In message , andrew
writes
I have heard that trapping a layer of relatively
static air can work to minimise condensation but at the risk of
encouraging rot in the wood:-(

This is my worry but I will try a section with tyvek, the difficult decision
is whether to spend the considerable extra time laying the material up to
down between the rafters, tight to the roof. With the hope any runoff will
be down the tyvex and out via the eaves.

When I had some slates off our roof I noticed that the roofing felt
(laid over the rafters) still sagged enough to cause potential puddling
where it lifted over the gutter board. Difficult to imagine doing this
as retrofit down near the eaves!

When we bought the yard opposite the farmhouse, we found lots of office
type suspended ceilings. T section steel hung on wires with 4' x 1'6"
insulated slabs between. Long gone now but a possible cheap source of
fireproof rigid insulation.

regards
--
Tim Lamb