IWI and Condensation (PIR and Studwork)

[noseall]

ah OK, so you have stud work fixed to the wall, PIR friction fitted between them then battens fitted on top of the PIR for a service voice, and thin PIR around cabling.

RE polythene, do you install this on the cold side of the insulation i.e. on the wall or the warm side (under the battens).

No. The battens go on the outside of the first 50mm layer and are fixed through. It just ensures continuity of insulation, tight against the wall.

 

[mrrusty]

but we want the flexibility to be able to fill is retrospectively

I would seriously think about this. When we IWI our small cavity wall, I discussed with our BCO about his view on interstitial condensation. We both came to the conclusion that when it occurred the ventilation in the cavity would evaporate it. Our cavity is very well ventilated. And so it has proved - we have had no issues.

Early cavity walls were designed to be ventilated, and the original purpose was to keep the inner skin, and hence the ends of joists dry and rot free. The cavity was not there initially for insulation.

 

[mrrusty]

The beauty with separates is the joints can be staggered. There have been instances where the flex or memory of the bound boards has cause fine cracking, even with scrim. We now use the 100mm orange scrim on those boards and plenty of mechanical fixings (post dab).

We haven't had any issues, but I take the point. I would certainly take your suggestions on board (!) next time I do something similar. always interesting to hear thoughts from the more experienced people...