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Thinnest wall type with best U Value

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Hi Guys


I have a single storey structure at the rear of my house with an internal width of 2.2m.

I'm looking to extend on top of this structure.

I understand modern insulation standards require 100mm+ PIR.

I don't want to go down the route of double skin wall with cavity because it is going to eat in to my already very narrow space.


Which wall type will allow me to hit U values while taking up the least amount of space?

I was thinking timber frame


single brick wall with 100mm PIR and cladding on the outside.





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extensions have less strict regs applying so you can get away with 100mm cav  and mineral batts (cheapest). New build you can still use 100mm and celotex but this is a worse and mor expensive build solution than 150mm cav and mineral batts.

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As @Dave Jones says extensions are different, and you can often offset deficiencies in one area with increases in others..I think your renovating a terrace? So EWI is not an option? 

The mantra of insulate and improve air tightness still applies. Design the small space efficiently (built in furniture/ bespoke furniture etc) to see if you can shrink the area to accommodate more insulation. Will 150 / 200mm make that much difference?

Drawings and spec of the space usage may help with some ideas.



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