You are taking a chance if you run the cat5 outside. Cat5 is UTP, meaning it is unshielded. Since it is unshielded, ANY, interference will disrupt your connection, creating packet loss, or maybe even no connection at all.
If you really want to run the cable outside I suggest to look for any possible disruptions before you go through the trouble and find out you are getting 70% packet loss. Possibilities are powerlines, flouscent lights, etc. Also, try to run the cable as high as you can, this will also help reduce the possibilities of disruptions. Perhaps under overhangs using little hooks.