Mixing airtight and breathable

[Jilly]

I'm doing a stable conversion so can't be a purist.

 

The roof has to be a cold vaulted ceiling with a steel ridge beam (which will have to be independently insulated (methods suggestions welcome), so needs a VCL to stop condensation from the kitchen. 

 

However, the external walls (140mm block) have 70mm Celotex EWI with breathable membrane. (It's sufficient for building regs 0.22 W/m2K)

 

I'd like to put a bit of hippy breathable internal wall insulation, I'm thinking 25mm cork board. All of this is sold to go with lime plasters, but will it be ok with gypsum plasterboard as no one seems keen to use lime at a sensible cost?

[Cpd]

I can’t see the point putting breathable insulation on the inside if the outside in Calotex….. 

if your concerned about off gassing then make sure your vcl is fully taped and sealed. 
what’s the point in using cork if you then cover it with a vcl and then plasterboard ?

just trying to find the logic in your suggestion, not trying to shoot your idea down 

[Jilly]

Yes, I am worried about off gassing, I've got asthma.

I could just have the external walls plastered instead but was trying to make the walls warmer feeling.

I wasn't going to put a vcl on the wall (only on the vaulted ceiling). Is that daft...?

 

 

[Cpd]

The idea of a vcl(I think)  Is to prevent any moisture created inside the house getting trapped inside the building fabric and potentiality causing problems.

it would be normal to have the vcl all the way from the ground floor up and linked into the ceiling vcl. 
But I am unsure how it works when your insulation is on the outside but I am sure someone will know !

 

I guess e question would be how breathable will the actual wall build up be from the inside and will it actually perform as predicted as it can only breath into the house. 

 

Paint

plasterboard

cork insulation 

block work 

external insulation 

something else ! 

 

It’s all above my pay grade but I think you need to have a clear understanding of how the wall will perform when you have two layers of insulation sandwiching a block work wall in. 
Maybe you can use the cork but then stick a vcl up behind the plaster board…. It costs very little and should be easy. But then if you have a vcl would this not prevent off gassing from 25mm of calotex which would be cheaper and easier to instal maybe…. 
 

sorry not much help but maybe some questions to follow up. 

 

 

 

 

 

 

 

 

[jfb]

I might be wrong but I think minimum U-value for domestic is 0.16. For non domestic it is 0.22. These figures are for new build, not renovation but I believe a barn conversion is effectively a new build - but I could be wrong.

can you not just go for 100mm external insulation? No need for internal insulation then. Then either render straight onto the wall or vcl/ battens for service void / plasterboard.