Andre90

This is the exact issue I have. I was thinking to cut the 40mm PIR from the inside and render it useless (for airflow) and then put 200mm PIR in between the rafters My rafters are 200mm deep but because the PIR has been added on top with an extra batten, that would leave more than 50mm for airflow. its such a waste of insulation though.

I realise this myself now hence why I am trying to find a solution to the mistake that has been made. Since whats been created is a hybrid roof, does anyone have any knowledge of what would happen if I was to put 150mm in between the rafters? Aparently its fine to match the insulation of the warm deck in between the rafters, so me putting 40mm in between is perfectly ok. Only problem is that would only achieve a U value of 0.25wm2 To put it simply the mistake thats been made is the 1/3 to 2/3 rule has been done the other way round.

so you are saying to have external and internal PIR as well as in between the rafters?

Well my wonderful builder has built it the way he has now (40mm PIR above rafters). And to be fair I dont blame him, when he asked about insulation BC always would say "refer to SAP" so he got used to just following SAP as that is what building control was interested in. I didn't question it because I didn't know any better. My question is, if I put 150mm PIR between the rafters as well as the 40mm thats above (hybrid roof), will a VCL work?

I have attached the sap report but also an online calculator which doesnt let me implement the buildup in the SAP report. My main concern is condensation/rot. As this doesn't conform with the 2/3 external and 1/3 internal rule

Hi everyone, My name is Andre and I’ve signed up as I’m in the process of a new build house. The new building regs are driving me mad and I’m the go between all parties, BC, architect, SAP calc guy, builder and engineer. Naturally it’s in my interest for everything to go right but my experience has been minimal thus far. Hoping to get some advice on issues I’m facing. The biggest one so far is that they’ve built the pitched roof with a hybrid method. The SAP guy had put 40mm on top of three rafters and 150mm in between. So the builders followed this. I’ve come to find out that’s it’s meant to be the other way round. The council accepted the SAP calculations and I didn’t think much of it. It’s only when I researched something else that I came across the 1/3 - 2/3 rule and then dug deep into the rabbit hole. It’s really stressing me out. How can I remedy the situation without ripping the whole roof down? I will naturally put a vapour control layer below the rafters. Will this be enough to correct the mistake? I don’t understand how the PIR in between the rafters being 150mm is an issue but 40mm is fine. The 40mm PIR above the rafters surely will keep these rafters warm enough for condensation not to occur? any help would be much appreciated