Insulating from underneath suspended floor

[fernicarry]

Hi All,

One of the jobs I've been meaning to get around to is insulating under the downstairs floors to help control drafts through the floorboards. I have decent access to the solum for about 2/3 of the main part of the house so plan to do the work from underneath. Realistically I'm not going to go to the trouble of lifting boards just to insulate.

Initially I bought a load of thermafleece with the intention of suspending it between the joists. We used this in our kitchen refurb but for that we had the floor up (was a chipboard floor so no historic fabric to lose) by hanging it on 'strawberry' nets slung across the joists. With hindsight this could probably have benefited from a bit more attention to detail as the UFH we subsequently put down does take a while to get going but the comfort levels are undoubtably better than before.

I also have a pallet load of Homatherm holtzflex wood fibre batts for use in the bathroom I'm redoing. This room has also been gutted with walls removed and boards lifted -- plaster was coming off in dinner-plate sized chunks and boards had been got at by woodworm -- so straightforward enough to cut to batts to size (but v. messy!) and insert into the stud work and between joists from inside the room.

It occurred to me that the holtzflex might be easier to fit under the ground floor by cutting pieces to a snug fit and then pushing up between the joists. The reality is that this has been a bit of a nightmare. One of the bigger problems is the joists having twisted over the years so that the gap at the bottom of the joist is smaller than at the top and the batts tend to just crumble apart if you bash them into place with any enthusiasm. Also a lot of the joist ends have been repaired with new pieces bolted on making spacing even more irregular. After two afternoon's work I'm about 1% done on the accessible part of the floor and rapidly losing enthusiasm.

So now I'm reconsidering my options. Maybe go back to the thermafleece but need a way to hold it firmly against the underside of the boards to prevent air gaps? The joist spacings are all over the place with a lot being wider a bit wider than the 400mm fleece but no where near as wide as the 600 so efficiently using the fleece will be a bit challenging. Or maybe cut the wood fibre batts under size to make it possible to maneuver them into position then fix battens to hold them up? Should I be worried about a 0.5 or 1cm gap around them, and if so best way to fill this gap?

Any suggestions on techniques for this or other materials I might want to try?

Btw solum is dry and well ventilated and joists in good condition so I don't believe I'm going to introduce any new problems by doing this. I'll also ensure I don't prevent air circulation up the inside of the external walls, in fact I'm digging out a lot of debris from the gap as I go along.

Cheers, Alan

 

[Penners]

Should I be worried about a 0.5 or 1cm gap around them, and if so best way to fill this gap?

Yes, it would be advisable to try and get your insulation fitted tightly between the joists. If you were able to cut some long wedge-shaped pieces of the wood fibre, perhaps you could force these into the gaps after the batts have been put in place. Alternatively, fill the gaps with polyurethane foam. I know this isn't breathable, but in that application I really don't think this should be a worry.

If you decide to abandon the wood fibre batts, personally I would use a polyisocyanurate batt, such as Celotex or Kingspan, and again I would use polyurethane foam to fill any installation gaps. I'm sure some people would thoroughly disapprove of this non-breathing material, but as I said above, in this application I really can't see that breathability is going to make any difference (provided your subfloor void is properly ventilated).

 

[Feltwell]

A differing view I'm afraid. I have also insulated suspended floors from below, with similar problems. I ended up using ordinary rockwool loft insulation - it's springy enough that a bit of variance in the joist width doesn't really matter, cut it slightly oversize and stuff it up! I'd hate to try to do the job with Kingspan/Celotex or similar - getting the boards down there would be difficult, getting them cut to fit well whilst working in the cramped, dirty underfloor space would be a nightmare. I wouldn't fancy going around with squirty foam afterwards either, it's such horrible messy stuff.

If I did the job again I'd do it the same way but I'd use Thermafleece or one of the loft insulations made from recycled plastic - i.e. one that's nicer to skin and lungs than Rockwool! B&Q sell a recycled plastic one that's quite stiff and holds it's shape well, not so easy to tear as Rockwool but cuts easily enough with an old bread knife.

