-15 would kill most mold.
The problem is, it would not have any effect on the spores. Mold spores can survive very extreme environmental conditions. If the spores survive, the mold will come back as soon as conditions are right -- moisture, a bit of heat, and some food. The food is still there, once the heat is back on and there is just a bit of moisture you can expect to see the mold thrive once again.