Insulation between rafters

[aswadeem]

I have been informed by the building inspector that the rafter insulation must achieve a U value of 0.2., therefore I must use 145 mm Kinspan which I think is a bit excessive. However, to comply with existing building regs are their any other products that achive the u value stated above, and that are considerably thinner, because I dont really want to lose any head space due to using the kingspan product. Also, the building control officer also stated that I must cross batten the kingspan, which I am not quite sure what he meant. Although I have manage to drop the ceiling and put new beams in, and fitted the stairs, I am at another hurdle in which I require some clarity.

Cheers

 

[freddiemercurystwin]

You cant get much better than kingspan so I'd think not, some people's local authorities allow tri-iso in addition to a single layer of kingspan but your BC officer sounds like a stickler so that's prob not an option. Cross batten the kingspan just means if the first layer runs parallel with the rafters the second layer should run at 90degrees to the first layer, its to minimise draughts. Joints should be taped as well.

 

[aswadeem]

Thanks for that valuable info. However, does that mean that I must use to layers of kingspan that makes up the 145 mm thickness. ie 80mm kingspan in between the rafter and 65mm kingspan cross batten.

 

[freddiemercurystwin]

Yes, provided you leave your 50mm airspace under the felt.

 

[aswadeem]

Concerning the 50 mm air space, doe this still applies when you have breathable felt, because I mentioned this to the building inspector, and he responded that I can place the kingspan close up to the felt, due to the fact I have used breathable felt, as long as their was a bit of slack on the breathable felt which is fixed on the rafters, is this correct otherwise I will ring the building office before I commence work.

 

[freddiemercurystwin]

If you’re using a breathable membrane that is correct. Any moisture that gets through the insulation can escape through the membrane rather than being drawn out through the airgap.