Room Temperature

Ideal is 20-25 degree Celsius and 60-80% humidity. Just for your information, with lower temperature ink viscosity increase as a result drop formation start to change. Usually below 15degrees ink become too think and you would not able to get print results. When it too hot then ink become more "watery" again you will have some kind of over sprays.
Another point to warry about with low humidity you increase static electricity on the media. With HIGH humidity material will absorb water in to it.

 

If it's a permanent move you can install a ceiling or wall mounted natural gas or propane heater. I have one in my 3 car garage which uses an 80# propane tank just outside the garage. I used to use it to heat my shop a few years back and it was set at 50 all winter long when not in use. I've attached a pic of a similar unit to mine. Obviously, it needs to be vented but you could build some makeshift walls or use heavy plastic to make the area smaller to conserve heat if it's a big warehouse. Maybe an electric baseboard heater would work also.