Aerogel is well expensive, any breathable insulation will let moisture through to the masonry and it will condense on the plaster, I wouldn’t use wood-fibre if moisture was a worry.

I can’t see how any of this would meet building regulations and it seems it should.

Bay windows are difficult, would you consider having vapour barriers?

We have used Aerogel window reveal lining boards e.g.
https://www.insulationsuperstore.co.uk/product/spacetherm-wl-wall-liner-aerogel-insulation-7-2m2-pack.html?gclid=CjwKCAiA55mPBhBOEiwANmzoQkkUi8ZHlfVo9j6UAqavsn4X4HBMGMkGzLFRG72JwwPHZo8rtC08HRoCJVcQAvD_BwE

to line the reveals on the ventilation slits in our barn conversion (now fitted with glazed units to provide very small windows). There wasn't much space so this product was a good solution to keep these and make a feature of them. if you were too obsessed then you wouldn't have bothered keeping these but we think they add interest to a conversion scheme where there wasn't a lot else of interest that could be kept and made a feature of.

It's early days yet because all the plastering and decorating isn't finished yet, but they do tend to be condensation magnets when there's a lot of moisture around so they are obviously the coldest place. However when its drier they seem okay and once the MVHR is up and running and the place is finished and dried out I'm hoping that they will be just fine.

The stuff we used blunts your tools when cutting it by the way. I bought a pack of 25 jigsaw blades to cut it up and only got about 3 or 4 cuts before the teeth were blunt (as in actually worn away as opposed to just dulled). The dust is superfine and desiccant, I cut it outdoors after the first cut!

Thanks all for your comments,

Dominic, the windows look great. I think I would have made the same choice to keep them! Interesting that you say its a condensation magnet, have you got pretty high u-values elsewhere? I hope they do work fine once the rest of the work is complete - you'll have to let me know. Thanks for the tips on cutting them as well, I actually thought they looked like they would be a lot easier to install than the wood fibre boards, but it seems perhaps not!

Will, thanks, I did read that actually (I think you might have posted it on a different discussion!) And there was no mention of damp, which is hopefully positive. Thanks for the link.

tony, moisture isn't really an issue at the moment. We get a lot of condensation but we have single glazed windows! In terms of regs, it'll depend what we can get away with using internal wall insulation. We obviously dont want to go too thick to risk interstitial condensation. I have contacted a few suppliers of the wood fibre to see what they recommend, but general advice seems to be 60-80mm. As well as that its a balance between retaining as many features as possible and getting the best thermal results.