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. 

[Mr Punter]

A more important factor will be the loss in floor area, which if it is 1.9% on a floor area of 160m2 would be about 3m2, which could be worth between £6,000 and £12,000 depending on location.

[JohnMo]

1 hour ago, Mr Punter said:

A more important factor will be the loss in floor area, which if it is 1.9% on a floor area of 160m2 would be about 3m2, which could be worth between £6,000 and £12,000 depending on location.

Add it the outside if you wanted. Do people in the UK buy by the ft² or m², thought they bought by number of bedrooms and that was about it. Hence big builders building shoebox sized houses with loads of bedrooms.

[Oz07]

1 hour ago, Mr Punter said:

A more important factor will be the loss in floor area, which if it is 1.9% on a floor area of 160m2 would be about 3m2, which could be worth between £6,000 and £12,000 depending on location.

I wouldn't be selling it as for my home but I just can't see the justification for the wider cavity. 50 year payback is insane. Also 3m2 is enough for a little ensuite or cloakroom, it adds up. 

[Oz07]

1 minute ago, JohnMo said:

Add it the outside if you wanted. Do people in the UK buy by the ft² or m², thought they bought by number of bedrooms and that was about it. Hence big builders building shoebox sized houses with loads of bedrooms.

Generally valued per sq ft or m. Round my way approx 3500 m2.

 

More important are the numbers though would it really make such little difference like 20 quid a year in energy?

[DannyT]

Anyone interested in 200mm cavity wall ties, some for sale in the market place on here 😉

Ive had to reduce down to a 150mm cavity due to foundations already been in so can’t build any closer to edge of concrete to pass building control.

disappointed but I’ll ad 25mm PIR to the inside to gain a little back.
 

[newbridgewolf]

We have a 200mm cavity brick and block build in progress. Before you committ, work out how you are going to source 200mm batt insulation, wall ties, insulated lintels, cavity closures, and so on. Although there are more  products for 200mm cavities now than ever, several items are still special order leading to delays and costs you might not be prepared to wait or pay for. We have had lots of issues with these issues and would have built quicker had we gone for 150mm cavity. 

[Oz07]

On 11/02/2026 at 19:15, DannyT said:

Anyone interested in 200mm cavity wall ties, some for sale in the market place on here 😉

Ive had to reduce down to a 150mm cavity due to foundations already been in so can’t build any closer to edge of concrete to pass building control.

disappointed but I’ll ad 25mm PIR to the inside to gain a little back.
 

Have you done the sums whether its worth doing the pir?

[JohnMo]

Or don't do a cavity at all, and do externally or internally insulated.

[Oz07]

29 minutes ago, JohnMo said:

Or don't do a cavity at all, and do externally or internally insulated.

Cavity has its benefits. Very mainstream when it comes to finance, insurance and local skills. It's not always the best solution, up your way TF is more common.