Insulation question explained

lagi

Could someone please explain this to me please.

Taken from building reg. spec sheet.

Sloping ceilings: 12.5mm foil back plasterboard over 90mm 'Celotex tuff R' rigid insulation batts between rafters with 45mm 'Celotex tuff R' under where rafters are at 400mm ctrs, 40mm where rafters are at 600mm ctrs.

I understand the plasterboard stuff, but confused with the 90/45 & 40mm stuff, just measured rafters and they are 400mm ctrs, some a bit less where skylights have been fitted.

WestonDave

Subject to better advice from a pro, it reads to me like you stuff the 90mm between the rafters (which makes sense as most rafters are about 90-100mm) - you then stick the 40 or 45mm stuff over it to cover the 90mm stuff and the rafters, and then you put the plasterboard over that - i.e. 3 layers. Normally you are supposed to put 270mm of insulation in a loft so its not surprising that they want more than just what would fit between the rafters (i.e. 100mm max)

I'm also guessing that the difference in thickness allowed depending on the rafter spacing is because wood (i.e. rafter) is less effective as an insulation than Celotex - so if the rafters are closer together you need more insulation on top to counter the fact you have more wood and less Celotex under the 40/45mm layer.

Cue someone who really knows what they are talking about shooting me down big time!

adaze

I read it slightly differently and I didn't do the "overlapping" when I redid our roof. I read it as 90mm insulation in between rafters, with 45mm insulation under the rafters only (i.e. not a second layer over the whole roof area).

Im not really sure why there is a need to insulate under the rafter itself, I would have thought the rafter would provide some insulation. Which it probably does and this is equivalent to the "other" 45mm of insulation.... maybe it does make sense

I would really like to see a comparison of insualtion that show what other insulation is equivalent to the "270mm wool type insulation", i.e. presumably 90mm celotex = 270 wool?? How about the foil backed, bubblewrap stuff with loads of layers, that must need a lot less....

TallTree

I had to do this for our loft conversion/extension.

You will also need to fit battens on the rafters so that the 90mm insulation does not go too close to your roof felt. The gap between felt and insulation must be maintained so that air can circulate. I think that the gap should be 50mm according to regs, so your rafters have to be deep enough to start with.

Also you will probably need to tape any joins of the insulation with aluminium foil tape. I found that the local insulation supplier was cheaper for the tape than screwfix.

I got a quote from a builder to do the work, but decided to do it myself to cut costs. Its worth getting the builder to do it, just so that you don't have to carry all the materials up 2 flights of stairs!! Knackering and time consuming work.

lagi

Thanks, will be calling the building inspector out hopefully next week to look at roof. Will ask him exactly what he wants as rafters are 150 thick. If its true that you have to leave 50mm air gap this will allow me to get 100mm in there, provided your allowed to add thicker stuff. Shame as i have seen some 120mm stuff pretty cheap locally.

beaker141

The 50mm airgap is variable depending on the construction of the roof.

I have left a 25mm airgap on mine, with 150mm rafters as we had the whole roof relaid and used a special breathable felt which eliminates the 50mm. All that was needed in my construction was a 25mm airgap to allow it to drape.

I am generally using 120mm Celotex, except where the old 75mm rafter is also left in place where I am using a 50mm and then a 60mm to save notching a 120mm piece along the entire length.

Over that I am then going to use Kingspan K17/K18 with 25mm insulation topped with 12.5mm plasterboard.

Branded equiv of what we used is Kingspan Nilvent, i think the Kingspan pdfs have lots of examples of what roofs you might have and what will meet the regs.