I think you are worrying to much about the heat.  You realize that 70F is only 21C.  Here in Texas last summer I was running a bunch of GPU miners.  They run at 70C which is 158F.  The hot Texas summer day of 115F (46.1) air was enough to cool the miners down.

This year I have a bunch of S1 Antminers and we have already had high 95+F days so far.  The S1's did fine with just some outside air being blown into the room with the miners and the hot air leaving the room.  In the main mining room with the building closed up, the room was at 115F (46.1C).  I opened the doors and windows to get the air circulating and the room dropped to near outside ambient temp in no time.  

I don't see that much of a problem here when it gets to summer temps at 115F (46.1C), just keep the air moving with outside air and have several replacement miner fans on hand as fans fail when you don't want them to.  If you cannot create the air exchange from outside with the inside, then you need to rethink you setup.  You need air flow from outside to inside to outside.

This reminds me of the late 80's and early 90's when small and medium businesses where having their computers networked.  I could not count how many times I was called in to troubleshoot network problems only to find a server or router closet.  That was the dumbest idea I seen done over and over by inexperienced so called "tech's".  Putting electronic equipment that produces heat into a closed off, normally not air cooled, closet. You have to have air exchanging to keep the equipment running.  Same with the miners.