I added Havlock wool to the ceiling under the OVRLND headliner. Super easy to undo/redo the headliner, I like the material OVRLND used, it's foam backed and provides both a bit of insulation and breathability. There are rivnuts at each end and velcro holding the sides of the liner. We added a single layer of wool for now (2" ~R7-value). I think the headliner is snug enough to hold the wool batts in place, I'll see in a couple months. A single box of Havlock (100sq ft) provides more than enough to insulate probably any OVRLND size, with my Gladiator bed I'm going to have lots extra.
The wool is very easy to work with, cut with scissors, and can quickly pull off tufts to fill in any gaps. With two people it took us about an hour. Even though the wool is non-toxic you'll want a mask and eye-protection for the install, it drops some wool dust/fibers. Not the cheapest option out there ($150/100sqft), but I was really intrigued by using such a good natural insulator, has great mildew+moisture control over anything else and the Havlock stuff isn't chemically treated.
I still have to settle on what to cover the walls with over the wool, so that's a project for later. Breathability is key since wool has it's own moisture control, I'm looking at headliner material, auto carpeting, DWR treated canvas, or even marine carpet. The marine carpet is tempting because it is foam backed and would insulate the metal beams really well without the need for additional foam/neoprene tape.
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