Metal Roofed Cabin in Hawaii

2 years ago, I bought a 1-room 20x24' cabin, hand-built (read "unpermitted") in 1969, on pier and post, of single-wall tongue & groove redwood, with a corrugated iron roof, all of which is quite common in this area.. The roof is on 1x4s which are on 4x6 beams. There is nothing else - no insulation, ceiling, etc. It used to leak, but I applied 2 coats of Gardner Sta-Kool Cool Roof Elastomeric Coating, which stopped the leaks. Now I have noticed a serious increase in condensation (or it could be just the unusually wet weather we're having). If I could afford it, I'd just put on a new roof, but that is several years down the road in my budget. What would be the most cost-effective way to reduce the condensation? I live 4000' up the side of a volcano, so there can be a serious temperature gradient between night and day, from high 30's to low 80's. FYI: the 4x6s are 4 feet apart, the 1x4s 18-24" apart. (Told you it was hand-built... His hand-built windows are quite charming, however!)

Thanks for your comments. The roof is 1 slope, no ridge. There are eave vents, and my windows are usually open (from Alaska; I like the cold). Plus this cabin is in no way air tight. So ventilation isn't the problem. A dehumidifier would run all day and not make a difference. Any suggestions on a source for the foam? A lot of places won't ship to Hawaii, can't go by truck.. So far, I can't find any that will.

What is the humidity level like at night? You can spray foam it if you want but it will need to be covered with an intumescent barrier.