Hello Beacon, sounds like a nice project, but 150mm wool insulation doesn't come close to meeting the legal minimum in England or Scotland. Best practice would be 400-500mm of wool, or thinner layer of polystyrene or polyurethane sheet insulation.

You can buy 'loft legs' like stilts to raise the boards up above the insulation, or make your own.

If the loft floor is stiff enough for your taste (not too bouncy) then personally I wouldn't add extra joists, which are extra weight and thermal bridges and a nuisance to get up the ladder. Victorian roof timbers were smaller than modern standards but have stood the test of time. If you did want to strengthen the existing, then the new joists would need to tie into the roof/wall structure at both ends, not just rest on the existing.

My experience was the wool comes compressed in the packaging and didn't 'fluff up' as thick as was printed on the outside, so go for more of it rather than less. It can be encouraged by hand but then sags over time.

Once the loft is boarded and full of stored stuff, it is very difficult to add more insulation, so do a good job of it just once!

Don't forget to fix draughts in the ceilings while there's still access, round the hatch and the lights and where cables come up through internal walls. Also leave good ventilation gap under the slates where the insulation goes into the eaves, and insulate the pipes and tanks.

You've presumably thought about whether to insulate the floor of the loft, or to insulate under the roof rafters incase you subsequently extend into the loft space.

Edit to add: the work is easier if there is plenty of ventilation and lighting in the loft, both will be needed long-term, so worth sorting out early!

For me min 300 quilt 50mm ventilation space , new deck above that

Personal advice is don’t store stuff in the loft🙂

''How much space would be best to leave between the eaves for ventilation? Is it best to taper the insualtion parallel to the roof by the eaves?''

Min 25mm, and yes, taper it. I appreciate you wanting to use breathable materials - does that include not wanting to use petrochemical insulants? I ask because you could cut wedges of, say, graphite expanded polystyrene (EPS), with a better lambda value, for use at 'point' of the eaves.

Interesting point. It is behind the same layer of plasterboard that, for example, 125mm of PIR would be on a sloping room-in-roof ceiling, so I cannot see how that's different, but apart from that comparison I cannot give a definitive answer.

I have 500mm of loft insulation 200 between 300 crossways, 45x47 posts to top of insulation at 600c/c with 50x47 rails and a deck allowing 50mm ventilation