u value pitched

ptogher14

Evening everybody,
What's the required u value on a pitched roof on an extension to meet building regs? And what build up can achieve this?

delfagio

0.16 u-value I think from memory. You will find in the Part L regs if you google it online.

Regards build up and insulation, do you have an Arch or Engineer on the extension build that can work this out for you?

ptogher14

I'm not building, just having a conversation with someone. You hardly know what u value a 63mm slab on its own will get you?

Metric Tensor

A slab of what?

Anyway - nowhere near close to 0.16.

ptogher14

A 63mm insulated board. I called into a friend today who is getting an extension built. Lean to type extension off the back of the house with a pitched roof. The "builder" hadn't put any insulation in between the rafters. When I pointed it out to my mate the so called builder said it didn't need any as he was using a 63mm insulated slab.

Gumbo

Remember that extensions only have to meet the element u value so it's much lower (0.25 IIRC) than a new build. Still though, very foolish of your mate to do this.

Also, the u value is calculated by taking a section through the complete build up, so the tiles, felt, joists and then the 63mm board.

ptogher14

My mate doesn't know any better. He's relying on the builder. As I was pointing out now is the time to do it. Why not insulate inbetween the rafters? Regs or no regs you're going to get the benefit from the extra insulation.

Metric Tensor

Also worth noting that a 63mm board only has 50mm of insulation on it!

I haven't done the calculation but I can't see the build up described getting 0.25.

Seems like madness not to insulate between the rafters - leaving a proper air gap of course!

BryanF

Where is the vapour barrier / dew point? Is the roof space vented?

ptogher14

BryanF wrote: »

Where is the vapour barrier / dew point? Is the roof space vented?

There's a 7 inch rafter, felt and tile.