150 vs 200mm cavity.

[Oz07]

Was weighing up this choice recently leaning towards 200mm cavity. I've built last 2 houses at 150mm. I plumbed the numbers into Google AI just to see what it thought on a 8x11m box 2 storey house. If the info it gave me back is right then I'd probably stick at 150mm. I thought it wouldn't be too much dearer but I think things will add up. I do remember someone @Mr Punter? Saying it was best to internally insulate the additional 50mm to keep everything more standard, footing width, ties, closers, lintels etc. The energy savings just don't seem worth it 50+ year payback and if you are constrained on overall size of dwelling your losing 2 or 3% in net floorspace. Anyone else make this decision and why?

 

[JohnMo]

Your form factor is good, it's a rectangular box. So U value effect is different from a poor form factor (like our house).

 

I would look at a slight bigger scale than just the walls, so floor windows/doors and roof. Get a bigger picture.

Note: Passivhaus required U value changes with form factor.

[Oz07]

Yeh i think my last place had poor form factor it was a T shape and a bungalow. Also the cold bridge at truss to wall wasn't great as didn't go with eco trusses so insulation was quite tight here. Air tightness was great but mvhr was over ventilating I did diy commission then paid for someone reccomend by bpc to commission, they were clueless. 

 

Anyway forgetting windows for now because I'm just looking at whether its worth increasing insulation width. I don't think it is.