It's not something we want to think about but if an emergency does happen because of a blown transformer or a thunderstorm knocking the power out, it's a good idea to know how to keep your rats cool in the summer when air conditioning isn't available.
Frozen Water Bottles
I always keep a couple of plastic water bottles filled with water in my freezer in case of an emergency. This method will work for short amounts of time where you need to keep your rats cool. Just take a couple of frozen water bottles and wrap them in a light cloth and place them in your rat cage. The rats will be able to sit or lay next to them and keep cool. You do need to make sure to wrap the frozen water bottles in a light cloth to absorb the condensation so the cage doesn't get wet and to keep the rats from getting frost bite if they lay right on the frozen bottles.
Ice Air Conditioning
This method works well for times when you need to keep the rats cooler for longer periods of time. You will need a hammock or hammocks, a large plastic pet igloo and a bag of ice.
Hang the hammocks in the cage from the top of the bars so that the rats can easily get into them. Place an unopened bag of ice or ice packs on top of the cage and a plastic igloo on top of the ice so that the igloo covers the whole bag of ice. The Plastic pet igloo will act like a refrigerator and keep the cold air from going anywhere but down, right into the hammocks.
Just place the rats in the hammocks and they will have a cool place to lay until the ice melts. When the ice melts, just replace the bags with new bags of ice.
If your rats already like water and are used to it, you can fill a tub with about an inch of cool water and let them run around in it. They will get wet and it will cool them off. Don't force your rats to stay in a tub of water if they are scared or not wanting to be in there. You can always hold your rat and gently rub a little water on his back while you pet him and get the same effect.
Rats like to be kept at temperatures between 60F and 76F, but they will tolerate cooler temperatures much easier than warmer temperatures because they can snuggle or burrow into their bedding to keep warm.