Rule of thumb ahead of SAP calc.

[Nakajo]

Ahead of commissioning a proper SAP calc, I'm trying to construct a rough rule-of-thumb, so that I can be reasonably sure I don't have to massively revise any details/wall thicknesses etc. for a new-build resi project.

The doors and windows I want to specify fail the target U-value by 25%, and represent approximately 27% of the exposed wall surface. Instinctively (and I accept that my instincts may be way off) this means that if I overprovide the walls by 10% of their target value, I'll hit my overall target:

Arithmetic: ( 0.27 x (1 - 0.25) ) + ( 1.1 * 0.73 ) = 1

Again instinctively, this means increasing insulation thickness by 10%.

I can see that I've made a series of very crude conjectures above, but are they too crude to be useful?

 

[freddiemercurystwin]

Sounds reasonable but does it really matter?

 

[Nakajo]

The project is in a conservation area, so I have to get the Planning Authority to sign-off on the proposed window profiles beforehand. I could swap non-compliant profiles with identical, part L compliant profiles, but at a massively increased cost!

For now, I'm just going to add 25mm to the walls, and hope that I can grab a bit of that back later (it's a small site, so every inch counts).

 

[freddiemercurystwin]

Why can't you just push the button the SAP's now?

 

[Nakajo]

Fair point. I think I will!