I've heard that the byproduct of burned gas is water (TRUE) and that is what's causing the growth of mildrew on the walls (PROBABLY NOT). I had some of the vents closed in the house and have now opened them for a better air exchange (PROBABLY A GOOD IDEA, BUT WHERE?). Someone told me that I also need more insulation in the attic to help the problem (POSSIBLY). What do you make of this?
Sue, things like where you are in the world, whether you have a basement, crawl or slab, how much insulation you have in the attic and in a crawlspace are what someone would need to know to really solve problems. I don't want to jump to conclusions either but I wonder if anyone has been up in your attic and looked for signs of water leaks and wet insulation there. That would be important to know.
You have to be our eyes, ears and noses for solving problems. Was the mildew pretty much confined to the one room? Or the room under that room? What is the roof like at that area? Anything special like a valley there?
One thing that is true is that mildew requires something to be on and moisture to grow. There are always mildew spores in the air ready to start growing. If you don't give them humidity, they will not grow. The humidity from a shower will allow a little bit of growth in most bathrooms but it will not have much affect several rooms away.
My guess is that you have a water leak somewhere in the roof, soffitt, eaves area? Or an ice dam that set it off with this cold winter? Or something of that nature. You are correct that a big release of humidity from a basement or something could do it, but I'd still look at water from above first.