breather membrane

Garage Ceiling:
Same as before but with plasterboard applied to underside of joists for garage ceiling.

I want to maintain a breathable build up as I can't guarantee vapour proof layer on flooring above - airflow is reasonable from below due to garage and crawlspace with a couple of airvents on the side and I've cleared out all the crap that had built up in the crawlspace over the years.

Looking for general feedback and in particular whether a) airgap between garage ceiling and depth of insulation will give me any condensation issues or whether better alternative to plasterboard (fire rated) b) whether breather membrane above insulation is overkill to aid in airtightness?

Appreciate any help

Regards

Daniel

Thanks Tony - by the void being draughty and cold - the logic of applying the breather membrane to the gaps between the joist ends was hoping to remove that issue (well not the cold bit!) but should reduce airflow when the garage ceiling is back on - crawlspace will still be an issue.

In respect of vapour barrier because I won't be lifting the floorboards I didn't want to make the joists the only permeable element of the structure hence the breather membrane instead.

Thanks Tony - so if I've understood it you believe best logic is to forget the breather membrane above the insulation, and just insulate and support with netting over the crawlspace which should allow airflow to remove condensation, and over the garage full fill depth of joists with insulation and plasterboard ceiling to support it?

Yes Daniel, Will’s method looks good, I would use string to temporarily support the quilt and do the netting in a seperate operation. Poly MUST be on top (warm side) if you get that.