Crawlspace Encapsulation Guide for Homeowners

If your home inspection report linked you to this page, the crawlspace may have an encapsulation system that is incomplete, improperly installed, damaged, or otherwise not functioning as a complete moisture control system.

Crawlspace encapsulation is more than simply placing plastic over the soil. A properly designed system controls bulk water, ground moisture, outside air infiltration, and humidity within the crawlspace.

The information below is based in part on published building science guidance from the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Energy (DOE). These recommendations should not be confused with a requirement that every existing crawlspace be encapsulated. Applicable building codes, manufacturer requirements, pest management requirements, and appropriate methods vary depending on the home and location.

Why Crawlspace Moisture Matters

Crawlspaces are particularly susceptible to moisture because they are located directly above the ground and below the living space. Moisture can enter from several sources at the same time.

Common sources include:

• Moisture evaporating from exposed soil
• Poor exterior grading and drainage
• Roof runoff or downspouts discharging near the foundation
• Groundwater or seasonal water intrusion
• Plumbing leaks
• Condensation on ductwork, piping, or framing
• Humid outdoor air entering through foundation vents and other openings

Excessive crawlspace moisture can contribute to fungal growth, wood decay, corrosion, musty odors, deterioration of insulation and other building materials, and elevated humidity inside the home.

What Is Crawlspace Encapsulation?

Encapsulation converts a traditionally vented crawlspace into a substantially sealed and moisture controlled space. The ground vapor retarder is only one component of that system.

A complete encapsulation system typically addresses:

• Exterior water and groundwater management
• Ground moisture
• Foundation vents and outside air leakage
• Foundation walls and interior piers
• Penetrations through the vapor retarder
• Thermal insulation
• Humidity control or conditioning

Simply laying polyethylene over the soil should not be confused with complete crawlspace encapsulation.

1. Control Water Before Encapsulating

Encapsulation should not be used to conceal or compensate for an active water intrusion problem. Bulk water should be controlled before the crawlspace is sealed.

Depending on the property, this may include:

• Correcting improper grading
• Extending downspouts away from the foundation
• Repairing plumbing leaks
• Correcting foundation or drainage problems
• Installing appropriate perimeter drainage where needed
• Providing a sump system where groundwater conditions require one

Standing water beneath an installed vapor retarder is still a moisture problem and should be investigated and corrected.

2. Completely Cover Exposed Soil

A continuous ground vapor retarder should cover exposed earth throughout the crawlspace. The material should be durable enough for its intended use and installed over a reasonably prepared surface to reduce punctures and damage.

DOE building science guidance describes using polyethylene or another approved vapor retarder over the crawlspace floor. Current DOE guidance discusses a minimum 6-mil polyethylene ground cover, while other programs and encapsulation systems may require more durable materials.

Membrane thickness alone does not determine whether a crawlspace is properly encapsulated. Coverage, sealing, durability, wall and pier integration, and moisture control all matter.

Missing sections, significant tears, large openings, or exposed soil interrupt the ground moisture barrier.

3. Overlap and Seal the Seams

Separate sections of the vapor retarder should not simply be laid loosely over one another. DOE guidance describes overlapping seams approximately 6 to 12 inches and sealing them with compatible tape, sealant, or another appropriate material.

EPA Indoor AirPlus guidance likewise addresses sealing or taping seams and penetrations to maintain continuity of the ground moisture barrier.

Loose, unsealed overlaps are not equivalent to a sealed encapsulation system.

4. Integrate the Vapor Retarder With Foundation Walls

For a fully encapsulated crawlspace, the ground vapor retarder should extend to the perimeter and be appropriately sealed and secured at foundation walls as part of the continuous moisture and air control system.

DOE guidance illustrates extending the ground cover at least 6 inches up walls and piers and securing it. EPA Indoor AirPlus guidance also addresses sealing the vapor retarder to crawlspace walls and piers.

The membrane should not simply be loosely turned up against the foundation wall.

A visible area may need to remain near the top of foundation walls where termite inspection requirements apply. The appropriate inspection gap depends on local requirements and pest management practices.

5. Seal Piers, Columns, and Penetrations

Interior support piers, columns, plumbing penetrations, structural supports, and other objects passing through the membrane are common weak points in encapsulation systems.

The vapor retarder should be fitted and sealed around these areas so the ground moisture barrier remains substantially continuous.

Common deficiencies observed during home inspections include:

• Plastic loosely cut around support columns
• Large openings around plumbing pipes
• Unsealed membrane around masonry piers
• Torn membrane around structural supports
• Unsealed seams in difficult access areas

What a Continuous Ground Moisture Barrier Looks Like

Summary based on U.S. Department of Energy Building Science Education guidance for crawlspace ground moisture control. See the federal references near the bottom of this page.

6. Seal Foundation Vents and Outside Air Openings

A closed or encapsulated crawlspace is fundamentally different from a traditionally vented crawlspace. When the crawlspace is intentionally converted to a closed system, foundation vents and significant exterior air openings are generally sealed as part of the enclosure.

