Crawl Space Ventilation – Good or Bad?

Crawl Space Ventilation – Good or Bad?

Crawl Space Venting Often Causes Condensation, Mold Growth, Poor Indoor Air Quality and Higher Energy Bills.

Venting on a Hot Summer Day

When we say relative humidity we mean how full of water the air is relative to the maximum amount of water it can hold at a certain temperature. Think of what happens when you take a soda can out of a refrigerator on a warm summer day. Water droplets form on the exterior of the can and the can “sweats.”  This is because you have reached dew point on the cans surface.

When you allow warm moist air into a cool damp or moderately damp crawlspace through vents you are overwhelming the crawlspace air with excess water vapor. You will eventually reach dew which causes water droplets to form on pipes, concrete, framing, wiring, etc.  The eventual result will usually mean severe water damage, mold damage and dangerous mold growth in the crawlspace.

The source of the cooling air in the crawl space is the earth, and the source of the warm air coming in is the crawl space vents or doors, so the surfaces in your crawl space are always colder than the air in a crawl space.

Now, on a summer day, there is condensation and the crawl space walls get wet, the dirt surface of the floor gets wet, the air ducts get wet – especially if we have the air conditioning on because the ducts are cold – and the cold water pipes get wet. These surfaces are the coldest. Our floor joists, girders, sill plates and insulation all get wet as well.  And as the insulation gets wet, it often develops mold on the paper backing and eventually falls down to the crawl space floor.

High humidity in a crawl space causes any porous materials to soak up water vapor from the air. There is a direct link between the relative humidity and wood moisture content. Wood in a damp environment will become damp too – and damp wood will also support mold growth.

All of these wet surfaces in a crawl space will eventually have to dry too at some point. So say you have the hot summer days that cause condensation in the crawl space, then you have four or five days that are cooler and mild. Is the problem over? Not a chance. After the hot days we are left with an area with multiple wet saturated surfaces. They dry into the crawl space air over the next weeks and months, and meanwhile, mold and wood destroying fungi are taking over your house.


Venting on a Spring or Fall Day

If a day is 72 degrees outside and it is humid out such as 80%, then allowing this air into a crawl space will also cause condensation. 80% relative humidity (RH) air-cooled ten degrees increases its RH by 22%, which added to the 80% RH is over 100%.  This means condensation in the crawl space. Is this an extremely hot day? Absolutely not, it’s a typical room-temperature day, yet we are left with a wet crawl space. Any time humidity levels exceed 60% there is a significant risk of mold activity occurring.

Venting on a Cool or Winter Day

If the RH of air goes up when we cool it, it goes down when we heat it. If a crawl space is vented in the winter and 35-degree air is mixed with 60% RH and air is warmed in a 62-degree crawl space, the RH goes to 3%. With this dry air we can begin to dry our crawl space. The dry cold air mixes with the crawl space air and cools the crawl space.  As a result we have water evaporating from the earth into the crawl space air so we never actually achieve 3% RH in our crawl space, but materials dry out and there is no condensation. The new problem now is cold floors, cold drafts, freezing pipes and increased energy costs.

Exposed earth contributes a lot of water vapor into the crawl space air. The earth is damp, and as that damp soil releases moisture into the air, the water vapor moves upward into the house. In most climates where there are dirt crawl spaces, you can never dry the earth and this invisible stream of water vapor from the exposed soils in a crawl space goes on indefinitely. In fact, water itself does very little to destroy a home with a dirt crawl space. The water seldom – if ever – touches any of the parts of a house that gets ruined, like floor joists, sub floor, and sill plates. It’s the water vapor, also known as relative humidity (RH), that causes mold and destroys the house.

The solution?

In our experience a permanent vapor barrier sealed at the seams combined with an adequate dehumidifier system set to a humidistat is the best approach to prevent moisture from taking over your home. It is usually best to close all vents and air seal the perimeter around rim joist and sills. Also make sure the area is free of leaking pipes and if needed have a working sump pump in low areas. It is also wise to improve exterior drainage and to install gutters to drive water away from the structure.

To learn more check out this article from  To Vent or Not to Vent

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