I would like to lower my indoor house temperature as low as possible to save heating costs in the winter when I'm not home. I was wondering what the safest lowest temperature could be before damaging any interior structures.
Please consider installing an electronic programmable setback thermostat. It will take out the guess work (and it doesn't forget!)
Some would say that in an ideal situation, you should shoot for a maximum swing of 3 degrees (Celcius). This means that if you set the thermostat down to 16°C during the night, you should be setting it at no more than 19°C during the day. This will give you the maximum benefit of energy savings and not make your furnace work so hard (and burn more gas) to get back up to the daytime temperature.
Also, it's a good idea to maintain a moisture reading of 50 – 55% Relative Humidity within your home with outside temperature 0°C to +10°C and 40% with outside temperature of 0°C to -12°C.
A bathroom exhaust fan controlled by a dehumidistat can control this for you. A “Relative Humidity Indicator” will help you with your moisture level readings. You can pick one up at most home improvement stores.