I agree that painting the white "stuff" will help keep it from leaving crumbles. I saw a ceiling like this in a church. They painted the "stuff" sky blue and left the rafters wood color. Their "stuff" had a matte finish (probably because the "stuff" soaked up the paint.) I have also seen this kind of ceiling where the "stuff" was covered by tongue-in-groove boards, left natural. I wonder if those bamboo window shades would fit between your rafters. You would have to buy a lot of them. It would certainly give the ceiling some beach "vibes." I wonder if the Habitat for Humanity ReStore might have some? Check places that sell builder's overstock. You might be surprised what is available for a reasonable price. Best wishes.

HU-348388819 is yours unfinished chipboard as appears to be in the original poster's pics, or is it something else? There were some interesting materials used in ceilings of that era which could be worth preserving. I've seen them mentioned in sales listings as valuable original features, though the names of the exact materials escape me at the moment. On the other hand, with any unfamiliar materials of that era you need to check if it contains asbestos. Another thing to consider is your climate, as the insulation is likely to be inadequate in a ceiling with exposed beams.

Cloudyhaze....I think all of the options look good, but it just depends on the look you personally wish to have for your house, because painting the ceiling white will COMPLETELY change the look. My sister has just bought a place that had brick walls and exposed ceilings that had the beams painted mission brown and the ceiling was in some form of raw chipboard type material. She wanted to brighten the place up before she moved in and got it all painted white. It looks beautiful, but I personally would have thought of other options before painting the brick etc. She got a quote for plastering (sheeting) the sections in between the exposed ceiling beams and it was a lot more expensive than painting. If you are keen to paint just the panelling in the ceiling, then I would get additional quotes. I am no painter but I would suspect if it has shellac on it, it will just need an initial sealer on it, before putting on the 2 coats of flat ceiling paint. I think all of the options will look beautiful, because the high ceilings are the feature.........it just depends on whether you are going for a more rustic look, mid-century look , more modern look...I am not sure of what all the designs are called. Good Luck. Your house is gorgeous. My sister lives on Gold Coast BTW.

You never mentioned your budget that you're willing to spend. You could add a thicker fill backed insulation blanket - 50mm, 75mm or 100mm are the common thicknesses. Steep roof pitch so would need scaffolding, harnesses, etc.. which adds up quickly. Probably still not enough insulation though but probably cheapest option. Insulation under existing timber linings then reline over the top of that as some have suggested. Working at height though so would need scaffold set up again and working overhead is not fun nor easy. You could replace the roof with a sandwich panel roof. Try Versiclad or Kingspan or comparable. They have a polystyrene type of insulation between an outer metal roof sheet and a metal inner sheet. These are heat laminated together. Again different thickness available from 50mm up to 180mm is common. The 180mm has a R rating of 4.7 which is quite good. Again steep pitch, etc., probably need new ridge capping, etc.. Probably most expensive option...... Keep us all posted on your decision though!