Work on the roof

Now the velux are in we are still waiting for the roofer to come and fix some leaks (he says soon for that) as well as install two new A frames to support the roof when we remove the stairs structure (not until December or January he says).
Despite that we are able to get on with working towards getting the insulation done. The first thing to do is to remove all the old and crumbly bits of wood where past damp or insect damage has make the outside crumbly. We are doing this with a wire brush on the angle grinder (by 'we' I obviously mean Doug - I have been driving around buying masks and overalls and sprayers as well as emptying out the trailer which was full still of the wall from upstairs).

Crumbly surface and loose bark need removing

Capricorn beetle damage. Look at the size of the holes! They
can eat a piece of wood away quite quickly - apparently they
don't eat the hard core of the wood - so quickly anyway.

Once this is finished (in a week or two) we need to spray the beams with an evil concoction called Xylothene and then we will be ready to install the insulation.
We have finally settled on wood fiber insulation and ordered it. It was a hard decision as it is more expensive than the alternatives. However we decided that in view of how hot the upstairs gets with the black slates sucking the heat in, we would splash out for it to keep the bedrooms cool. Wood fiber is meant to have a good 'decrement delay'. This means that even though it has similar insulation qualities according to common measures (eg u value) as fiber glass or rock wool, per thickness, the heat in summer is slower to get from one side to the other. This is related to thermal mass and how stone walls keep the inside of the house cool. It is also meant to cope with getting damp well and continue to insulate - this is bound to happen in windy rain storms.
It will be interesting to see how it works out! Very important we install it well with not a single tiny gap to get the benefit.
We have been reading about how to install the insulation once we get it. The general idea is to attach it to our rafters and then use a metal framework in front of it to attach plaster board to. In the UK I think this is often done with wood but in France they mostly use metal. In the systems we prefer the metal is attached to the rafters with plastic to avoid thermal bridging.
We have also been reading about vapour and air barriers. Riveting stuff. Our current plan is to install the plasterboard carefully with tape and airtight seals everywhere something pierces it (eg wires, beams, walls) and use it as an air barrier. So it will stop warm damp air from the house from getting out in to the insulation and roof space where it can cause beams and wood to rot over time. The roof is also well ventilated with the vents we have had installed so if damp does get in it should dry. The final part then is that we will probably paint with a vapour barrier paint to stop any damp in the internal air soaking through the plasterboard. We may not bother with this though given the ventilation above the insulation.
We spent some time fretting about whether we should have a vapour permeable but air tight-ish layer outside the insulation - like you wear a windproof over a wool jumper, but we decided it was unnecessary as wood fiber is reasonably air tight itself. More like a windstopper fleece perhaps!

We are very excited as it feels like we have really started work now. It may get worse before it gets better... dust EVERYWHERE is fun.