Experiences with PIR floor insulation

[mostly_wrong]

Hi all,

Sorry for another floor insulation thread! I have an 1880s semi, solid wall property with most of the ground floor suspended timber floors. I was primed to have a go at insulating them figuring that this would be a reasonable return on investment to do myself, however reading through the ecological building systems guidance on 'best practice' it notes PIR's "...its inability to enable vapour to be absorbed and gradually dispersed".

I would usually go for something like wood fibre, but the thermal properties of PIR can't be matched, as i'm only working with 2x4 joists so only looking for c.80mm of insulation supported by battens with a vapor control layer over the top. The fact that it's best practice guidance on a website trying to sell me something else makes me a bit skeptical, maybe i'm too cynical. I accept it will be a pain to cut to size but the point around vapor permeability worries me more. Does anyone have any direct experience of the long term impact from doing this?

My floor is pretty standard: it's a small ish void over soil solum with 2x4 joists held up across two sleeper walls. At the moment I just have the original boards covered by a t&g board which i'm going to replace afterwards.

 

[fernicarry]

You don't really need a lot of insulation under a suspended floor since heat doesn't tend to go downwards. I only put 100mm natural insulation under my floor. In places its 80 or 50 depending on what sizes I had lying around. The biggest improvement from the exercise IMO is reducing drafts between the floor boards. The comfort improvement was quite noticeable when I first did it.

Keeping old buildings vapour permeable is a good general principle to follow on the assumption that water can and will find its way in eventually so you need to give it a way to get back out. Modern systems focus on making buildings vapour tight with membranes and tapes and assume that water will never get in. Traditional buildings can move around seasonally due to more shallow footings so modern materials that aren't intended to accomodate movement won't perform properly. Its really two distinct approaches so you have to be mindful mixing one with the other.

From a practical perspective natural insulation usually has a bit more give in it than PIR which makes it better suited to fitting up against old joists that probably aren't particularly parallel or square. Its not the sort of job you want to revisit.

 

[paulc]

You don't really need a lot of insulation under a suspended floor since heat doesn't tend to go downwards. I only put 100mm natural insulation under my floor. In places its 80 or 50 depending on what sizes I had lying around. The biggest improvement from the exercise IMO is reducing drafts between the floor boards. The comfort improvement was quite noticeable when I first did it.

I put down 5mm of woodfibre (laminate underlay) on top of the floor in one room here. It certainly killed all the cold draughts coming up through the gaps between boards where British Gas had trashed the tongue & grooves when putting in a gas pipe. Combined with sealing gaps around the perimeter and installing a new stove, it is nice & warm in there now.

 

[mostly_wrong]

Thanks very much both, I've been reading through Toby Newell's detailed post from a previous thread and am on the cusp of chickening out altogether. The original floorboards don't look anything special, and they aren't t&g but the cut clasp nails are a pain and will stay with the joist and potentially split all of the boards. As much as I see this as a fairly low hanging bit of DIY with a good return, the condition of the joists is pretty good as far as i've seen through the bits cut out by the electricians, so i'm inclined to leave it be. That does mean I don't get the opportunity to clean out all of the dead rats from the solum area .

I'm going to get a new floor put down over the originals, replacing the current t&g overboards. I'll put slivers between the original gaps to try and limit draughts, hopefully that combined with an underlay should help a bit.