Adding EWI to existing cavity wall containing rock wool

[Jilly]

We have gone with the old regs and a cavity wall containing 100mm rock wool. However, we have to clad externally with feather boarding so have room for external wall insulation (EWI), and could fit about 70mm PIR. I'm thinking this is a good thing. Are there any reasons not to do this? Is it likely to cause interstitial condensation, or is that more likely with internal wall insulation (IWI) ?

 

 

Edited November 2, 2022 by Jilly

[Redbeard]

I.C is generally more likely with IWI. PIR is not used so much for EWI, although I know some have used it. Have you thought of using 70mm graphite EPS?

 

The issue you could have is condensation on the inside of the outer skin, as you will have cooled it down with the 100 Rockwool.

 

What you could end up with is the wall equivalent of a Hybrid Warm Roof (some insulation between the timbers and some on the outside). The rough rule-of-thumb with hybrid warm roofs is that 66% of the R value should be on the outside and 33% between the timbers.  Your 100 Rockwool and 70 gEPS would not cut this particular mustard.