These aluminum cases are considered waterproof. Even though there is no guarantee that water will not damage the Mezuzah, it is nevertheless the best option for outdoors with exposure to rain and snow. It is noteworthy to mention what the Da’as Kedoshim writes in the name of the Ba’al Shem Tov that it’s preferable not to use "metal" cases. The reason given is that just as the Mizbe’ach was not supposed to have any metal used in its construction since the Mizbe’ach’s purpose is to lengthen a person’s life while metal shortens it, the same analogy applies to Mezuzah. But he then writes that it seems that the Ba’al Shem Tov meant strictly Barzel which is “iron” and not all metal. Therefore, many are of the opinion that our cases which are made out of aluminum, do not fall into the category of Barzel.