The Dreaded Mold

There are a few things I don’t know about roof ventilation:

1. Why is the amount of ventilation required by the International Building Code based on square footage of attic space not volume of attic space?  Shouldn’t a steep roof with a huge volume of air require more ventilation than a very flat roof with almost no volume of air?

2. As long as you have a modern HVAC system and proper outside venting of your clothes dryer and bathroom, etc., why is venting an attic in a warm humid climate a good thing?  Isn’t the most difficult job of an AC unit to take moisture out of the air, and if that is the case why bring in a constant flow of new moisture?

3. If you have R49 as recommended by the US Department of Energy, in your ceiling and live in a mild climate, does it really matter if your attic is a little warmer than the weather outside?

If anyone can answer these questions for me I’m all ears.

There is one thing I do know do about attic ventilation:

If you make changes to an older home to make it more energy efficient and you live in a climate where it gets cold in the winter, you probably should make changes to the attic ventilation as well. Homes that were built before the energy crisis of the 1970’s rarely had moisture ( and mold)  issues related to ventilation. Back when I was a kid houses breathed. There weren’t double and triple paned windows, weather stripping was rare, and homeowners didn’t shoot expandable foam into every nook and cranny. Take one of those old homes and tighten it up everywhere and suddenly there is nowhere for all the humidity that we generate from our showers, clothes dryers, dishwasher, boiling water, etc to go.  Well, I guess there is some place…your attic.

I think all of us, at least those of us in the South and the Midwest, intuitively know that warm air can hold more humidity than cold air.  You just have to go outside a couple of times in each season to figure that out. Now when warm moist air in your attic hits a cooler surface it condenses and turns to water. That is because the cold air that is adjacent to the cold surface can’t hold as much water as the warm air.  When the temperature and humidity differential is great enough you can have a very damp attic. Not only is this bad for the structure, it is also a perfect breeding environment for the “Dreaded Mold”.



The best way to prevent the “dreaded mold” is with proper ventilation. In the picture below we see radio talk show host Mark Clement installing a continuous ridge vent in a new Bellaforté by DaVinci steep slope roofing installation. You can see more helpful tips from Mark and his wife, Theresa, at