Mattsville

Yes that will be right - 75mm is what I'm calling a deep screed (could use 50mm self levelling, but I have space for neither). Turns out XPS cement coated boards only need a few mm of fibre reinforced self levelling screed so I've gone with 10mm XPS cement coated backer boards (Jackoboard) underneath 18mm XPS cement coated routed boards for 12mm pipe. It's far from ideal but I'm reasonably happy having 25mm odd insulation for this small area. Much better than the nothing I had under electric UFH before.

I can't use PIR because it would need a deep screed on top of it as it's not structural boards. Or so I've been told? I'm also now being told that nu heat boards need to be fixed directly to a concrete floor and can't be put on top of insulation, and that XPS boards with pipework on top would need a 50mm liquid or a layer of plywood on top so that's not ideal either.

PIR isn't an option as there's no room for a deep screed on top. So my options are 12mm XPS + superfoil, 20mm XPS or something else that is around 20mm deep and doesn't require a deep screed on top. Seems odd that some of the big companies recommend and advertise superfoil for under floor insulation. Surely that shouldn't be allowed if it's useless? I realise it'll never get near their claimed values, but if it's completely useless that's another thing. Matt

A step isn't an option. The new extension is approx. 30m2 and the existing kitchen is about 18m2. I would consider not having any heating in the old kitchen part but there is a barrier at high level (steel supporting previous back wall) and low level (kitchen island/peninsula) so I don't think the air would circulate much into the kitchen part. I think I'll zone the UFH so worst case we can not use the UFH in the kitchen area and install a rad somewhere, but I'd still like to put the best insulation I can under the UFH. Even 10mm of XPS alone will be a lot better than nothing. Tthe kitchen is already designed and paid for and there isn't a brilliant place for a rad, so I don't mind spending money on this. I considered some vacuum insulation board but can't find any prices or stock. Matt

Hi All, I'm currently halfway through a kitchen extension, and in the new floor we have 150mm of insulation over the concrete slab and will put in underfloor heating in 75mm screed on top. The plan was to take up the existing kitchen floor and do the same, however we've just discovered the kitchen slab is part of a raft foundation, so we can't touch it. This means we have no room for adding thick insulation and screed - we only have about 40-50mm depth which will include the flooring. We want LVT or SRC tiles which are around 6mm deep for the flooring. I've been looking into low profile UFH and it seems we can still have a wet system, so I'm just trying to work out the best way of insulating, given the depth restrictions. I believe our pipework (in a liquid screed) can be as little as 20mm, so we have about 20mm for possible insulation. Nu-heat recommend a system that is 22mm deep in total with no insulation underneath and they say we'd lose very little heat downwards, but I can't see how this is right. Can't find any R values for their system online. I've also been recommended to use just a 6mm superfoil insulation under the liquid screed, which would be better than nothing... but I'm wondering if I can also include a 10mm XPS insulation board, or 10mm Jackoboard or something? So the build up from bottom to top would be concrete slab > 10mm XPS > 6mm Superfoil > 20mm UFH pipework in liquid screed > 6mm vinyl tiles. Has anyone done similar? Does anyone know if combining the XPS and superfoil like this is a good/bad idea? I realise this isn't ideal and we wanted much more insulation, however at this point there's not a lot we can do. Thanks for the help! Matt

Roof pitch is 25 degrees. I think you're probably right about the builders, however I think it's too late for me on this one to change the roof construction significantly. Just got to work out which of the options left to me is going to be acceptable. It seems like surface mounted spots with just holes for cables will be acceptable, and probably shallow flush mounted spots within the insulated plasterboard will be fine too. If we get fire rated spots then we don't need ventilation to cool them. I'm not really sure how essential a VCL is in any case - the ventilation above the rafters should clear any condesation that get through cable holes in the PIR shouldn't it?

I'd never be happy with that, there'd be no light in the middle of the room. I'd rather have pendant lights (or bar mounted spots) at the apex but even those are going to need holes in the VCL for the cables.

