Foundation Vent Myths in Regard to Weatherization

While driving around and giving bids to many folks who are looking to take the edge off their winter heating bills, I have seen a large number of people who seem to think that foam blocking their foundation vents is a good idea in regards to weatherization. While it would go to reason that any blockage of cool air in or under the house may increase comfort and lessen the heating load, it should be stressed that covering foundation vents is a horrible idea in Oregon or any cool wet climate. Foundation vents are meant to allow air to evaporate the moisture within the crawlspace areas. Nearly every crawlspace I have seen has some sort of moisture problem. Energy trust and most incentive programs require a vapor barrier in these areas to help contain the moisture within the ground. Building Codes have long stressed the importance of vapor barriers in these spaces as well.
What happens when you trap the existing moisture from these areas within the building cavity or crawlspace and do not allow proper airflow to help remove the moisture? Imagine dumping a cup of water in your refrigerator and sealing it closed for a few months! Mildew and mold are often seen in these situations and can lead to serious health problems and increase many other problems that are often seen such as dry rot within the sub floor and framing.
Remember that your insulation is your thermal barrier for the house. Not your crawlspace vents!