More Reasons Why Venting Your Crawl Space is a Bad Idea

You may think that venting your crawl space is only bad if the temperatures are drastically hot and humid.  However, it’s bad to have an open vent in your crawl space no matter what the temperature is like outside.

If it is a beautiful spring day at 72 degrees outside with 80% humidity, the outside air will still produce condensation in the crawl space.  If the air with 80% relative humidity (RH) is cooled 10 degrees, its RH increases by 22%, putting the overall RH over 100%.  Even if the outside air was only at 60% humidity, it would still increase the RH by 22% to 82%.  As long as it’s less than 100%, it’s okay, right? Wrong.  Mold and rot happen at anything over 70% RH, and some can even thrive at less than that.  Just because the outside air is room temperature, doesn’t mean it’s safe for your crawl space.

If it’s a cold winter day with less humidity in the air, it’s still dangerous for your crawl space.  The RH of air goes up when it’s cooled and down when it’s heated.  So if your vented crawl space brings in 35-degree air with 60% RH in the winter, the air is then warmed to the 62-degree crawl space, and the RH drops to 3%.  The dry air will dry out the crawl space, meaning no condensation.  That’s a good thing, right?  It is if you like high energy bills, cold floors, and drafts in your home.  When it’s cold out, vents let in cold air under your feet, which costs you even more in heating bills.