Insulation Thickness: When Does More Become Pointless?

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There will be diminishing returns of course, but at what thickness is it pointless doing more.

When we built our timber frame house in 1995, 100mm was very much the norm. We went for 150mm, but there was also talk of 'spaceframes' being 300mm or more thick.

At what thickness of insulation does reducing the U-value stop? I know there are variables, but generally? Only asking from interest.

Ta

CG
 
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If you compare 300mm Dritherm 37 loft insulation with 400mm of the same the u-value goes from 0.12 to 0.09. You could say a 25% increase in thermal value.
500mm takes you to 0.07.
600mm = 0.06
700mm = 0.05
800mm = 0.05
 
More insulation will always reduce heat loss, but after 50-100mm full cavity fill PIR, or 1-200mm of rockwool, the losses from other parts of the house, windows, drafts etc are much higher, so it's pointless adding thicker insulation.

Even when you've tried to address other losses then Passivehaus is probably the point where thicker makes no sense under any circumstances.
 
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It's not straight forward to think just in terms of insulation, as more heat is lost due to air leakages than through insulation to current standards. So if it's about time and money then it may pay to invest it in dealing with air leakage and other design options rather than additional insulation on it's own.
 

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