Fermentis recommends 59-75 degrees for us-05. I know it works like a champ in the high 50s, I use it at those temps for a lot of IIPA's. Still, I'd guess much below that 59 degree mark, and it may have a hard time kicking off.
Listen to Revvy, if you're unsure, take a reading. That's really the only way you'll know what's going on.