Why Does My House Have a Crawlspace?

If you’ve ever asked yourself, “Why does my home have a crawlspace instead of a basement or slab?” you’re not alone. Crawlspaces are common, especially in certain regions of the country, and their existence is often rooted in practicality, local conditions, and cost-efficiency. But the logic behind crawlspaces has evolved over time. Let’s break down why they were built, what problems they’ve caused, and the modern approach to making them healthy, dry, and energy-efficient.

Why Homes Have Crawlspaces

There are a few key reasons why builders have historically chosen crawlspaces over basements or slabs:

Cost

Crawlspaces are cheaper and faster to build than full basements. They require less excavation, less concrete, and fewer materials, making them appealing in areas where affordability and construction speed are a priority.

Soil and Substrate Issues

In areas with dense clay or bedrock, digging deep for a basement may be too difficult or expensive. A shallow foundation like a crawlspace helps builders avoid these challenges.

Water Table Concerns

Homes built in areas with a high water table (i.e., water close to the surface) are at greater risk for flooding and groundwater intrusion. Crawlspaces provide an elevated foundation, keeping the living space above moisture-prone soil.

The Original Crawlspace Logic (And What Went Wrong)

In the early days of crawlspace construction, homes were often built without vents. This trapped moisture rising from the ground, leading to mold growth, wood rot, pest infestations, and poor energy efficiency. To fix the problem, builders began adding vents to promote airflow, believing it would keep the space dry. But here’s what really happened…

In the Winter

Thanks to the Stack Effect (air rising through the home), cold outdoor air was pulled into the crawlspace through the vents. This made the floors cold and the home less energy efficient. Builders started insulating the crawlspace ceiling with fiberglass… but this only helped a little. Why? Because fiberglass insulates against heat, not airflow. Cold air still moved through the insulation and into the living space.

In the Summer

Warm, humid air entered the crawlspace. Since crawlspaces are cooler than the outdoor air, that warm air condensed, dumping moisture into the space and causing:

• Soaked insulation
• Sweating ductwork
• Mold growth
• Damaged wood and stored items
• Higher energy bills
• Poor indoor air quality

In fact, up to 60% of the air you breathe in your home comes from the crawlspace. Closing the door to the crawlspace won’t keep that unhealthy air from entering your home.

The Modern Approach to Crawlspaces

To truly fix the problems caused by outdated crawlspace construction, experts now recommend a 5-step approach:

Step 1: Address Groundwater Seepage

Install a perforated drain tile system (French drain) around the interior perimeter of the crawlspace. Pair it with a sump pump system to collect and discharge groundwater before it can enter the space.

Step 2: Air Seal the Crawlspace

Keep outside air out by sealing:

• Around pipes and penetrations in the foundation
• Joist cavities at the band board with spray foam or rigid insulation
• Crawlspace vents
• The access door with an airtight, insulated crawlspace door

Step 3: Isolate the Home from the Earth

Even if your crawlspace has gravel, moisture and bacteria can still rise through it. Cover the floor and walls with a heavy-duty vapor barrier (plastic liner). Be sure to:

• Overlap and seal every seam
• Extend the barrier up the walls
• Seal it securely to the foundation

This step blocks moisture, mold spores, and radon from entering the home.

Step 4: Control Humidity and Improve Air Quality

Install a crawlspace-rated dehumidifier… but not the ones sold at big box stores. These standard units are only effective at 80°F and 60% humidity, while crawlspaces are usually much cooler. Instead, use a unit from SantaFe, AprilAire, or GeneralAire, which are built for below-grade spaces. They:

• Handle large spaces
• Work in cooler temperatures
• Filter and clean the air

Step 5: Repair the Damage

Before sealing up the space, address any existing issues:

• Replace or repair rotted wood
• Strengthen sagging floors
• Replace sill plates or band boards as needed
• Perform mold remediation and apply mold-resistant coatings if needed

Final Thoughts

Crawlspaces were built to save time, money, and work around soil and water challenges, but outdated thinking has led to big problems in comfort, energy efficiency, and health. Thankfully, modern crawlspace encapsulation and moisture control methods can turn your crawlspace from a liability into an asset.

If you suspect your crawlspace is contributing to poor indoor air quality, high energy bills, or foundation issues, don’t wait. An expert evaluation can help you understand exactly what’s going on and how to fix it for good.