Adding extra (internal) wall insulation

[Jilly]

My stable conversion is built as 140mm block and we have plans accepted by BC for external (PIR) insulation on 70mm (U value 0.23 W/m2K on BC drawings). This has already been installed by John Wayne and friends. 

 

I'm thinking about adding extra wall insulation, my options/concerns are:

 

Interstitial condensation. How do I mitigate?

 

I have some spare sheeps wool insulation (think the builder has got the wrong stuff as it looks very compressed in the bags) can it be compressed and used by battening against the walls, then plaster board? Need to do something to prevent sagging.

 

Other option is insulation backed plasterboard, we weren't going for airtightness, except in the pitched roof, but maybe I need to think about it and explore the whole MHVR thing..

 

Or just proper plaster rather than dot and dab will be more airtight?

 

Clay plaster looks great, am I being bonkers here?

 

I'm wondering if the walls would act like a big thermal store. Will IWI stop this benefit? Does it matter? Will nice warm walls override that concern?

 

Awaiting Steamy Tea's physics lesson...?

Edited May 28, 2021 by Jilly

[A_L]

49 minutes ago, Jilly said:

external (PIR) insulation on 70mm (U value 0.23 W/m2K on BC drawings)

 

What block specifically as this is quite low. Typical lightweight aggregate block gives about 0.26

 

53 minutes ago, Jilly said:

Interstitial condensation. How do I mitigate?

 

Up to 50mm of fibrous insulation would be OK. A polythene VCL or foil backed plasterboard would add reassurance.

 

57 minutes ago, Jilly said:

can it be compressed and used by battening against the walls,

 

Compressing insulation destroys its properties.

[Jilly]

Edited May 28, 2021 by Jilly

[Jilly]

Above is what it says on the BC drawings, however, the blocks seem like pretty heavy concrete to me, not very 'lite' or airy. 

[A_L]

@Jilly , K5 is a phenolic insulation with slightly better insulation characteristics than PIR. With Thermalite as described the U-value is correct, even slightly conservative.