Dual leaf brick wall, what size cavity for full fill batts?

[allthatpebbledash]

Just another follow up topic from my previous as even though the nature of the question is same, new facts.

 

Say you were building a new cavity wall using dual leaf brick. New foundations. Width of wall to be constructed not an issue, as long as it’s not like 600 or something.

 

What size cavity would you have if you were intending to full fill with mineral wool batts?

 

Wall would need to achieve 0.18w/ but lower the better. The question is what’s the sweet spot for performance vs cost vs cavity size?

 

I’ve learnt DriTherm 37 is more cost effective to buy per sqm than the 32 slabs. Almost 50% cheaper online in some places. But when I run a calculation it shows I would need almost double the quantity to achieve the required performance. Kind of makes it the same then and not as cost effective as I’d read on here. Or is there something I’ve not factored in? Thanks.

 

Edited yesterday at 17:26 by allthatpebbledash
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[Mr Punter]

If you want decent insulation the Dritherm 32 are the ones to use.  In the great scheme of things will not make much difference.  Have you priced the actual difference in £?  150 cavity is simple as it works with digger bucket for foundations, standard wide lintels and standard long wall ties and standard cavity closers.  Quite why you would build in brick on the inner leaf I have no idea unless you want an exposed brick inside, which I would not recommend as it will cause big problems with services and airtightness.