Hi all,

new on site although read quite a few threads in stalking mode I'll debut by opening up a possible can of worms here (based on prev threads) though I haven't found anything analogous to my situation in the threads so far (apologies for the length a few things to explain) ...
insulating a beam & block floor!
My house is a late 1960s bungalow built on a hill with B&B floor but the voids are huge - in fact roughly 25% under the house is garage, another 25% a store room/workshop and the height to the bottom of floor here is well over 2m; the height at back of house is less dropping to as little as maybe 1.35m in 1 corner. The bottom of the B&B floor is around 390mm above ground level at the back ( over 2m at front) beams sit on the internal brick leaf on a dpc and solid concrete cavity fill below dpc, insulated cavity above. The ground under the cavities is either concrete (50%) or dry earth.

The floor itself is currently un-insulated and no dpm, total depth to top of (newest) tile surface around 200mm. I know the general recommendations seem to be insulate above but 50mm (or greater) celotex plus boarding on top then a finish is going to play havoc with my door openings which are currently 1960mm; ceiling height at 2410mm is less of a concern but there are also 2 fairly new (and expensive) external doors to consider.

My initial thoughts were some kind of solid insulation bonded to the bottom of the B&B floor given the access is so good but I'm picking up warning signs about ventilation under the beams to avoid moisture build-up and the fact that thermal bridging may be an issue!

So in a nutshell any bright ideas? Does my floor still count as a ground floor given the size of the voids?
I'm due to start a new kitchen installation in a few weeks so want to sort the approach by then.

Thanks for reading!

Far less of an issue than if it were a timber floor. If you can get it 'tight' enough it's good. I cannot see where thermal bridging will be a significant issue. A former client is doing something similar.

What about getting a company in to use spray foam on the bottom of the B&B floor?

Whatever you do below the floor there will be a cold bridge at the edges where it sits on the wall. Hence the normal advice to put the insulation above.

I would have thought the standard EWI systems would be the easiest. fix your chosen thickness of EPS to the underside of the B&B with the normal adhesive, making sure no gaps and good fit/seal around the edges, then a top coat (on the underside!) as per normal EWI - that is glass mesh and adhesive and then thin coat render painted on if you want a colour. No need for mechanical fixings. Probably the easiest solution and all items readily available. It could be a bit messy doing the mesh coat so wear a wide brimmed hat if you're not good at plastering ceilings!
I'M no expert on condensation but as Nick said above - less of a problem that with a timber floor.
I just went to
http://www.vesma.com/tutorial/uvalue01/uvalue01.htm
to check out U values and the application also has a condensation analysis. Selected 'floor' as the building element, entered concrete from the Layer pull down menu, 200mm thickness and the default temp. values showed a condensation risk. Entered 5mm EPS as the (next) outside layer and the condensation risk went away. So from that I would presume any amount of insulation would mitigate the condensation risk. (you can use this application to see what you need for your target 'U' value (or see what 'U' value you get for the size of your wallet!)

Posted By: ringiIs the underside of the B&B floor level?

All the ones I have seen have the bottom of the beams lower then the bottom of the blocks.

I think that's probably true except for the ones that are already insulated as built, using weirdly shaped EPS 'blocks'.

I expect the spaces could be filled with loft roll or something to cut down any convection currents, or otherwise could be used to circulate heated air under the floor ...

Posted By: Jeff BWould that matter as long as the insulation is well sealed at the edges/joins?

Airspaces inside insulation lead to the phenomenon known as thermal bypass, where air movement, even just circulation within the space, causes extra heatloss.

I think that this situation is not likely to lead to a large thermal bypass, but adding some extra insulation to fill the gaps and so help to reduce air movement as I suggested is best practice.

djh - OK then. I suppose I would be concerned about the size of the gap and how convenient/easy it would be to fill with additional insulation.

Ringi - I admit that whenever I think of additional insulation works I immediately switch into DIY mode and therefore spray-on insulation would not enter my thinking! Whilst I'm sure it would be efficient wouldn't that look a bit out of place on the ceiling of a garage and workshop? Would it need to be plaster-boarded over after?

I suppose it is impossible to prove 100% but I have always been able to well seal joins/edges in rigid insulation using acrylic mastic and self-adhesive aluminium tape. The OP's floor is tiled so presumably there is a concrete screed over the B&B system - would there be any serious gaps in that?

Posted By: shandyd05My house is a late 1960s bungalow built on a hill with B&B floor but the voids are huge - in fact roughly 25% under the house is garage, another 25% a store room/workshop and the height to the bottom of floor here is well over 2m; the height at back of house is less dropping to as little as maybe 1.35m in 1 corner. The bottom of the B&B floor is around 390mm above ground level at the back ( over 2m at front) beams sit on the internal brick leaf on a dpc and solid concrete cavity fill below dpc, insulated cavity above. The ground under the cavities is either concrete (50%) or dry earth.

You could consider insulating the vertical walls under the slab, using rigid foam.
("conditioned crawlspace" approach)
(The area involved might work out much less than the underside of the slab).

gg

shandyd05 - IMO it will be a lot easier if you put battens over the Celotex and screw through both into the B&B. This will give you plenty of places to screw/nail your plasterboard to. Also you could create a void space for cables, or fill the gaps between the battens with additional Celotex.

Jeff B