Cavity wall and insulation Building regs

I've finally got planning permission for a single story kitchen extension, thanks largely to Woody's advice.
Now for the building regs, i believe the requirements are now 100mm cavity, I will use 100mm concrete blocks on the outer leaf and thermalite on the inner leaf, I intend using Kingspan insulation, do I use 75mm and leave a 25mm gap?
Also the rafters for the roof, the span from wall to wall is 2,700mm internally, the pitch about 25°, I will be using Redland double Roman tiles on 2" batons and breathable felt, could anyone advice me on the size of the rafters and the insulation I require please.
Many thanks in advance.

Most of the rigid PIR board manufacturers ask for a 50mm clear cavity some let it go down to 25mm in a sheltered location. Some have 95/97mm thick full fill PIR boards. You pays ya money and takes ya choice. Lots of info on their websites.

As for the rafter size there are span tables available free online. Otherwise that span of 2.7m is that the actual rafter span or the depth of the room? Insulation for roof.......oodles of information on all the manufacturers websites, it depends what type of insulation you want to use, where you want to put it etc.

Why are you thinking of going rigid PIR board in your 100mm cavity? Why not use regular 100mm rock wall insulation? Will work out a damn site cheaper..... Kingspan and the likes are bloody expensive.

Rafter will be just over 3m so a 5x2 @ 400mm or 450mm centres or a 6x2 @ any spacing up to 600mm.

You really need to consult your choice of insulation manufacturer to see what type and thickness of insulation for the roof. You don't say if you want a insulation on top, between or under rafters or on the ceiling joists - you need to decide on a roof design first.

For the wall cavity, most makers will require a 50mm clear cavity, but some can go down to less. Bear in mind that this type of insulation is not only more expensive than quilt, but is less tolerant of poor construction/fixing and blocked cavities. But yes it is more efficient than others for any given thickness. Full fill cavity may not be suitable for very exposed location though. Again decide on a design and criteria first before selecting any type of insulation.

^woody^ said:

For the wall cavity, most makers will require a 50mm clear cavity, but some can go down to less. Bear in mind that this type of insulation is not only more expensive than quilt, but is less tolerant of poor construction/fixing and blocked cavities. But yes it is more efficient than others for any given thickness. Full fill cavity may not be suitable for very exposed location though. Again decide on a design and criteria first before selecting any type of insulation.

Click to expand...

What are the pros/cons of full fill (assuming it isn't exposed location) given that full fill is cheaper?

I have now instructed an architect to draw up and submit the plans to Building Regs, the cheapest quote I got in Bristol was £1,500 which I thought was a bit steep, I found one across the bridge that is over 50% cheaper.
I would like to thank you all for your assistance I've had over the past couple of months, it gave me the ammunition to shoot the case officer and her line manager