What U-value are you aiming at?

Best bang for your buck with insulation is glass/rockwool/ecowool containing recycled material but that would increase thickness.

Do you mean 4x2 and 5x2 rafters?

Cheers Gents

No the rafters are a mix of 4x2 and 4x5, its an oast house roof from around 1907. The room sizes are pretty generous and losing a bit of space is not an issue there are already some 4x4 purlins which makes adding the board under the rafters pretty straightforward and no loss of space in practical terms.
The building is listed and council want a proven breathable system on the masonry walls if i’m to do any insulation, thicknesses in these rooms is a bit more critical and will end up somewhere between 60 and 120 mm. To be frank if I wasn’t doing it for myself i’d likely just insulate the roof spaces and forget the rest, instead,putting the cash into the wow factor for prospective buyers.
The specs i’ve had suggested bring the insulation levels upto that would be required bty building control in a normal refurb ( though as its listed i could do next to nothing) , however all being well i’m here for the longterm and reasonable levels (ie close to those required by building regs) are where i’m aiming for but there will be more than a few areas where it’ll not be practicable, nothing can be added to the exterior, and some rooms are too small/ odd shaped to make thick iwi work whilst retaining the buildings history.
Double glazing has been refused outright, so there’ll be secondary glazing where it can be made reasonably unobtrusive.

Its a square oast.Managed to get the cowl up before xmas, it’s been in the way for months since i finished it. More than a few delays getting the oast stripped and reslated and then finding someone to do the leadwork. Apologies the earlier explanation was a bit confused, the roof structure is/will be ventilated (min 50mm air gap ventilation at low and high level) and so in that area there is no need for the insulation itself to be breathable, the breathability is required in respect of the masonry walls, the suspended ground floors and eventually the ground floor in what was the barn the oast is attached to.

Posted By: Artigliothe roof structure is/will be ventilated (min 50mm air gap ventilation at low and high level) and so in that area there is no need for the insulation itself to be breathable, the breathability is required in respect of the masonry walls,

OK roof sorted - How do you get breathability on the masonry walls if as is usual you put a VCL on the warm side of the insulation? Never mind Celotex, Kingspan or other XPS as the insulation.

As per djh , the masonry spec is using woodfibre bonded internally and finished with naumit plasters, no vcl in these areas, though a breathable water repellant treatment has been recommended for the outside surfaces, apparently its not noticeable once dried ( if thats the route i take a test area will be done first), expect its similar to a “masonry water repellant” i used on some old weathered chimney stacks i repaired /repointed, 4 years on they look fine.
Suspended floors at ground level are another area to look at, designer would prefer to backfill and use asome sort of foam glass based system. The underfloor voids are currently poorly ventilated (older end has no ventilation but a 30” void under the joists, though floor has been replaced at some point as old timbers had rotted away, in newer end (1930’s void is only 7” but does have some ventilation) everything seems sound.

Posted By: Artiglioa breathable water repellant treatment has been recommended for the outside surfaces, apparently its not noticeable once dried

I can vouch for that. I used some on the clay-paint-on-lime-plaster in our showers and it works.

Posted By: ArtiglioSuspended floors at ground level are another area to look at, designer would prefer to backfill and use asome sort of foam glass based system. The underfloor voids are currently poorly ventilated (older end has no ventilation but a 30” void under the joists, though floor has been replaced at some point as old timbers had rotted away, in newer end (1930’s void is only 7” but does have some ventilation) everything seems sound.

The biggest problem with insulating a suspended floor is the belly ache of getting all the floorboards up. If your floors aren't original I guess that's not such a problem. I've only got a few of my boards up and even though it was necessary it felt like total vandalism. I was as careful as I could be and I hated every moment of it.