On Fri, 03 Apr 2009 01:14:27 +0100, Tim S wrote:

As I understand it, sarking is for windproofing, not water proofing a
tiled roof?

No it's secondary waterproofing to guard against broken/slipped tiles or
heavy rain/wind blowing water in between the tiles. Don't under estimate
the power of the wind to blow water up hill...

Is cellotex/kingspan a closed cell foam? Will it absorb moisture if used
as you suggest? Remember youi can only get at the face of any joins to
seal them against wet as well. To save stripping the roof to replace the
sarking I guess you could put a membrane over the rafters tape any joins
then the insulation on top. Remembering to get overlap between rows of
membrane the right way round...

--
Cheers
Dave.