24 March 2021 at 8:36 pm #62993Philippa Birch-WoodParticipant
Hi fellow AECB members
I am hoping this is the place where someone is going to tell me they have done this all before 😉
I have a house that was built in 1964 which is very tired and in need of some TLC. We are moving the kitchen to the rear of the house and thought it was worth wrapping in some works to improve air tightness. The floor at the front of the house is solid concrete and at the rear it is suspended timber with airbricks ventilating the subfloor (only on two sides of the room) and there is a chimney which we will need to remove.
There seems to be two trains of thought on this:
1) fill the floor in –
Remove the floor joists, fill with hardcore, then line with membrane taped at the perimeter, then insulation (contractor would use “ecoboard” something like this – https://www.insulationsuperstore.co.uk/browse/insulation/insulation-board/ecotherm-insulation/eco-versal.html) , underfloor heating then a concrete screed.
( I would be interested in a lower embodied carbon alternative/ less plasticky alternative if this is the way to go)
2) Leave the joists in place and sag a breathable membrane between them and fill with loose fill natural insulation. top with wood fibre board (making sure to leave 150mm of air gap in the subfloor)
We are intending to externally insulate our cavity walls (that are filled with polystyrene balls) so I am unsure if the later is appropriate as I suspect we will need to block up the air bricks and create a french drain detail? Also I may struggle with head space in the lounge as the ceiling is already pretty low.
Any advice would be much appreciated – on the method and on the materials.
7 May 2021 at 9:40 am #63347X WookeyParticipant
My 1960s retrofit already has concrete floors so I’ve not done this personally, but as no-one else has offered an opinion, here’s mine.
Unless the space is too deep to make it practical, I’d fill it in. so you can get a properly insulated floor with some thermal mass. and get decent airtightness. Building control people get very difficult about airbricks, airtightness and suspended timber floors so getting rid of it is both technically and ‘politically’ better.
Hopefully the space is just right for 200-300mm of insulation. I default to XPS rather than PUR underground just in case it ever got soaked (flooding), but that’s carbon-intensive too, so if you’d prefer to avoid that foamglass agreegate is very green (but also expensive when I looked). https://www.mikewye.co.uk/product-category/brands/geocell-foam-glass/ You’d just pour that in instead of the hardcore, then put a floor on top. Lithotherm UFH floor blocks directly on top of the foamglass is one way to do concreteless solid floor: https://www.backtoearth.co.uk/product/underfloor-heating-tiles/ I’ve not tried it myself but I’ve seen one and it looks like a reasonable plan. Backtoearth are great so I’d certainly have a chat to them about it.
You could take the view that having this nice space to fill in with some insulation is a bonus. Dealing with an existing concrete floor is a pain because digging it up is a big job and insulation on top is technically limiting, especially with UFH.
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