I have had massive mildrew in the upstairs bedroom of my house in the winter. I have had the gas furnace checked and there does not seem to be a problem with the exchange-whatever. I've heard that the byproduct of burned gas is water and that is what's causing the growth of mildrew on the walls. I had some of the vents closed in the house and have now opened them for a better air exchange. Someone told me that I also need more insulation in the attic to help the problem. What do you make of this? Thanks!!!!!