Originally Posted by Brian Carlton

Something is not right. Are you stating that the house dropped three degrees in six hours, without any furnace in operation, with an ambient of -6°F?

Either you have some unbelievable insulation in that house or there is some truth being stretched. 2 x 6 walls with R-19 would not be able to hold a house to a .5°/hour temperature drop at -6°F. ambient.

In fact, at this temperature, the average 2,000 sq. ft. house in MI would be sucking down somewhere around 70,000 BTU/hr. If you are losing 70,000 BTU/hr, the house is going to drop by much more than .5° per hour.