In article , The Natural
Philosopher writes
wrote:
Having looked at Kingspan TP10 literature it seems to suggest that it
is rafter level insulation with no condensation risk - does this mean
no need for a gap and the ability to put in a thicker depth of
insulation or is 50mm enough? Is TP10 the best stuff for between
rafters insulation? Thanks.

I think what the condensation bit means is that it is foil covered, and
impermeable. It doesn't mean that you don't need a gap.

You need to ventilate those rafters for sure. The air above will be wet
in humid weather, and at night you may well get condensation running
down the top surface as the temp goes below dew point. That has to be
allowed to run away down to the eaves and the residue evaporate.

Polyisocyanurate is the best commercial material for insulation there
is. Kingspan and celotex ate the current dogs ********.


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
roof that expects heat to bleed from below to help keep it dry, and then
seal that heat off..things change. And you have no sarking..so its
slightly unusual.

Although it does take up more space I agree with your last suggestion,
if the rafters do get wet for any reason and there is material between
them you will have damp timber in an enclosed trench which will have no
opportunity to dry out, potentially leading to rot problems.
--
fred
Plusnet - I hope you like vanilla