I have a height restriction so i am going to have a pitched roof to flat crown roof at the top. The roof pitch is only 25 degrees. I like a warm roof, but it's just not going to happen. It's going to be a pain to insulate the loft due to the total height of the truss being about 1200mm internally. So the roof is hipped on all sides rising to the crown roof in the middle. I will provide a 50mm airgap up the rafter to above the height of the internal, floor insulation, which will either be fluffy stuff, or blown in insulation. There will also be continuous ventilation equivalent in the soffit of 50mm, and i may even put some ventilation over the facia. I will also ensure that the breather membrane will be a nice air leaky one. 

My question is do i need to try and do anything in relation to the underside of the flat roof section ? Once the trusses are on, i will use firing pieces above the truss timbers in order to provide a fall on all 4 sides, onto the respective pitch roof sections. The firings will then have 18mm OSB3 on top, and epdm glued on top of this. 

So providing i can get enough air up into the loft from all 4 sides of the roof will i need to do anything to the underside of the OSB.

I would have thought that with enough ventilation (that you do propose) it will be ok. I will say however I have come across cold lofts in the past with lots of condensation due to warm moist air escaping from within the house, so make sure your ceilings are well sealed. As you said it’s going to be a bugger to insulate tho 🤯. Could you not insulate with fluffy stuff just before you membrane and batten?

Edited August 22, 20233 yr by joe90

@joe90 I could try. Im waiting for a quote for blown in insulation. One problem is i'm going to have a load of MVHR ducts up there, so i would rather they were under the insulation.