Thermal Bridging of an Insulated Flat Roof Through the Front Fascia

Dear Steven ‘Steve Roofer’,

I have watched a few of your videos in regards to flat ‘warm roofs’ on YouTube and have found them to be of great guidance with my current new build extension.

Accordingly, I would be most grateful if you would answer a couple of questions that I have regarding your video posted on YouTube entitled ‘Thermal Bridging of a insulated flat roof through the front fascia’.

Firstly, you emphasise the importance of installing insulation between the fascia board, stating that this area is often overlooked, but how much insulation should be laid from the fascia board going inwards?

Secondly, if I had a single PIR board on top of my decking would I be able to place 100mm knauf wool in the fascia area (and only in the fascia area) – combining two different types of insulation for two different areas? If so, how far inwards should I go from the fascia board?

Lastly, what level of PIR insulation would be sufficient for the warm roof with your structure above the decking? When I say ‘sufficient’ I mean in terms of (a) the minimum depth required for building regulations for new build warm roof type extensions and (b) an optimal level.
There is an awful lot of literature online about U-values and different types of roof structures but very little clear and conclusive guidance on the types and possible combinations that can be used to achieve minimum building regulations and at what depths. My proposed roof structure is the same as the one in your above mentioned video.

At the moment I have been told that 150mm Kingspan on top of my decking would be sufficient but there are not many retailers (other than online retailers) that sell 150mm Kingspan – suggesting that this depth is not commonly used for your type of warm roof.

I appreciate that you must receive many emails like this, receiving no benefit to yourself. I would greatly appreciate your expertise on this.

Kind regards,