2 x 75mm Rafters bolted together loft conversion?

[gasbusters]

Building control approved it with 75mm between and 50mm over.

They never actually checked it, just looked and said it was OK. I could have used nothing in between and 25mm over and they would have never known.



Are they recommending a full layer of insulation under the rafters as well?

Theyve said 120mm between rafters with 25mm/or plasterboard which has a layer of insulation, over rafters . Dont get why i need so much is needed

 

[Ian H]

That’s it a lot!!!

Personally i’d rather have thicker layer over the rafters rather than in between. You could probably calculate the performance of 120mm in/25mm over compared to 75mm in/75mm over or 100mm in/50mm over.

We have a wardrobe in the loft and I put a thinner insulation over the rafters in there and it’s colder for sure.

 

[Leofric]

Insulation suppliers like Celotex have data sheets showing ' U ' values for different configurations of insulation.