Thinking about the roof assembly for my tiny house on wheels. Height is limited, but I think I can do about a 10″ thick roof.  I was going to do a 2×6 rafter with Havelock wool in the cavity (R19) with 4 inches of some combination of polyiso, xps, or rockwool above the sheathing. BUT I have 2×4 walls so might be easier to do a 2×4 rafter and just get extra of the same R13 wool for the wall cavities with 6″ of some combo of iso/xps/eps/rockwool… What combos would you all recommend?

Also this house will have plenty of heat but no AC so I am concerned about keeping things cool in the summer, especially in the loft. The foil-faced poly iso sounds good for that, but from what I am reading I need a membrane between any outsulation and the actual roofing material? I found a vapor permeable staple on foil pretty cheap — could I use that on top of whatever membrane I use? Also how would I ensure the airgap necessary for the foil to work? Furring strips just like on my wall assembly?

I’m in climate zone 5, maybe moving the house to climate zone 4 later in life.

Roofs are so confounding….