insulating over old concrete floor

[Tom__40]

Hi. I need to insulate the floor in my utility. It's an unheated single storey outbuilding attached to the back of the main house - an end of terrace with cavity walls. The roof has recently been replaced and insulated (it was corrugated asbestos sheets), and I plan to add external wall insulation for the walls (waiting on planning as I want to have render on top). Because I removed the door between the utility and the main house (the surveyor said it was a hazard as it was one of those nearly all glass ones, untoughened, and near the bottom of the stairs - it's a 50s house), building control say I'm required to insulate the floor which is currently just a concrete slab, which likely doesn't have a damp proof membrane. I will need to do a building notice soon anyway as am adding a shower room to the utility, however I'm unsure on a couple of details on doing the floor. Just wondering if anyone knows if something like the below (the first diagram in the link) would be acceptable to BC - painting on a liquid DPM, frame fixing the timber battens to the floor, PIR between the battens, vapour control layer and then screwing the chipboard to the battens through the vapour control membrane? I want a nice solid floor with minimal/no bounce to it. I'm unclear on whether the frame fixing element and screwing the chipboard down would be ok, as would obviously breach the DPM and vapour control layer. Not sure how I'd get it solid without doing so though! I plan to ask BC as well, but thought I'd see if anyone had done similar?

Insulating Concrete Floors: New build and Retrofit Expert Guidance

Insulating Concrete Floors: New build and Retrofit Expert Guidance

www.designsindetail.com

 

[Deluks]

If you've removed a door between house and outbuilding, then you'd better get a new door, or this might be viewed as an extension, and all elements of it will have to meet building regulations.
You could have had the glass replaced with toughened glass, or used a sticky backed glazing film to make the glass a bit safer, as temporary measures.
Your best bet is to get a uPVC replacement door fitted.

If you are actually converting this space to be part of the house, then run all your proposals by building control to make sure they are happy, particularly in regard to your new roof construction and insulation specs.

If you can't get 100mm of floor insulation in, they might make you dig up the floor and do it all properly.
Assuming the door is reinstated. Then you won't need to insulate the floor, if you want to you can float a chipboard floor onto rigid insulation. Plastic sheet DPM (1200 gauge, not any old polythene) glue the chipboard together.

If there are any air bricks from the house in this space, you won't be able to cover them up, and if converting the space you would need to duct them to the outside, which would probably mean digging up the floor.

 

[Tom__40]

Thanks for the reply. They're happy with the roof construction and insulation, but the guy from BC who came round today (to sign off the roof) said he wasn't sure he'd be able to sign the roof off because of the door removal, he's going to get back to me. I don't think they'll make me fit 100mm from what he said though - he implied 50mm PIR would be ok, the utility floor itself is ~90mm below the main house. No air bricks at all in this bit of the house - it's just concrete slab on the ground with single skin wall at the moment. And a now nicely insulated roof!

Just had a closer look at the floor and to lay any kind of PIR I'd need to level it first unless I fix wood joists/battens and insulation in between. I reckon I'll lose 10mm height on the levelling if I go that route. 50mm should still be doable though just about. I just don't like the idea of a floor that isn't fixed down properly, I'm much more belt and braces approach - I like things fixed rock solid in place!

 

[stevie888]

It is written into the regs that the payback period has to be assessed before automatically requiring an element to be thermally upgraded and BCOs have always been pragmatic in these cases. In you case I would look to 20 and then dense screed. Considerably better than nothing and bear in mind that heat rises so the floor is the lowest loss zone.

 

[Tom__40]

Thanks for the reply, I may well put it to them. So I'm envisioning something like paint on DPM, then PIR, tape joints, ?expansion gap at the perimeter of the PIR filled with some kind of foam tape (not sure if this would be needed for the PIR?), presumably prime the PIR as they usually have that foil surface (the liquid DPM I have in mind is also an excellent primer), then a flowing screed for maybe another 40mm. From what I google about it, normal sand/cement screed you need 65mm thick minimum to avoid cracking, but there seem to be various modified ones that have a lower minimum thickness I could use

 

[endecotp]

I just don't like the idea of a floor that isn't fixed down properly, I'm much more belt and braces approach - I like things fixed rock solid in place!

Chipboard (or OSB) floating on PIR is nice and solid. Having said that, I vaguely recall some caution about using that construction where you have heavy kitchen appliances - but I could be imagining that.

What sort of levelling is needed? You can knock off any high spots in the concrete, and fill dips with sand. Or is there a slope?

 

[Tom__40]

Not had a complete proper look, but put a straight edge around 2.4m long on the floor and the ends were about 10mm off the ground, middle touching. Nothing a bit of floor leveller won't fix, and if I have to liquid DPM anyway the one I'd use would also act as a primer for the leveller.

 

[endecotp]

Not had a complete proper look, but put a straight edge around 2.4m long on the floor and the ends were about 10mm off the ground, middle touching. Nothing a bit of floor leveller won't fix, and if I have to liquid DPM anyway the one I'd use would also act as a primer for the leveller.

Sand for levelling.
Thick plastic DPM sheet.
PIR.
Thinner plastic sheet as a “slip layer”.
T&G OSB or chipboard.
Final floor covering.

A Guide to Constructing a Floating Floor

A floating floor is a construction where chipboard, gypsum or cement fibre board floor panels are 'floated' across the top of the rigid insulation layer. Read more to find out what insulation thickness is required and how to insulate and install a floating floor.

www.kingspan.com

 

[Tom__40]

Thanks, that's definitely one option. I've been doing some googling and have also now found what might be another one...there are apparently screeds that incorporate EPS beads in, with thermal conductivity similar to mineral wool insulation. Have e-mailed a couple of companies to see what the cost is like though - might be prohibitive, we'll see! Threshold value I ideally need to hit would be a U of 0.7, the thermal screeds wouldn't be as good U value wise as the PIR, but I think could still reach it, and would be a stronger floor.