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.
Yes, 70F outside is far from a crisis for the miners, it's just a harbinger of what is to come. Perhaps you like your house at 85 to 95F; I, and especially my wife, do not. We are Minnesotans. We are used to temps of 10F outside, not 100F.
I keep my miners in the garage. The garage doesn't have any windows. I prefer not to advertise the fact that I have lots of nice electronics in there. With the door closed and it's upwards of 70F outside, my miners (Ten S1s and six Cubes) bring the garage to 85F in under a half hour and the miners' temp sensors quickly head north of 75C. The cubes drop 10GH apiece at the high clock rate. They don't have temp gauges that I can see via the web interface. When the Cubes go over whatever is their maximum temp, they drop down to the low clock rate. The error rate of the S1s increases from under 100 per hour to several ten thousand per hour. The number of invalid/discarded/rejected shares jumps from tens to tens of thousands. Then the chips on all the miners randomly start getting flagged with X's a few at a time. So no, the miners are not happy at 50C or higher ambient temps. When it hits 80F and 90F outside, forget it, the garage turns into an oven.
Secondly, I doubt your miners are doing "just fine." I'd be interested what temp the S1s are reporting on their miner status web page. If the room is already 50+C, I'd bet they are running at 70 to 80+C. At those temps you can look forward to cumulatively degrading hashing rates and/or chips failing. Me, being an electrical design engineer, I want my miners to perform as best they can for as long as they can. Right now, when they are operating below 50C, they get hashing rates around 220GH consistently, with no permanent X's on any chips.
Lastly, again, being an electronics design engineer, I tend to be a little anal about keeping my electronics running in the "low to mid" temp range stated on a devices data sheet, if I can, and stay far away from the "absolute maximum" temp range.
So, all those who like to bake their miners and shorten their useful life, you can line up with AbiTxGroup.
I'll keep it cool thank you.