I don't like to keep them cooler than 65, or warmer than 75. We've been keeping ours in a cooler part of the house (65) and there are some definite sneezes going round.

To offset this, if you use a pelleted bedding, give them a lot of shavings to nest in and an oil filled (or other type) radiator when available.

Ralph also uses pine shavings, and a lot of it, and he also keeps them in smaller tubs with more rats in each tub. I don't agree with the cramming amd super confined spaces, but it will keep the rats warmer.