Hi, All! So, I'll be attempting a 5 tier wedding cake for my cousin for an OUTDOOR reception July 4th weekend here in North Carolina - HIGH HEAT, HIGH HUMIDITY!! The cake will be covered in fondant, but curious about what would be the best medium to use UNDER the fondant. Would it be more stability to go with Italian Meringue buttercream -or- Ganache? I know nothing is "fool proof" in heat and humidity, but just want to give this cake it's best chance ;) I plan on keeping the cake in a fridge as long as I can before transporting it to the site (abt 5 min away). Thank you all in advance!! 

I would do a dummy cake and insert a small section of real cake into that for the bride and groom to cut into. There is nothing that holds up in high heat. The other thing to do is make a completely different kind of cake that isn't "decorated" traditionally with frosting. Make it more like a dessert cake with unfrosted sides and place pretty fruits or flowers on it to decorate.

The catch with keeping the cake chilled until it goes outside is that then you get condensation that makes the moisture problem worse.  I'd suggest removing from the fridge several hours early, continuing to keep it in a cool, air-conditioned room until time to move it outdoors.

Ah, yes - the condensation :( Good point!

you might wont to check out Planetcake.com they deal with high heat you could even contact them for info. good luck
 

AShortening based "butter"cream holds up better in high heat but it's not half as tasty as any meringue based buttercream.

I would seriously consider Sharon Zambito's buttercreme icing. She lives in Loiusiana and would know alot about humidity and cakes. She is very easy to get hold of and will answer any questions.

Has anyone made Italian Meringue and substituted a small bit of shortening (say 1/4 cup) for some of the unsalted butter? I've heard that makes the Italian Meringue even more stable...

AA pastry chef suggests using up to 10% shortening for stability. Blend the butter & shortening together before they get incorporated into your meringue. Still don't know if that will help enough.

I'm a Pastry Chef............and I disagree. 10% shortening will get you nothing. Tell the rest of the 90% butter not to melt, it won't listen! Now if you went with 100% meringue that would hold up in the heat, totally.

So are you talking about using an actual Italian Meringue and not an Italian Meringue Buttercream?

What about doing a Swiss meringue buttercream with hi-ratio in place of the butter?  Would that give the workability of regular SMBC but with more stability?  

I would personally do a ganache under the fondant. 

A

Chocolate melts in the 90's. Add cream to it as in 'ganache' it melts in the high 80's.

What part of NC?  Average temp in Blowing Rock is 76 degrees in July.  The coast is 90 degrees.  Major difference.  I live in central NC and it's pretty hot here in July.  Is there anyway the cake could be cut indoors and still served outside?  I wouldn't risk any type of icing at 90 degrees.

I had this exact same scenario a few years back... :(  I live in Arkansas and it ended up being 104 degrees.  The biggest thing I would recommend is that you make sure that the cake is perfectly level that you have plenty of dowel rods and that it is on a very level table!  The wedding reception was in a barn and the table was on shavings and not level at all.  Looking back I know that I should have brought a level and insisted on moving the table but I had told the wedding planner that it needed to be on concrete so I wasn't expecting it.  So the sad ending to my story is that after about an hour and a half or maybe two hours the cake fell over and a friend of mine there caught it ( I had already gone home at that point) :(  The only good thing was that it was for a friend who I wasn't charging so she was super nice about it and didn't even tell me about what happened but the man who caught it did.

So just a warning...having everything level is so important always but especially in high heat!!

 How about a very thin layer of apricot jam under fondant?  I haven't had a reason to try this, but have seen it mentioned.  (it is the traditional way to cover fruitcake although sometimes with marzipan). As long as your fillings are not particularly perishable that would be a relatively heat resistant cake.  Carma Ticino Tropic is a wonderful fondant that works particularly well in higher temps and humidity.  HTH