The default way to insulate between the ribs using kingspan or simlar, then ply over, and line accordingly.
But the NV200 is quite low, and we thought every cm would help, so we did it a more complex and long winded way, which we hope adds character!
We insuated the ribs with 7mm foam insulation.
We battened in each recess 18×11 wood bonded in, then filled all the gaps with 3mm foil in the recesses and then 7mm foam insuation over the rest to “fill it in”. Then, all joins were taped with silver foil tape to create a vapour barrier. The ribs were then carpet lined.
After all that, we put a full width piece of 7mm foam over the existing insulation and battens.
Then we made a bit of ply to perfectly fit the gaps and drilled holes ready for screwing into the battens (marked before covering over), and then countersunk the holes, and we planned to insert screws and then carpet line. Where the screws were we planned to cut the carpet to allow us to screw in then move carpet back and reglue. However, that didn’t work – it went badly 🙁 Instead, I glued the ply in readiness, then screwed it in, then put on the carpet in situ and tidied up. A right b***ch of a job!
Quite long winded, especially as you need to hide all the wires, and work around the lips, and also the carpet needs doing surrounding the roof. But, this way gives us at least 14mm of top (but expensive) insulation and a neat finish with good character. The screwed ply (12+ screws per width) will hold the insulation if needed, but it is glued or self adhesive. Also where there are lighting electrics, we only had a single layer to make the cables accessible. We are also confident this gives us a virtually perfect vapour barrier which is essential.
(Also, as you can see, we can only do 1/2 of the roof as we don’t know the size of the bed and hence width of the wardrobe yet and as the roof needs to go around this….)