Crawlspace Ventilation: Essential Insights to Better Guide Your Clients

Welcome to the Inspection Insider. Here, seasoned home inspectors share their technical expertise and experience to help you improve your own inspections. 

In this post, retired inspector Tom Lauhon discusses crawlspace ventilation. As a home inspector of 36 years, educator at the Midwest Inspectors Institute for 30 years, and prior member of the Examination Board of Professional Home Inspectors (EBPHI), Lauhon has a wealth of experience and knowledge. He has conducted training and guest lectures at inspection conferences nationwide. His knowledge has been featured in various books and publications, including his own column in the Kansas City Star. Lauhon has also served as an expert witness on inspection cases across the country. 

Now, here’s Lauhon with his article on crawlspace ventilation systems.

Do you really need ventilation in a crawlspace?

Yes!  No. Maybe?

As a home inspector, you’re expected to examine crawlspaces, or the space under the first floor and the Mother Earth beneath it. Understanding crawlspace ventilation systems, techniques, benefits, and drawbacks helps you recognize existing defects and the potential for future harm.

Aside from building codes mandating crawlspace vents, why do builders ventilate crawlspaces? How are they doing it? And what are some common mistakes builders and contractors make installing ventilation in crawlspaces? Read on to find out.

Why does a crawlspace need ventilation?

To reach the heart of the issue, we need to discuss humidity.

Crawlspace ventilation is installed to reduce humidity, or the amount of moisture that the air can hold before condensing and falling as water. By decreasing humidity in the soil, venting can prevent condensation and moisture buildup in crawlspaces. Preventing dampness is important because excess moisture can lead to mold, mildew, and wood rot. 

But where is the humidity coming from? Is it humidity from the ambient air outside the house? Or is it groundwater coming to the surface? Is it human interference created by cooking, cleaning, and showering? 

The answer is YES!  Humidity comes from all three sources.

Considering human interference, alone, cooking, cleaning, bathing, and even breathing create moisture. Just how much humidity are people producing?

• Every person in your home produces about 0.4 liters of water vapor per day just by exhaling.
• Showering creates about 0.25 to 0.5 liters of water vapor, though exhaust fans evacuate some of that vapor if people use them.
• Boiling water produces over 10 milliliters of water vapor every 10 minutes. Brewing a simple cup of coffee without even boiling creates one milliliter of vapor.
• A typical laundry load with cool water creates 50 milliliters of water. Hot water cycles triple that amount.

These amounts seem harmless. But when you consider that amount of water over years, these water vapor stats are significant. 

So now the question is how to minimize accessibility from these sources. 

How do builders minimize humidity in crawlspaces?

Moisture from the exterior is usually the easiest way for water to enter a crawlspace. An improper slope, where the ground around the foundation allows water to drain toward the house and collect at the foundation wall, is a frequent culprit. The moisture can transpire through the wall and appear as water drops on the interior side of the concrete or concrete block walls. This slope can also cause foundation wall issues due to freezing and thawing, which can result in extensive structural repairs.

One way builders minimize crawlspace humidity is by preventing a bad slope. For context, code calls for a half-inch ground slope six feet from the foundation. However, the Federal Housing Administration (FHA) says to extend the slope to 10 feet.  Roof downspouts should also direct the water at least six feet from the foundation.

Groundwater coming to the surface can be an issue in most geographic locations. The simplest and least expensive way to eliminate it would be to put a vapor barrier on the crawlspace floor. Plastic sheeting of at least  six-millimeter-thick material with twelve-inch taped seams and adhered outer walls is the best option, in my experience.

What about human-produced humidity? To prevent man-made water vapor from getting into the crawlspace, many builders and homeowners install fiberglass vapor-backed insulation in the joist cavities. This will only prevent moisture intrusion if it’s installed with the paper back in contact with the subfloor. Unfortunately, we rarely see this installed correctly. Instead, we often find the insulation installed with the paper back side down. 

Besides crawlspace ventilation, how else do builders remove humidity?

When improper installations happen, crawlspace ventilation systems can be a great help. The problem is, homes rarely ever meet minimum crawlspace ventilation requirements. 

Builders use simple crawlspace ventilation calculations to determine how much venting is required. For years, construction practices have required one square foot of net-free ventilation for every 150 square feet of floor space, with vents in opposing corners of the crawlspace without the soil covered with a vapor barrier. For example, a 1,500 square foot crawlspace would call for 10 square feet of net-free ventilation space.  In my over 30 years inspecting, I never saw anywhere close to that much venting. (And I feel sure you haven’t, either.)

So what is the solution to insufficient crawlspace ventilation? Modern thinking says to seal the crawlspace by:

1. Covering the floor with a vapor barrier.
2. Lining the interior of the walls with one and a half inches of foam covering.
3. Covering the exterior vents.
4. Adding a floor grill to the main level floor at both ends of the house.

Combined, these four practices temper crawlspace humidity and, as a by-product, eliminate cold floors near the exterior walls.

Should your clients install ventilation in crawlspaces or not?

To vent or not to vent? Either ventilate correctly or don’t ventilate to the exterior.

We hope you enjoyed Tom Lauhon’s article on crawlspace ventilation. Expert knowledge can help you navigate crawlspaces and prevent claims with confidence—as can your insurance.

Whether you’re accused of missing snakes in a crawlspace or mold from insufficient crawlspace ventilation, or even if your crawlspace robot causes property damage, InspectorPro protects against common claims. Learn how our home inspector insurance solutions provide peace of mind.

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