Building control application made before the latest change. But what now?

[jeds]

I've seen this spec advised by several inspectors, yet the Kingspan U-value calculator says you can have a 50mm clear cavity and 75 PIR board against aac block, and you still get 0.18.

I'd be very wary of using a 10mm cavity; using partial-fill board in any case requires more care than - say - full-fill batts, but when you're down to a 10mm cavity, that must require extra care and are many bricklayers up to that?

Cavity walls were common from after WW1 and they were introduced to prevent water penetration to the inner skin (not, incidentally, to improve thermal insulation). The 2" cavity has been widely used for over a century and is 2" for a specific reason - to reduce the risk of mortar bridging.

That is correct Tony, kingspan u-value calculator makes 75mm partial fill K8 board U=0.18. However, I had this picked up by building control who argued that it does not come out as 0.18. I can't remember what they thought it was but they were adament that according to their BRE calculator it was not 0.18? Now, I have two different u-value calculators - software - and they both come out at 0.20. I got it agreed in the end - luckily because a lot of walling was already built - but it does seem a bit odd. I also double, double checked everything to make sure the values were identical.

I have been giving builders a couple of options on this. The favourite has been 100mm Dritherm 34 full fill cavity + 52.5 thermal laminate dot add dab. This comes out at 0.18. The next preferred option is EcoCavity full fill PIR 115mm (requires a 125mm cavity) which gives a u-value of 0.15.

PS, I've specced Ecocavity and Kingspan full fill (with the 5 to 10mm gap) many times over the last couple of years and I've never had a problem with it. Builders quite like it as well - easy to use and doesn't take up too much time.