I have a Wedding Cake and Groom Cake to decorate for the middle of August. The Bride's cake is a 5 tier and the groom cake is Chocolate. The Wedding will be outside under a Barn Shed with no electricity for cool air. What frosting is the best to hold up in the humidity? The bride doesn't want any fondant. And need some ideas as to keep the cakes cool when I don't have enough refrigerator space to hold all these layers up to transporting to the wedding.
What about marzipan or marshmallow or an Italian meringue ?
Google tells me that people make "Crisco" based frosting to stand up on heat and humidity.
(I don't make cakes and never wedding cakes) I would skip frosting altogether, and use powdered sugar (flavor and/or colored) or use ground nuts (with honey) to cover the cakes and use hard sugar flowers for decorations.
Me think that you should arrange to have fridge space; we are talking about food here, and food safety is important; you do not want to either poison the guests or have a mess of a cake because of heat and humidity.
In August you will have not one, but two "enemies" to keep in check: Heat and humidity.
Heat gives you trouble with every kind of food that gets runny when not cooled, so for some frostings refrigeration or at least a cool space to store is crucial.
But as soon as you take your cool cake out (to be displayed or transported) without cooling...
Humidity can cause condensation on the surface, which can range from "dull" surfaces to "beads" of condensation to bleeding of colours or even affect the consistency of the icing itself if it reacts with lots of liquid on the surface.
So if I read your post correctly, there certainly will be at least one, possibly many stretches of time where the cake can't be cooled And that inevitably will lead to condensation and it's side effects if it is cooled inbetween.
-> There is one alternative, though: Don't refrigerate at all.
Cakes are not "spoils-super-fast-and-needs-constant-refrigeration" foods if certain restrictions are kept in mind:
- The cake itself isn't critical. But unless you plan on doing a night-shift (bad idea...) you will want to bake the layers some time in advance. Either freeze or choose something that doesn't get stale quickly. Thaw completely before use.
- The filling must be stable and safe without refrigeration, so whipped cream, light ganache or those "dairy-with-gelatine" types are out. A heavy ganache will work (think "truffles") and so will cooked fruit fillings like jam. Shortening-based "buttercreams" could be ok, but I have no personal experience with these; I suppose it depends a bit on the temperature on the day of the event. "Fruit-and-gelatine" should be ok if made quite freshly, i.e. early enough to set properly but late enough to be still fresh. Whatever you choose, don't use too much.
- For the outer layer, fondant, marzipan and royal icing are the obvious choices. All of them can remain unrefrigerated. All of them have their pros and cons.
- Your bride doesn't want fondant. Ok, never mess with the bride... (and admittedly I detest the stuff myself).
- Marzipan is a love-hate-food, not everyone likes it. And - main disadvantage - it isn't pure white. And even tinted marzipan typically has a somewhat muted colour. Not necessarily bad, though, just with some restrictions.
- Royal icing can be a bit tricky to work with to get a smooth surface (pouring, drying for a day, light sanding, next layer, dry again...) and absolutely needs a cake-plus-filling base that can "survive" for a few days. In very high humidity, RI can be hygroscopic, that is, it "soaks up" moisture from the air and starts to get soft. Especially small details and decorations are at risk. So it needs to be stored in a dry place until display. But that is often easier to manage than cooling - unless you expect heavy rain or thunderstorms. But the cake in this case would be the least of your worries, IMHO.
So if I were in your position I'd suggest a very traditional (British?) fruitcake or a carrot cake (choose a recipe on the drier side, not too wet), filled with layers of jam or lemon curd or similar. Then brushed with a thin layer of jam (as glue) and a covering layer of marzipan to seal the inner cake for freshness and finally a double or triple covering of royal icing. Have fun!