This sounds more like colder temps are increasing draft and possibly not moisture in the firebrick. The wood still may be suspect if the glass is blackening.I've been able to achieve overall great temperatures and have not used oil for our heating needs since temperatures started going down (thermostat is set to 40). Last night the temperature got down to 32 degrees but inside I was hovering at a consistent 71 degrees without issue. I actually had to turn down the fan and air control because the house was getting far too hot. At one point I had the house at 78 degrees and just couldn't take it anymore. The insert had made it's point. My house is a piece of cake to heat for this thing.
Any significant concerns about heating the house are basically gone. My focus will now be on making the house tighter.