I pushed the Rockwool up between the joists, and then meant to staple chicken wire under the joists to support it. I didn't finish the job on day 1, and, err, day 2 never came... The Rockwool has actually stayed in place just by it's own grip on the joists for quite some time now, despite the best efforts of children bouncing about up above!

 

[Tony.]

I also used rock wool batts
600x1200mm cut about 1/2" bigger than the joist width and squashed in. Friction holds mine in place.

Horrible job but only had 18" so crawling on back pushing it up.

Well worth the effort

 

[fernicarry]

Thanks for the replies. Glad to hear its worth doing.

Surprised to hear that rock wool holds its shape well enough to stay up but I really don't fancy wrestling with that. I'd also prefer to keep use something breathable. Isn't there a risk of interstitial condensation between a non-breathable material and the joists and boards?

I think on my next attempt I will cut the wood fibre batts in half length wise and then slot them together into position. This should also make it a bit easier to get them to fit over the bolts that are protruding from the repaired joists. Means a lot of extra cutting though and the place is already knee deep in the fluff that produces....

I shall report back once I've summoned up the enthusiasm to have another go.

-Alan

 

[Penners]

A differing view I'm afraid. I have also insulated suspended floors from below, with similar problems. I ended up using ordinary rockwool loft insulation - it's springy enough that a bit of variance in the joist width doesn't really matter, cut it slightly oversize and stuff it up! I'd hate to try to do the job with Kingspan/Celotex or similar - getting the boards down there would be difficult, getting them cut to fit well whilst working in the cramped, dirty underfloor space would be a nightmare. I wouldn't fancy going around with squirty foam afterwards either, it's such horrible messy stuff.

How dare you disagree with me! What's more, how dare you be correct?!!

Seriously, I think all of Feltwell's comments are absolutely valid, assuming that access to the underfloor void is cramped. However, your tapering joists obviously make a friction fit (of any rigid material) difficult to achieve, so I would still consider wedges or foam for securing such materials. If you use foam, do so moderately and be careful not to let it drip on you!

 

[Zebra]

the place is already knee deep in the fluff that produces....

Keep that fluff fernicarry! I've found that mixed with lime putty, it makes an excellent filler!

 

[noj]

I'm wondering if stapling a breather membrane on the underside of the joists could be good whatever the insulation - vapour open, but air tight(ish). Would keep it all breathable and stop anything dropping but act as an extra barrier to draughts coming through?

 

[Feltwell]

I'm wondering if stapling a breather membrane on the underside of the joists could be good whatever the insulation - vapour open, but air tight(ish). Would keep it all breathable and stop anything dropping but act as an extra barrier to draughts coming through?

I don't know how air tight it would be. When we had a breathable roofing membrane as our only, temporary roof covering, you could feel the breeze coming straight through it.

 

[agbagb]

I'm wondering if stapling a breather membrane on the underside of the joists could be good whatever the insulation - vapour open, but air tight(ish). Would keep it all breathable and stop anything dropping but act as an extra barrier to draughts coming through?

That is my preferred option and how I've done ours (cheap roof insulation and membrane) . It certainly works, air tightness will depend on the detailing, lapped joints, taping, sealing. Its probably easier to get full air tightness from the room side.

Many years ago someone boarded the joists with hardboard and the joists suffered so much, many had to be replaced. After seeing how bad it can get, I would definitely go for something breathable.

 

[fernicarry]

That is my preferred option and how I've done ours (cheap roof insulation and membrane) . It certainly works, air tightness will depend on the detailing, lapped joints, taping, sealing. Its probably easier to get full air tightness from the room side.

The people who sold me the thermafleece recommended I get a breathable membrane as well. The explanation was more to stop debris falling between the boards and clogging the insulation.

Agbagb, did you fit the membrane from below? Did you fold it onto the joists at all or just keep it flat on the underside of the boards? Did you tape along the length of the joists?

How about detailing around the edges of the room? I'd want to fold the membrane up to meet the bottom edge of the plaster but that means getting the skirtings off.

 

[agbagb]

The membrane holds the insulation up and stops a lot of the draughts. So it is fixed to the underneath of the joists, not the boards.