Garden Room Warm Roof

[SpaceWorm]

Alright ladies,

I am planning a garden office build with a flat roof within 1m of a boundary and want to avoid a cold roof and the issues that come with them, especially as I want downlights. I have more than enough 50mm PIR board that I have pulled out of my roof (crap, non-compliant loft room done by the previous owner) and also want to maximise internal headroom without breaching the 2.5m rule.

I am proposing to construct a warm roof, with some of the insulation between the joists, as per the detail below. Does this detail work?

 

[John D v2.0]

Looks ok to me, that's called a hybrid flat roof.
Garden Room won't be particularly humid and the top of the joists won't be particularly cold.

 

[Notch7]

Use foil tape around the joists to continue the insulation foil face - that will act as a VCL, which will help as it’s hard to seal around the downlighters