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?
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?