if your snake is eating on a schedule, pooping on a schedule, and to the best of your knowledge, healthy and acting like a corn snake should, then you probably do not need additional heat.
Corn snakes do best with a heat gradient so they can choose for themselves. The highest temp should not exceed 88 and the lowest should not go below 70. (Most Corn snake owners shoot for about 85 & 75, but if it varies a few degrees, no problem.)
So you are concerned about temps lower than 70....... especially at night. (Remember, a corn snakes temp should not go below 70).....
So what happens to corn snakes in the wild if the temp goes below 70? It obviously does, right?
A snake will become sluggish if it gets too cold, it could slow down its metabolism and might even hibernate if it gets cold and stays cold. But you were referring to the "Ambient Air" around your snake. As long as he has that "Hot Spot" he can go to that is 84, then there should be no worries.
Remember, it's "His Choice" if he hangs out in 65 degree air, sits in a 85 degrees Hide box, or soaks in a water bowl. As long as you provide all 3, he will pick out where he goes.
Now having said that, I was also concerned with temps getting "Too Low" as you are. So I actually have 2 heat pads under my tank. 1 is set for 85 (the hot side) and 1 is set for 74 (the cool side). That way even if my room air gets cooler I can rest assured he does not go below 74. But a lot of people get by with just 1 heat pad and they do just fine.