Loft insulation - best option with 200mm available space?

[Niscars]

I want to upgrade the loft insulation in my semi. The house has had dormers fitted and the remaining loft has been cross battened (2 x 100mm) and a thin layer of insulation laid (current situation is shown in the photo below). Wooden floorboards (22mm) have then been nailed on top.

The Loft is just used for general storage.

I think that the obvious first step is to clear out the old and lay 100mm of Rockwool directly on top of the ceiling. I could then easily lay another 100mm on top of that (at right angles) inside the upper framework and screw the floorboards back - but that would leave me short of the required 270mm.

In order to get 270mm (Rockwool), I would have to add 70mm to the upper battens. Possible, but there maybe a weight issue. I don't think that I can sensibly use loft legs because it's floorboards not sheets of chipboard.

Can I add Kingspan (or something else) instead of the second layer of Rockwool. I assume it could then be much thinner than the 170mm of Rockwool. Can they be safely mixed?

Can someone advise how much difference this will make please. Is it even worth doing more than 2 x layers of 100mm (plus the floorboards). That's at least 4 times what I currently have.

Any advice appreciated. Thanks.

 

[cdbe]

You can mix. Some cavity bats have better performance than mineral wool. Personally, these days, I'd probably go for 2 layers of tightly fitted (and/or foamed in) 100mm pir.