There is a " spray on " expanded PU foam which is insulating and water proof
( provided the skin is not broken ) Good for winter.

In Malaysia, we buy a corrugated zinc sheet which has this foam already
applied. It's sound proof, weather proof and keeps cool.

Another alternative is to stick on sheet foam onto the plywood. Seal the
seams with duct tape. Expanded polystyrene foam is cheap and good but will
break and tear from itchy fingers of kids. Large sheets of bubble wrap also
works well. Stick to plywood with contact adhesive or for expanded
polystyrene a non solvent glue.

Do not use bitumen.....you'll regret it. Epoxy is good adhesive and
waterproofing but so is normal paint ! Epoxy will cost 10 x more.


Have fun.