Insulating external solid wall

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any comments on the following plan?

I have a 9" solid wall to my house and have ample internal space so was planning to insulate. It's back to bare wall at the moment due other work in progress.

I was going to build a stud wall/ false wall about 25mm of existing with 4x2 and cavity fill with 100mm celotex.

Any other way to do it?
 
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So no air cavity? Ok I can do that was just concerned about condensation and damp issues.

I'll vapour seal the internal wall anyway before boarding out so I guess that should solve the issue.

Thanks woody star as usual!
 
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You can get plasterboard glued to celotex at a cost, then you can just stick it to the wall directly.
Although with that much insulation inside you're going to have a condensation risk whatever you do. That's why in roofs you always have a ventilated void or breathable membrane.
 
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That John I did look at that it's it's quite an expensive way to go.

I won't be able to vent the new cavity so hopefully a Vapour barrier will be enough.
 
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If you reduce to 50mm celotex it would be safer. The second 50mm doesn't make a big saving compared with the first anyway.
Celotex have a load of calculators for u value and condensation risk, use them.
 
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If you stop most air getting in, and cover cold surfaces with insulation, then there is no condensation risk.
 
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Only twice as much
:LOL:sorry I should have put more detail, yes the r value is double for the insulation so 5m²/w instead of 2.5 but his gas bill won't half because firstly the solid wall will add 0.5 to both so 5.5 instead of 3.
But actually the real difference would be from all the other elements that won't change eg windows and thermal bridges, not to mention ventilation losses.
You should always focus the extra insulation on the areas that are losing the most heat, and after 50mm insulation in the wall, that'll probably be the floor or ventilation.
Hope that makes sense!
 
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Ok so 4x2 stud wall tight to the brick wall filled with 100mm celotex overlaid with 10mm celotex sheets tapes at edges and 12.5mm plasterboard and skim.

Sounds like a solid plan thanks.
 
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I've just been insulating double solid brick walls, internally.

25x50mm battens, then 50mm celotex on top of battens, aluminium tape all joints, then plasterboard and skim.
Consensus seemed to be, to have an air gap behind boards.

It's time consuming in a old house, where walls are not flat.

Oh, and you need long fixings 70mm for insulation and 80mm for plasterboards.

I always wonder about the where the law of diminishing returns are with insulation, can't help wondering why fridges and freezers don't have insulation thicker than 40/50mm?
 
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I always wonder about the where the law of diminishing returns are with insulation, can't help wondering why fridges and freezers don't have insulation thicker than 40/50mm?
because there's an eu standard to meet, and that's the amount needed. Most people would compare fridges based on the usable space. It's a bit like the fact they specifically made vacuum cleaner to be as inefficient as possible (and noisy) because people compared vacuum cleaners based on their input power. Then the EU banned unnecessarily powerful cleaners and made the manufacturers include a label for how good they were, and suddenly they became a lot better. Same happened with light bulbs, now we have decent LEDs rather than those terrible compact fluorescents.
 

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