Down collapse in high humidity

In my experience, the loss of loft from humidity isn't that significant from spring thru fall. Certainly manageable for weekend or week-long trips. If you can keep it dry from rain during the day, and avoid copious sweating at night, you should be fine. If possible, you can take a little time here and there to air out the bag in the sunshine during a lunch break or while setting up camp.

-Mark

For me with the conditions stated (rain and 80-90% humidity) I would choose a light synthetic fill bag because even though synthetic fill is a bit heavier than down it provide the insurance that if your bag does get wet it will dry faster and you will be able to sleep dry the next night. (In high humidity people tend to sweat more so even though you might be able to keep it from the rain you might still experience absorbing moisture from your bag).

I've been using down bags for 25 years or so in SE USA and never really had any loft-loss issues to speak of. That being said, you have to manage things a little more when using a down bag. I keep a pack towel handy, and when I wake up at various times at night, I wipe my bag shell and the tarp over me. I never, ever breath into the bag while sleeping. I also air the bag out in the morning and at rest stops. Make sure you have a totally waterproof stuffsack; I like a small sil pack liner with a trashbag inside, allowing the bag to remain relatively uncompressed in the pack. Unless it's really cold, I think your body heat will keep the bag pretty dry while sleeping.

It doesn't rain every day on my trips in northern Michigan and Minnesota, but it's also not uncommon to have very high humidity and several days of rain. I only use down bags. After several days of heinous humidity I can typically tell that the bag has some moisture in it, but I don't typically notice any loss of warmth. All you need is a few hours of sunshine to dry the thing out; also, as long as you dress and vent properly, your body heat can help drive moisture out of the bag…