Hi Experts - Hope you can help with my next question. Very grateful for any advice.

 

Our extension is having 100mm insulation batons put as per building regs. I think that will mean the extension walls will have a U value of 0.24 W /m2k. (?)

 

My existing external walls are only 260 thick. 100mm Std Aircrete Block + 100mm Brick with 60mm cavity. We are looking at having Knauf Superfill 40 blown into the old walls (to blow out the now banned insulation in there which I don't think is doing anything). This should bring the U value to .38 W / m2k. I am then looking at installing 75mm savotherm laminated insulation board externally with render over the top of that. On my calcs that should provide a 0.21 W/m2k U value. 

 

After all that my question:

 

Should I be adding a VCL on the internal wall (with 25mm baton before plasterboard) to combat any potential damp issues? Or have I got this totally wrong? 

For what it's worth, ee did much the same to our 1960s bungalow: 

 

100mm brick, 60mm cavity filled with blown polystyrene beads, 100mm brick, 2 layers of Kingspan K5 50mm on the outside with staggered joins, rendered over with K-Rend.  

 

We put fresh 12mm plasterboard on the inside in preference to re-skimming the rather flaky old plaster.  No VCL fitted.

 

We have had no damp problems at all.  We do have whole house MVHR which helps prevent damp I believe.

 

I will probably take a similar approach on our next project, maybe with even more insulation on the outside, depending on the heat loss calcs.