Good evening buildingregstrackworld!

I’ve been getting a few quotes to fix up the flat roofed bits of our house. As part of this I want some proper insulation on the utility room roof, which currently has a worthless amount in the ceiling space, a long way off any vaguely recent regulations.

As I understood it, if we’re adding any insulation at all then it has to be to the current standard, ie. a u-value of 0.18 or better. So why have all the companies I’ve asked so far just wanted to put a random – and insufficient – thickness on? I think it has to be 140mm, but the quotes so far have been for 90, 70 or 50(!) mm.

Are the regs not as strict as I thought? Are they just making it up assuming most people will not know or care? Or do they know something I don’t that makes a thinner layer okay?

Depends on what type of construction it is really and which insulation material. My preference is a warm roof construction with grp. 100mm celotex is what building control always want from me for flat roofs and garage conversions

Dont know about the regs as i just put inulation as per the architects drawing and none has been under 100mm of the solid celotex type insulation for the last few years.
edit too slow

They may be using different types of insulation, some of which won’t need to be as thick as others for the same u-value. More realistically though, they are probably just making it up and have no real idea.

Try getting in touch with Kingspan customer support. They’re generally pretty good at recommending products if you give them the u-value you need to achieve, and the build-up you’ve got/want.

If you are upgrading the roof to meet current thermal requirements then the thickness of insulation will vary depending on its thermal conductivity. 120mm of Kingspan TR26 or similar will make the majority of roof build-ups achieve 0.18W/m2k. If there is already insulation below the roof deck this can be reduced. Make sure that whoever does the work also carries out a condensation prediction/risk analysis.

As has been said, they’re probably guessing there’s something there already.

Like funky monkey, I’d favour a warm roof design that should deal with cold bridging. If you’ve got the external height, I’d go for as much insulation as possible 150mm celotex or analogue and GRP or EPDM rubber. Might want to think about if you have a moisture barrier behind your ceiling plasterboard too.

If they’re removing whatever insulation you have there already, there won’t be many solutions that don’t involve 100mm Celotex / Kingspan PIR board, unless they were planning on using the fancy Kingspan foil wrapped Quantum board, but in a domestic scenario that’s highly unlikely.

BTW, if you are increasing the finished height of the roof through additional insulation, get them to check that there aren’t any cavity trays that are now below your up stands, as otherwise you’ll get water discharging to the interior rather than exterior.

Roofers at work are replacing 50mm with (I think) 160mm, but that’s on a concrete sub-roof
One layer of insulation with five layers in total including vapour control, movement, waterproof, etc
They’ve had to raise the skylights, increase the height of the sidings, not to mention the opening window on a higher part of the building that has a sill lower than 160mm…

As much as you can afford. Insulation (on a scale of diminishing returns) is a no-brainer.

If you want a speedy reply from a manufacturer contact Xtratherm with your proposed roof construction and ask them to specify the minimum insulation thickness for your target u value. Kingspan website is good for literature with pretty pictures though, and there are tables in there showing how much insulation you’ll need to achieve a certain u value. You can use insulation from any manufacturer as long as the thermal properties are the same, or similar, which most are unless you go for the more expensive stuff (which you’d only do if you had restricted space to build up the required thickness).

There’s a few ways to go about it but I’d probably have most of the insulation on top with insulated plasterboard inside, but it may depend on which roof cladding system you go for.

Thanks all, I feel a lot better informed now. We’re definitely going for a warm roof, there’s no issue with available height and I can’t be arsed to start faffing around with the ceilings inside. Given the costs of labour and all the other materials slightly thicker insulation doesn’t really make that much difference given it’s not a very large area.

+1 @ Splitlip

Make sure your warm roof really is a warm roof. I moved into a house with a flat ‘warm roof’ over the kitchen extension, with nothing at all between the ends of the rafters and the void under the old slates of the apex roof it joined. So all that lovely insulation doing nothing at all…twunts!!!