Sorry everyone, I said joists when I meant rafters. Yes, the 100mm PIR is between the rafters. This is already fitted. Also the insulated plasterboard has been delivered, but they could send it back I assume. Unfortunately ceiling height is still an issue - at the eaves the ceiling height is already a little low due to not wanting the roof to obstruct windows on the rest of the house, and also the roof is supported on oak trusses and it would look odd if we covered half the truss. I mean, I'd rather that than have a rotten roof in 20 years, but if we can avoid it by just having surface mounted spots then we will. The compromise suggested by BC is to cut out holes in the insulated plasterboard and halfway through the 100mm PIR between the rafters, just leaving 50mm left above the spots. This sounds like it leaves us with no VCL at all. Is this OK? As far as I can see other options we have are: 1. BC suggestion as above. Will this lead to a rotten roof as there's no VCL? Celotex say under no circumstances cut through the insulation, but if it just means a slight loss of insulation I'm not too worried. 2. Create VCL below 100mm PIR (either tape over rafters and joins or install a membrane) then cut holes in the insulated plasterboard only for shallow spots. We would still need holes in the VCL and 100mm PIR for power cables. Can LED spots be completely surrounded by PIR insulation? 3. 50mm batten below insulated plasterboard and then plaster below it creating a void for shallow spots. Should work fine but loses 62mm ceiling height and is a lot of work. 4. Keep roof construction as is and install surface mounted spots. This will mean cables have to pass through VCL. Is this acceptable? Celotex say drilling small holes for cables through PIR is fine. If you could let me know which of these options is acceptable I'd be grateful. I think all would get passed by BC, but that's no good if the roof would rot. Replacing rusty spots in 20 years I can probably live with if it's not dangerous and doesn't make the room really cold. Thanks for the ongoing help! Matt

Thanks for the reply - it's a pitched roof with a vaulted ceiling, so there is no loft or joists. Given the roof is already build, I'm willing to accept "acceptable" and not be too worried about "good" if I have to. What I'm not willing to accept is a roof that might rot in 20 years. It sounds like the roof construction is acceptable, but not if we put spotlights in. I believe the plasterboard can form the VCL if they tape the joints correctly, but then the downlights would ruin it. I'll have a look at surface mounted spotlights, but what about the cables to power them? I've been told it's ok to drill through the insulation to get power to spots, but what about the VCL? Matt

It's too late to put the insulation above the joists - the slate roof is already on and finished. Also the extra height would then mean blocking other windows on the house. The BC inspector seems happy enough with the construction, it is only the holes for the spotlights he has a problem with. Looking around it seems very common to use this type of cold roof construction and still acheive the U values required for buildings regs. Is this not right? An online u value calculation is telling me this gives a value on 0.19... which whilst it may not be quite what it should be, if the BC inspector is happy with it then I am. Do you think this isn't right? If we get 0.19 ish then that's fine by me, but I'm worried about no VCL causing condensation and rot, and also whether the spotlights will ruin all the insulation. Thanks, Matt

Hi All, Wonder if anyone can shed some light on this. I'm no builder so please bare with me. We've had a single storey pitched roof extension built with a vaulted ceiling. The roof construction as done by our builder, is a cold roof as follows: Slate Rood Breathable Membrane 200mm Joists with ventilation gap and 100mm Celotex inbetween plasterboard backed with 40mm Celotex I have sketched a profile - hope it all makes sense. They have got as far as fitting the Celotex between the joists and following a conversation with the building control officer I have some concerns - I'd appreciate some advice. 1. Do we need a VCL? The 100mm Celotex has a foil layer, as it looks like the plasterboard insulation does on either side of the insulation, but there has been no taping of joins and no plans to add a VCL. Is this ok? 2. We are having spotlights in the ceiling - the builder planned to leave gaps in the 100mm Celotex for them, but as building control has pointed out this means there are cold spots in the roof and the light is exposed to the vantilation gap and will get condensation on and rust. He advised cutting out halfway through the celotex to leave a space for the light but also 50mm of insulation above it. However this would mean cutting through the foil layer. Is this ok? From the research I've done it appears that neither is acceptable. Is there any way we can add a VCL and still have spotlights in the ceiling? Thanks very much for the help! Matt