In article ,
The Natural Philosopher writes:

Thinking more about this, if you have the space, Id be tempted to try
and mount the stoff OFF the rafters altogether - to make a 'tent' of
insulation underneath, because I am concerned that condensation above
the insulation layer may in time affect the rafters: when you take a

I did this with a lean-to construction roof (over my bathroom).
I fixed the underside of the rafters with pressure treated roofing
battens at right angles, and then fixed the Kingspan to those (or in
most cases through those into the rafters. I provided a vent in the gable
end above the Kingspan, and the roofing felt looped down enough to
provide further ventilation. The rafters showed some signs of rot which
had been repaired (before my time, probably when slates were replaced),
and I wanted to make sure they stayed very well ventilated.
My worry was not condensation, but any water ingress through the roof
which I would likely not notice one the Kingspan was in place, and
without ventilation might lead to serious rotting of the rafters.

--
Andrew Gabriel
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