I'm not sure if it would be enough heat to keep it completely dry but from my experience running motion on a Pi B, the CPU can get up to 70degC on a 30degC day

I'm not sure if it would be enough heat to keep it completely dry but from my experience running motion on a Pi B, the CPU can get up to 70degC on a 30degC day and I haven't seen it below 33degC which occurred on a -5degC night. This is with a standard ebay Pi case, however.