Venting your Crawl Space on a Fall Day

Passing air through your crawlspace can cause condensation and mold, even on Fall days. Let's imagine a day In the Fall around 72 degrees, along with an 80% relative humidity, (RH for short). When this air enters your cool crawl space it will cause condensation, which can rust your plumbing along with other problems. Let's do the math: If the relative humidity is 80% and we cool the air by 10 degrees F, then we have increased its RH by 22% for a new RH of 102%. Since this is over 100% RH, the air becomes unstable and begins to rain down in your crawlspace through condensation. It's not even a hot day by average, but it still causes condensation because you have not sealed your crawlspace off from the open earth. Even when it's not so humid outside, like an RH of 60%, it is still enough humidity to cause damage. Using the formula from earlier, if we add 22% and the 60% RH's we get a RH of 82%. Although not enough to cause condensation, it is still wet enough to grow mold. Mold begins to grow at a RH of 70%. Again, it is very important to properly seal your crawlspace off, even in the Fall.