How best to insulate the roof on an old farmhouse

Hello,
Could anyone tell me the best insulation for an old farmhouse with a lot of beams!
Thank you very much as everything on the site is dated quite far back and I would love any new info if at all possible!
Best,
Lucy

Are you putting on a new roof, or is this internal?

We are going to ‘raise’ the existing roof two foot so we dont have to loose all the old beams, does that make sense?
Thank you for replying!

Roof spaces/attics, ceilings etc?. Do you still want to see some of the beams, are you looking for “flat” finishes or something mors rustic? What’s the roof like?

Sorry, bit of a crossover. Big job raising the roof, at least it should be nice and flat when done. Breathable membrane over the joists, make sure the lower ends are sealed to prevent pests getting in. Rockwool between the joists, multilayer insulation under that fixed with vertical battens to the hoists and then plasterboarded to a suitable framework (i always find a wooden one works best in roof spaces) with an a ir space. Do the first fix wiring before plasterboarding. How do you intend to raise the roof half a metre ?

Are there 2 of you ?, see there appears to be a maura chambers too!

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I think multilayer insulation is not that efficient, and the aging of these materials is not known really. However, if you use it together with conventional rock or glass wool, I would say it is best to use it as a screen first below the roof, where it can reflect infrared of this potentially very hot surface (as a reflective surface - opposed to an air cell material like wool - it is more efficient when the temperature gradient is larger).
From a few experiences, the best is when you can afford to devote the attic to insulation. In warm regions, you will likely experience hot rooms otherwise.

Fitted properly so it seals the space from drafts multifoil is very effective as a radiant barrier to reflect the heat out in summer. Sealing it to the walls properly is essential to prevent heat loss or gain. But even the very thick 30+ layers does not do as much as thick rockwool but together they are great, so they dont require so much space 150-200mm of rockwool + 30 layer multifoil with air gaps and a vapour permeable membrane under the counterbattened vented tile roof works really well on ours and shows enough beams to keep it interesting.
Longevity of multifoil? Not an issue, left test pieces out in all weathers for 15 -16 years before I used it, no degradation from the aluminium or plasticised aluminium types.

Degradation of the aluminium foil may not be evident by visual inspection. The efficiency relies on reflectivity, which is highly dependent on the surface state, and even tarnishing of the metallic mirror by dirt may affect it.

Well in short it didnt happen, just as bright as the day I nailed it to a board and left it out in full sun which of course it never gets exposed to.

Thinking how much energy you would have saved if you had used it 16 years earlier😉

Hahaha, so many people said it would age badly, fall apart, oxidise and tarnish. But none of them actually tested them. It took a while for the re-roof project and being a holiday home very little if anything was wasted except some firewood in winter. Now if it had been a main house yes absolutely, its why I am so outspoken on such things.