In Florida, ambient temps and humidity levels are too high to store grain in the garage or shed--regardless of whether or not the container is sealed or "air tight." Storing the grain bags indoors is the only way to go.
I've had good success storing the bags purged of excess air (i.e., rolled up tight and taped shut) and placed inside my igloo coolers, mash tun cooler, and SS boil kettle. I also have a large x-mas storage bin for various specialty grain and crystal malt bags. Again, use the indoor space efficiently by storing the grain inside the coolers and boil kettle w/ lid on. No issues with critters, mold, or spoilage.