I have seen hundreds of totally burnt to a crisp mezuzos that were slow baked over a long period of time. It is often difficult to determine why one mezuzah in a cheap definitely not waterproof case survives year after year with no damage and another mezuzah in a solid metal (supposedly waterproof ) case turns into black powder, or black mush (if water got inside). Last week I may have made a bit of progress in understanding this issue. I checked two houses right next to each other. Both had an aluminum mezuzah case with a screw cap on the bottom. Both were on the garage door (garage doors usually have less protection from the sun than other doors as they don’t have screen doors or large overhangs), and both had a southern exposure to the sun. Sure enough, both were burnt to a crisp with nothing remaining except black powder. My conclusion: Aluminum plus southern exposure even without moisture (both of these cases were dry inside) will destroy a mezuzah. My favorite case for outside is a white plastic one piece with a plug on the bottom. Yes, they will turn yellow and look really bad (mezuzah cases should look nice, they are for a mitzvah) after a year or so, but they are much cheaper to replace than the klaf inside. More importantly, if the klaf is ruined, that means you weren’t doing the mitzvah of mezuzah. Check your mezuzos every three and one half years, and replace old outdoor cases when they no longer look like they are up to the task of protecting your mezuzos.