Leaving exterior vents open while attempting to dehumidify or condition the crawlspace allows outdoor air to continually enter and can undermine the moisture control system.

7. Control Humidity After the Crawlspace Is Sealed

Sealing a crawlspace does not automatically make it dry.

Once outside ventilation is eliminated, the crawlspace needs an appropriate means of moisture control. Depending on the design and applicable requirements, this may include conditioned air, mechanical ventilation, dehumidification, or another approved method.

EPA Indoor AirPlus and DOE guidance treat moisture management as part of the complete closed crawlspace system rather than relying on the ground membrane alone.

A permanently installed hygrometer or remote humidity monitor can also be useful for monitoring crawlspace conditions after encapsulation.

8. Insulation Should Match the Crawlspace Design

Traditional vented crawlspaces commonly have insulation installed between the floor framing above the crawlspace. When a crawlspace is converted to a properly designed closed and conditioned space, the thermal boundary may instead be moved to the crawlspace perimeter walls.

The proper insulation method depends on the crawlspace design, climate, mechanical equipment, energy requirements, and applicable building code.

Existing floor insulation should not simply be removed because ground plastic has been installed. The crawlspace should first be evaluated as a complete enclosure system.

9. Sump Pits and Other Openings Require Attention

Sump pits and similar openings can provide pathways for moisture and soil gases into an encapsulated crawlspace. Where the sump is located within the encapsulated enclosure, an appropriate sealed cover may be necessary and penetrations through the cover should be sealed as applicable.

The drainage system must still be capable of performing its intended function.

10. Combustion Appliances Require Special Consideration

Gas furnaces, water heaters, and other fuel burning appliances located within a crawlspace must be considered before converting the space from vented to sealed.

Atmospheric combustion appliances may depend on surrounding air for proper combustion. Closing foundation vents and substantially air sealing the crawlspace without evaluating combustion air requirements can create unsafe operating conditions.

A qualified HVAC contractor should evaluate combustion appliances when crawlspace modifications could affect combustion air, drafting, or venting.

Why Crawlspace Vents Can Contribute to Moisture Problems

Foundation vents were traditionally intended to remove crawlspace moisture by allowing outdoor air to circulate beneath the home. This approach does not perform equally well in every climate or season.

During warm, humid weather, outdoor air entering a cooler crawlspace can increase relative humidity. If surfaces within the crawlspace are below the dew point of the entering air, condensation can occur.

This can contribute to fungal growth, wood decay, corrosion, damaged insulation, and other moisture related conditions.

Research conducted through the U.S. Department of Energy Building America program has demonstrated moisture and energy performance benefits from properly designed closed crawlspaces under appropriate conditions.

Common Encapsulation Deficiencies Found During Home Inspections

An encapsulation system may appear complete at first glance while still containing important defects. Conditions we commonly identify include:

• Exposed soil
• Loose or unsealed membrane seams
• Torn or damaged vapor barrier material
• Vapor barrier not sealed or secured at foundation walls
• Unsealed masonry piers or columns
• Openings around plumbing and other penetrations
• Open foundation vents in an otherwise encapsulated crawlspace
• Standing water above or beneath the membrane
• Unsealed sump pits
• No apparent means of controlling crawlspace humidity
• Improper, missing, or damaged insulation
• Conditions preventing an appropriate termite inspection

The presence of one or more of these conditions does not necessarily mean the entire encapsulation system requires replacement. Repairs may be possible depending on the installation and conditions present.

If Your Inspection Report Linked You Here

Your inspection report identifies the specific condition observed at the property. This page provides additional information explaining why that condition may matter and how a complete crawlspace moisture control system generally functions.

A home inspection is a visual evaluation and does not certify an encapsulation system or determine compliance with every installation specification. When the encapsulation is incomplete, damaged, or improperly installed, evaluation and appropriate repairs by a qualified crawlspace, waterproofing, insulation, or HVAC contractor may be recommended depending on the condition present.

Federal Building Science References

The following federal resources provide additional technical information regarding crawlspace vapor retarders, moisture control, air sealing, and closed crawlspace construction:

• 
  U.S. Department of Energy – Crawlspace Capillary Break at Crawlspace Floors
  
• 
  U.S. Department of Energy – Building America: Unvented, Conditioned Crawlspaces
  
• 
  U.S. Environmental Protection Agency – Indoor AirPlus
  

These federal publications contain building science guidance and, in some cases, requirements for specific voluntary programs. They do not establish that every existing home is required to have an encapsulated crawlspace. Applicable building codes, manufacturer instructions, pest management requirements, and local regulations should also be considered.

The Bottom Line

Plastic on the ground does not necessarily mean a crawlspace has been properly encapsulated.

A complete encapsulation system manages bulk water, ground moisture, outside air, penetrations, insulation, and crawlspace humidity together. The vapor retarder should function as a continuous system rather than a collection of loose sheets covering portions of the soil.

When properly designed and installed, crawlspace encapsulation can provide an effective means of controlling moisture and protecting the building structure. When portions of the system are missing, damaged, or improperly installed, those deficiencies should be corrected so the system can perform as intended.