I am wanting to enter a cake into our local county fair. The rules say it can be a styrofoam base, I would love to try fondant, this will be the first time I have done this. The temps will probably be in the 90's in a covered building, no air. What would hold up for 2-3 days? Any suggestions, or real world experiences?
I entered a fair a few years ago and the weather conditions were the same as you've described.
The cake was fine for judging because that was done within 2 hrs. of delivery---BUT, the heat and humidity took a big toll on it over the display time and the fondant was basically melted off by the time I picked up. I won 1st place and Best of Show, but it sure was a sad site when I retrieved it!
The buttercream cakes didn't fare any better. The only ones that held up really well were the one's done completely in royal (and I WON'T do that--once was enough).
So, my advice is that if judging will be done immediately upon delivery, go for the fondant. If it won't be judged for a day, or so, I have no good advice.
If you want to use buttercream, my experience has been that IndyDebi's buttercream recipe holds up well to hot and humid weather. I think she says in the notes section of the recipe that it holds up for about 8 hours. So if the judging is soon (like Blakescakes was saying) then it will work for a while. After a couple of days, it sounds like it might melt down as well.