Foil insulation in roof?

[Jack07]

My BCO has asked for 150mm celotex insulation in my vaulted porch roof with 50mm air gap.

Would there be any benefit in tacking up foil insulation before the plasterboard? I know the foil will be compressed but is there any need for me to be concerned over potential bridging of cold through timbers to PB?

Only asking as a friend of mine in another county was told by his BCO to Insulate between the timbers and PB.

I could always fix insulated PB instead of regular

Thanks in advance

 

[^woody^]

You should insulate between and either over or under the rafters.
How much of each, depends on the type of insulation.

If you decide to use two different insulation products, you may likely have to justify the thicknesses by a calculation.

 

[Mr Chibs]

In my vaulted ceiling I was told 100mm in between (although I think they have increased it now) and 50mm below. And no air gap.
I have left a gap, as when I read up, it seems to be a good idea.

 

[noseall]

In my vaulted ceiling I was told 100mm in between (although I think they have increased it now) and 50mm below. And no air gap.
I have left a gap, as when I read up, it seems to be a good idea.

If the gap can be vented, then you should leave an air gap on the cold side.