Best way to insulate under floorboards

dominoman

My living room is always cold, even with the heating and log burner on.

The house is draughty, single glazed and with solid (uninsulated) walls. I have decided to make it better insulated one room at a time.

For the floors I was going to self-install polystyrene boards under the floor. Luckily there is a good side crawl space so I can easily get to it all. However, I've seen some pictures of sprayed foam insulation.

Has anyone used sprayed foam insulation. What are the pros/cons?

Just looking for some advice on where to start

phil24_7

I take it you have suspended floors with wooden joists?

If so these still need to breath to avoid them rotting. Stay away from spray foam at all costs.

There are conflicting thoughts on insulating suspended floors. Some say leave an air gap, but this lessens the effect of the insulation and you'll likely still end up with draughts.

Polystyrene is better suited to it than cellotex etc in my opinion but I think breathable insulation like mineral wool would probably be better again.

missbiggles1

Do you already have fitted carpets and underlay?

TheCyclingProgrammer

I spent quite some time researching this - never ended up doing it in our house once we bought it as no crawl space and I didn't really want to rip all the floorboards up - but the very short simple answer is: breathable insulation between the joists, with something to support them, a breather membrane below the insulation on the cold side to prevent drafts from reducing the insulation's efficiency whilst still letting the timbers breath and a vapour barrier above the insulation, on the warm side (normally between the joists and the floorboards) to minimise the passage of warm humid air into the cold (which will be even colder once you've insulated) cavity below which would increase the risk of interstitial condensation.

The vapour barrier needs to be properly detailed (normally lapped up the wall and taped with proper vapour tape, behind the skirting board) and you need to ensure good subfloor ventilation (make sure you have sufficient air bricks and that they aren't blocked). You'll also need to make sure all of your heating pipes are properly lagged to prevent freezing in winter.

There are several approaches you can take and there is an excellent overview from English Heritage here:
https://content.historicengland.org.uk/images-books/publications/eehb-insulation-suspended-timber-floors/eehb-insulation-suspended-floors.pdf/

Personally I've found that reducing drafts from the sub-floor to be beneficial but I'm not sure that the time/cost of properly insulating the sub-floor is worth it, certainly not in our house. We've got decent laminate downstairs now with a vapour check layer over the floorboards up the wall behind the skirting and wood fibre board underlay and this has virtually eliminated all drafts. We still have a slight draft from our built-in alcove cupboards as I built these directly onto a plinth on the floorboards and the carcasses have some cutouts for sockets. I opted not to cover the floorboards before fitting them in my case as one of the cupboards contains my AV equipment and the small flow of cold air from the sub-floor actually works as a passive ventilation stack for the equipment (with the shelves having a cutout at the back and a ventilation grill in the worktop).

saverbuyer

I've been thinking about doing this and have been reading up on the subject. Some say a vapor barrier is needed, some say don't bother. I think I'll go with mineral wool as it's so much easier to use.


I also noticed a few posts popping up on my facebook, with friends using spray foam. Reading around the subject, I don't think I'd touch it to be honest.

dominoman

missbiggles1 wrote: »

Do you already have fitted carpets and underlay?

No. The floors are bare wooden floorboards. We're going to re-sand them (they were last varnished 20+ years ago) but leave the top exposed.

There are gaps between the floorboards through which air rushes up because there is no barrier or insulation underneath

TheCyclingProgrammer

A vapour barrier will help reduce the risk of interstitial condensation but it really depends on the approach you take to insulating. If OP has lots of room to work underneath, they could fix rigid insulation like Celotex to the underside of the joists - effectively like a warm roof design but upside down. The joists are on the warm side and therefore should not be at risk from interstitial condensation (in theory).

Lots of useful information about this on the Green Building Forums, for instance:
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=2850

dominoman

TheCyclingProgrammer wrote: »

the very short simple answer is: breathable insulation between the joists, with something to support them, a breather membrane below the insulation on the cold side to prevent drafts from reducing the insulation's efficiency whilst still letting the timbers breath and a vapour barrier above the insulation, on the warm side (normally between the joists and the floorboards) to minimise the passage of warm humid air into the cold (which will be even colder once you've insulated) cavity below which would increase the risk of interstitial condensation.

Thanks for the great advice.

The hard part of this would be the vapour barrier above the insulation. I can't lift the floorboards without damaging them so was hoping to do all the work from underneath.

If I fill the gaps between the joists with breathable insulation, and then add a vapour-breathable membrane beneath that will that be OK? The membrane will be on the cold side, but any condensation forming above it on the warm side would be able to wick away into the sub floor space.

TheCyclingProgrammer

You could use rigid insulation between the joists up tight to the floorboards but you'll need to seal everything with foam to make a good air and vapour-tight seal. You could then top the insulation up with a breathable insulation below this, supported on nets or a breather membrane. Or you could just use rigid insulation below the joists keeping the floor structure entirely on the warm side. See that GBF thread above for more on this.

The best advice I could give you if you're really planning to do this yourself is sign up to the Green Building Forum (it costs about £5 IIRC) and get some advice on there, they really know their stuff over there.

System

I can't really see the point of a vapour barrier under the floorboards tbh - how likely is it that warm humid air, instead of rising, will be forcing it's way into the insulated floor and condensing on the timbers?
Even on new build I can't remember ever specifying a vapour barrier in that location, although taping insulation like celotex/kingspan at the joints usually provides a VCL of sorts...

TheCyclingProgrammer

Seems like a risk to me, but I'm no expert.