Hi All, wondered if you could advise please.
We’re planning to renovate our 1960s bungalow and incorporate various energy saving features. One issue I have been grappling with is how to insulate the uninsulated drafty timber suspended floor, but, I am now wondering whether it is worthwhile.
We plan to insulate the external walls (EWI), likely with 90mm EPS finished with render. After lots of reading, particularly older posts here, we’re thinking about running the EWI down to the foundation to eradicate the cold bridge essentially making a shallow insulated basement (though I guess there is still a bit of a cold bridge at the base of the internal cavity wall where it sits on the foundation). I assume this will make the current suspended floor void part of the warm envelope. It will involve blocking up the air bricks and bridging the DPM - is this a sticking point for us and building control?
What led us to this decision is that the void under the joists is only 85mm (between the concrete slab and joist) so we can’t insulate under the floor and leave the 150mm to satisfy building regs if we were to take a traditional approach (which is what we were initially considering).
The idea is to trench around the house, insulate (fixed with adhesive) and backfill with LECA (including the foundation footing face).
Having done this, we could fill the floor void but the incremental reduction in U value might not be justified (going from about 0.4 to 0.15, calculated using CIBSE ). Ironically having no void arguably negates the need for the air bricks - so that’s a thought, it avoids the potential argument with building control about blocking the air bricks.
We want to improve the level of insulation and reduction in infiltration losses caused by the air bricks.
I’ve attached a section through the floor space (1:10). Not sure of the thickness of concrete slab or foundation footing thickness.
Thoughts? And thanks very much in anticipation of any responses…

Have you seen the blurry jpg I posted several times on those threads, of the annotated drawing sheet which convinced our Building Inspector?

That's the one. Is it legible?

Not that I know of - I made it up, from what I know generally! But our Building Inspector said she understood - 'my husband is a physics teacher'!

I realise (failing memory) that the drawing I'm refering to was for a different job, where the solution was rather different, without subfloor void - but the principles still apply, sort of. But I think it was that drawing which convinced the Building Inspector on the subfloor-void job, like yours. Or maybe I just talked her through it.

VikingHouse (from Eire) of this forum (haven't heard from him for a while) does in fact fill the underfloor void with EPS bead - search for that, for more gd info.

I notice that in that drawing, I refered to the trench face as 'dehumidifier' for the whole void - just as currently discused in http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16271&page=1#Item_11

Posted By: WillInAberdeena cold absorbent well-drained soil surface, with a low vapour pressure. This draws down the humidity in the air above it

O'course the popular notion is that humidity and foul vapours rise into the house from the damp subfloor!

Posted By: WillInAberdeenI admire people with the courage of conviction to try the poly beads!

Thanks for the post. It makes sense to me. But I think all it says about the poly bead solution, or the VCL under the floorboards, is that people need to deal with it. i.e. install better glazing (double or triple, I don't know) and ensure good ventilation and insulate the walls etc. You can't treat one 'thermal element' and not expect to have to deal with the rest.

Thanks for all your comments people.
I'm not too worried about interstitial condensation within the wall, but the effect that blocking the air bricks will have on the floor void. I cannot add a layer of insulation between the floor joists as I only have 85mm beneath them and the concrete slab (building regs wants 150mm minimum).
This is a complete renovation of all rooms including rewiring and plumbing so if I do decide to fill the void (read if I am allowed to) then I will take appropriate precautions with the installation of cables and pipes.
I think the pragmatic solution is to sit down with Building Control and see how they respond. Personally I would be comfortable with insulating externally and not filling the void, even if I block up the air bricks - the concrete slab/solid ground is not a bad insulating layer, especially once EWI is installed down to the foundation. Or am I missing something?

Thanks djh, sensible feedback. And, yes, I'm leaning towards filling up the void. It won't cost much, the hassle of taking up the boards will be worth it and it should make an easier sell to Building Control.

We were told by our local Building Control that the underfloor either had to be solid or be ventilated. With a sheet polythene DPM on the soil, the space above could be filled completely around the joists and up to the floor effectively making the underfloor solid as required and bringing the floor into the insulated envelope as has been said.

Non combustible glass beads rather than EPS which either degrades or is aggressive, are slightly less insulative than EPS but much more insulative than Leca and as inert and indestructible as the latter. They are lighter than Leca and can be blown throughout the underfloor void to build up between the joists with a workshop vacuum cleaner.

For external situations geotextile would line the trench and the same beads, being pourable, will fill irregular spaces completely.

We have some expanded glass beads left over from a build and if you are interested in a sample we could supply.

Posted By: tonyFor me the idea of a dpm on top of the oversite on the cold side of the insulation is insane and could collect moisture on top of it, likely causing smells, slugs and eventually mould growth.

Agreed, something like a geotextile would seem a lot more appropriate on soil. But if there's already a concrete oversite then there's no need for anything extra, I think?

Thanks for the follow up comments djh and tony.
Yes, I have a concrete oversite so am not faced with a damp soil sub floor. I'm still not 100% convinced that filling the void is necessary - the slab/earth essentially becomes the insulating layer so the void would be 'warm'. That said happy to go with a pragmatic approach and fill it if it helps to pass muster with Building Control. Might use Loose Fill Cellulose Insulation rather than poly beads. The floors will have to come up, cabling rerouted and I want to clear all the builders' rubbish that has laid there since year dot (fag packets, butts, and all the debris they couldn't bury in the garden!). Fun...

Thanks Petlyn, I missed your choice of loose fill material i.e. expanded glass beads. I like the fact that they are non-combustible although they have a higher thermal conductivity (~0.07 W/mK) than EPS or Cellulose (~0.04 W/mK). The overall U value would only increase from 0.15 (with EPS) to 0.2 (with Glass beads) - potentially, if either the EPS or Loosefill Cellulose settles then they may be equal. I also like the fact they can be used around cabling - though are you saying cables would not need to be derated if they are routed through within? If not I don't see any advantage (I'd isolate cables from EPS if I use it)
The cost of the glass beads appears to be prohibitive (~£30 per 100 litres) cf Loosefill Cellulose (~£4 per 100 litres). Unless I have my maths wrong (double checked it). Higher thermal conductivity and cost - not sure the non-combustibility would sway me.