I'll bet this is it: a kerosene heater has been used to keep the
temperature in the addition around 50 F at night (so the drying plaster
does not freeze). The heater throws off a lot of carbon when it runs
low on kerosene, as it does around 6am.

Now we have an extra layer of dust on the instulation. Any reason to
think that this is a problem?

Thanks,

mh