I thought my experience with Building Control over my roof insulation might be helpful to others:
This was a full plans application and after some further information about window and door reveals and wall condensation risk assessments (solid wall + EWI) I got their approval. No question about the roof so I assumed that was all fine.

The roof was designed by a S.E but he did not specify the coverings and insulation. My build document to BC stated:
22mm Uditop sarking
200mm Omnifit Stud between rafters (200mm rafters)
70mm PIR to the underside of the rafters
VCL under PIR
Plasterboard under VCL.

Inspection was before VCL and plasterboard and went something like this:

BC said the construction didn’t match the (my) specification and because I hadn’t read it just before he arrived I just accepted this – first mistake.

BC: there will be a huge condensation risk of the wool sweating behind the sarking.

ME: there’s a VCL on the inside and the sarking is vapor permeable so where is the moisture going to come from?

BC: there should be a ventilated air space between the wool and sarking boards. There’s too much insulation! (this was clarified later in the context of “a ventilated air space roof” – 4 days later).

ME: there isn’t an air space! I have constructed it to the sarking board manufacturers recommendations

BC: I will need to see the manufacturers data sheet.

End of visit.

Emailed Uditop sarking board data sheets, the full plans build specification (which they had from day one) and a condensation risk analysis for the roof build-up.

Phone conversation with BC officer a couple of days later went something like this:

While BC was going through my build specification he was struggling to understand the written spec’. I explained the make-up of the roof and how it was as the manufacturers data sheet, and exactly as I had constructed it. [to be fair I didn’t draw the roof make-up and the written description was out of sequence, hence the confusion].

BC then looked at the manufacturers data sheet and immediately focused on a table talking about board thickness and maximum rafter spacing and said I was using more than 100mm of insulation. The table actually meant I could use 22mm boards up to a maximum rafter span of 850mm while 100mm boards could have a max spacing of 850mm! [I'm losing confidence!]

BC then moved to the construction examples in the data sheet and asked if I had used their proprietary insulation and I said no (I was tempted to say yes for an easier life because I knew what would follow). He also pointed out that the data sheet did not show PIR boards under the wool insulation

BC said he couldn't accept this because I have used a hybrid system that is different to the manufacturers data sheet. [manufacturers data sheet show their own insulation products but I had used omnifit as it's bit better insulator but has a similar vapour permeability. I had also included extra insulation below rafters which is not shown on the data sheet - I'm going down!!]

[I wanted to say they had the full plans 18 months ago so why wasn’t this questioned before I built it– but I held my tongue]

And finally…

I directed him to my condensation risk analysis and after I had talked through the numbers he was happy – to a degree.

BC said he would put copies in the folder and sign it off.

And finally I get an approval email:
I am willing to accept the details but must inform you that any defects likely to be caused by interstitial condensation in the future, will be at your own risk.

Any advice for the future?

Well done on getting approval.

I don't understand the CRA. The external temp of -1 °C seems way warmer than I would think prudent to allow for. The roof surface, whatever it is, should protect you against exposure to night sky temperature, but even sheltered air temperatures are likely to be lower. Where are you?

Posted By: ward32My logic being this would be equivalent to an internal temperature of 20DegC and an outside temperature of -18DegC (using existing weather model) - it would create the same vapour pressure driving force, but correct me if I'm wrong?

Equilibrium vapour pressure doesn't scale linearly with temperature; roughly speaking it doubles for each 10 °C increase. So you're playing safe in a way in that the vapour pressure difference you're using is greater but it hurts my head to think about what it's doing to the condensation risk on the way through.

Posted By: ward32I'm as confident as I can be that the structure will be free of interstitial condensation so on to the next job

good quote, I have decided to adopt it for *ME* !
LOL

gg