A proper humidifier solution (aka, humidor solution) consists of a 50/50 mix of propylene glycol and distilled water. There are a few key benefits of using a humidifier solution, most notably the anti-fungal properties of glycol and the ability of PG to keep water from evaporating beyond 70%. The latter benefit stabilizes the humidity in the sweet spot for cigar humidification.
There is much debate about whether one should use a glycol solution for their humibrick on a regular basis. Generally, if you are not using it regularly, it is a good idea to at least add some to a foam or crystal brick every 3-6 months. This will still help keep your humidity level consistent while fending off potential mold issues.
At City Cigar, we currently carry the Brigham brand Humidor Solution, available in 236 ml (8oz) or 473 ml (16oz) bottles. Both are priced under $20.
Make sure to pick up a bottle next time you’re in to keep your humidor in top condition!