The only thing I could find was a group on facebook that was recommended to me, and I was able to ask Stevi Auble, who did the Craftsy wafer paper flower class, and who uses a lot of wafer paper flowers, what her experience was. She said that she'd never had trouble with humidity affecting the flowers. That was promising, but she lives in San Diego, and the humidity there is generally less intense, shall we say, than it is here.
Now let's be clear, I'm not talking about heat. Humidity is a wild card, it can be high during cool weather (fog, for example), or during hot weather (like when you're in Florida and you feel like you've been hit in the face with a hot, wet towel when you leave the house). Humidity can also rise or sink when it rains, apparently. Dry heat won't affect wafer paper at all, so setting something up in the desert outside might melt the cake, but the flowers will still be okay.
The humidity today was pretty high because it was going to rain, so I thought I'd do my test. If I'm not sure how something will react under certain conditions I'll do a test run. I did it with heat and buttercream, and with isomalt and the refrigerator, so why not with this?
I looked up the weather report for the time I was doing this, which was around 9AM. The humidity before it started raining was at about 79%, which is fairly high but not tremendously so. By the afternoon here it had risen to 95%.
So here are the flowers when I put them outside on the porch. The calla and the roses were made from single-thickness wafer petals, and the orchids and tulip were made from double-layered petals so they were a little sturdier.
I put them on the front porch out of the direct rain if it started to fall, and went inside.
45 minutes later I checked on them, and they had started to sag. The calla had folded over onto itself, which didn't surprise me since it was the thinnest of all of them, The tulip had softened up and had started to sag, and the paper was soft. The orchid still looked okay but it was the thickest one so I wasn't surprised by that. I was hopeful that it would hold up for the long run.
Half an hour later I checked them again, and it was a little more tragic.
Everything was losing its shape and melting. The rose had held up best but was saggy and the edges were rolling in. The orchid had given up and the tulip was falling apart. Saddest of all was the calla, which had folded in on itself and was now a lump.
After a total of 2 1/2 hours in the humidity, which I'm assuming was at about 85% at this point, they had totally succumbed and were now just wet, lumpy and sad.
I went outside later when the humidity was at 95%, according to the weather channel, and this is what happened to a sheet of wafer paper (the video is about 3 minutes long):
So my basic advice is that if you have an outdoor reception in a humid month, wafer paper isn't going to be the best choice for flowers. If you can't be 100% sure that a cake will hold up under certain conditions, you shouldn't sell that cake to a client who will be putting it in those conditions. It's too risky.
And if you're not sure, do a test run to see what happens to the flowers before you put them in those conditions. It's not good to take advice about outdoor cakes from people if their weather conditions are totally different from yours, because their answer might be 100% correct for their area, but not apply to yours at all.
Kara Buntin owns A Cake To Remember LLC, online cake supplies at www.acaketoremember.com and www.acaketoremember.etsy.com