Insulating a sloping loft roof

[Makhaira]

Away from the horrors of plumbing projects, and back to something I enjoy - good old DIY and joinery.

I already have a horizontal layer of rockwool-type insulation in my loft, but I'm going to board it over (horizontal spars + loftboard) and fit a light to make it a more accessible storage space. I'm also tempted to make it a bit more hospitable up there by adding some insulation to the recesses between the roof joists.

My current plan is simply to put up rockwool or similar, and board over with a thin ply, partly to keep the insulation in place, but also to make it feel more like a "storage space" and less like a "deep dark alcove" of the house!

I've boarded and insulated lofts before, so I'm not batting an eyelid at that, but any "gotchas" to watch out for with the angled roof insulation?

 

[Shaz]sigh[]

If it's a cold roof, leave a 50mm gap from felt to insulation for ventilation. Rigid insulation is hilariously expensive so if you're not trying to 'ghetto loft conversion' it, keep your standard rock wool as you have now and board on stilts.

Edited, noticed you said rockwool in rafters... this would be somewhat difficult to ensure it stayed in place, possibly netting both sides to ensure a ventilation gap then board but it'd probably be less effective than rigid as you'll require a thicker layer in the rafters but will be limited by the ventilation gap.