This seems like a simple question with two possible answers. It is not. Open the foundation vents, close the foundation vents, one of the two has to be right. If you think that either option will keep the crawl space dry year round, you will likely be disappointed. Here is why.
Why a Crawl Space Is Ventilated in the First Place
The primary reason for ventilating a crawl space is to control moisture coming up from the ground or from other sources. Without some kind of moisture control, dry rot and other forms of biological degradation will occur.
WHAT HAPPENS WHEN CRAWL SPACE MOISTURE GOES UNCONTROLLED
- Dry rot and other forms of biological degradation in the wood structure
- Floors that could buckle or even collapse if the problem is left untreated
- Breathing air in the home that smells musty and unhealthy
The Rule Most of Us Were Taught About Ventilation
If you were like me, you were taught that good ventilation is sufficient to keep a crawl space dry and mold free. This is only true if the outside air is drier than the air already in the crawl space. For most of the year, this works very well. Unfortunately, this is not the case during hot, humid weather.
What Happens When Hot, Humid Air Enters the Crawl Space
Warm summer air carries a tremendous amount of water vapor
Hot, summertime air holds a tremendous amount of moisture in the form of water vapor. When this outside air flows into the crawl space, it will naturally cool down and come to equilibrium with the crawl space air.
Cool crawl space surfaces push that air past its dew point
If the crawl space temperature is below the dew point of the outside air, water vapor will condense into water liquid. If the crawl space is really cold and the outside air is really hot, the crawl space may turn into a sauna with water dripping from pipes, insulation, and even the wood floor joists.
Since the outside air is the source of the moisture, it makes no sense to bring in more air to dry things out.
So Why Not Just Close the Vents?
Okay. Let us just close the foundation vents and minimize the influx of outside air. But wait a minute. What about the moisture coming up from the ground? Now that the vents are closed, we still have a source of moisture intrusion. If we seal up the vents, how are we going to control moisture from the ground, or from storm water intrusion?
The Honest Answer: It Is a Trick Question
So what is the answer? During hot, humid weather neither opening the vents nor closing the vents will guarantee that the crawl space will stay dry. So, it is really a trick question with no correct answer.
What Actually Controls Crawl Space Humidity Year Round
If you want to control humidity year round in a crawl space in Tennessee or other humid climates, we recommend sealing the crawl space vents and installing a dehumidifier.
The Most Important Point to Remember
Ventilation alone will not keep a crawl space dry year round in a humid climate.
What to Do With Your Vents in June, July, and August
During June, July, and August you can open your crawl space vents or close them. It probably will not make much difference either way. Here at DocAir, we recommend keeping them open since this configuration will limit the maximum amount of humidity that can accumulate during worst case conditions.
ONE EXCEPTION YOU SHOULD NEVER IGNORE
Certainly, you should never close the vents if there is standing water in the crawl space or the ground is not covered with a continuous vapor barrier.
Key Takeaways
- The reason to ventilate a crawl space is to control moisture coming up from the ground or from other sources.
- Ventilation only dries a crawl space when the outside air is drier than the air already in the crawl space.
- In hot, humid weather the incoming air is the moisture source, so more air will not dry things out.
- Closing the vents does nothing about moisture from the ground or from storm water intrusion.
- Year round humidity control in Tennessee comes from sealed vents plus a dehumidifier.
- Never close the vents over standing water or over ground with no continuous vapor barrier.
Frequently Asked Questions
Will opening the vents dry out a damp crawl space in summer?
Not reliably. Ventilation only works when the outside air is drier than the air already in the crawl space. During hot, humid weather the outside air is the source of the moisture, so it makes no sense to bring in more air to dry things out.
Is it better to close the vents instead?
Closing the foundation vents minimizes the influx of outside air, but the moisture coming up from the ground is still there, along with storm water intrusion. During hot, humid weather neither opening the vents nor closing them will guarantee that the crawl space will stay dry.
Why does water drip from pipes and floor joists in the summer?
When hot outside air flows into the crawl space, it cools down and comes to equilibrium with the crawl space air. If the crawl space temperature is below the dew point of that outside air, the water vapor condenses into water liquid on pipes, insulation, and even the wood floor joists.
What controls crawl space humidity year round in Tennessee?
Sealing the crawl space vents and installing a dehumidifier. Ventilation alone will not keep a crawl space dry year round in a humid climate.
When should the vents never be closed?
Never close the vents if there is standing water in the crawl space, or if the ground is not covered with a continuous vapor barrier.
Not Sure What Is Happening Under Your Home?
If your floors feel damp, the air smells musty, or you have seen water on the crawl space ground, the vents are not the whole story. DocAir has been diagnosing moisture, ventilation, and indoor air quality problems in Nashville and Middle Tennessee homes since 2002. Call 615.373.2498 or request an evaluation and get a clear read on what your crawl space actually needs before you spend money on the wrong fix.
