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Water Management of Existing Crawlspace Floor

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Scope
Moisture problems must be dealt with before sealing and insulating a crawlspace
Moisture problems must be dealt with before sealing and insulating a crawlspace
Scope

Treat bulk water drainage issues and provide additional moisture control measures as needed to address site moisture issues before retrofitting a vented crawlspace to an unvented, insulated crawlspace.

See the Compliance Tab for links to related codes and standards and voluntary federal energy-efficiency program requirements.

Description
Description

When looking for opportunities to improve the energy efficiency of older single-family and multi-family residential buildings, sealing and insulating of vented crawlspaces is one retrofit measure that can improve the energy efficiency, comfort, and durability of the structure, particularly in climates with cold winters and/or hot humid summers. However, before any sealing and insulating of the crawlspace walls can take place, water drainage and moisture management issues in and around the crawlspace must be dealt with to ensure that the crawlspace will be dry and remain dry once enclosed.

See the guide Unvented Insulated Crawlspaces for more information on constructing sealed, insulated crawlspaces.

How to Address Water Management Issues in a Crawlspace

Several steps can be taken to address moisture issues in the crawlspace depending on the initial conditions at the site and the project budget:

  1. Reduce the chances of bulk intrusion of water into the crawlspace by re-grading the ground around the building to slope away from the structure, installing drainage pipes around exterior footings that drain to daylight or a drywell, and installing or modifying gutters and downspouts to ensure that they carry rainwater away from the foundation. Ensure that any existing concrete patios, sidewalks, or driveways slope away from the house. If they slope toward the house, consider replacing with pervious pavers or correctly sloped concrete, or install drains, for example at the base of a down-sloping driveway, to carry water away from the foundation.
  2. Block off and thoroughly seal crawlspace windows and vents with durable, waterproof materials that will prevent air and water leakage. Completely fill crawlspace window wells to above-grade levels.
  3. Install exhaust fans that will pull damp air out of the crawlspace, while indirectly pulling drier air from the dwelling unit into the crawlspace. This infusion of conditioned air will help to dry out the crawlspace.
  4. Install sump pumps in low-lying areas of the crawlspace floor that can be activated to carry away any water that accumulates at the crawlspace floor from seepage under the foundation walls or rising water table from periodic storm events. The sump pump should have a plastic pit that is 2-ft to 3-ft deep and perforated with holes drilled in the bottom so that if the water table rises (for example due to storm events), the water will be removed before it reaches the floor of the crawlspace. Use sump pumps that are equipped with gasketed, tight-fitting lids. For more on sump pumps, see the guide Drain or Sump Pump Installed in Basements or Crawlspaces.
  5. Install a vapor barrier of 6-mil or thicker polyethylene sheeting that completely covers the crawlspace floor and extends up the foundation walls and any piers by 8 to 12 inches, or preferably up to the exterior ground level. The plastic should be secured with pressure-treated wood furring strips, mechanical fasteners, or fiberglass mesh tape and duct mastic. Seams in the vapor barrier sheeting should be overlapped 6 to 12 inches and taped. This vapor barrier can be covered with a concrete slab for additional durability. For more information, see the guides Capillary Break at Crawlspace Floors - Polyethylene Lapped Walls and Piers and Secured in the Ground and Capillary Break Beneath Slab - Polyethylene Sheeting or Rigid Insulation.
Water-proof materials cover the exterior side of crawlspace windows in a crawlspace retrofit, while foil-faced rigid foams seals and insulates the inside face of the openings.
Figure 1. As part of a crawlspace retrofit from a vented to a sealed, insulated crawlspace, the crawlspace windows are sealed at the exterior with water-proofing materials and at the interior of the concrete block foundation wall with foil-faced rigid foam which provides insulation and air sealing.

 

An exhaust fan pulls damp air out of a retrofitted sealed crawlspace while drawing in dry air from the living space.
Figure 2. To remove moisture from a crawlspace that was sealed and insulated, an exhaust fan was installed in the crawlspace to draw damp air out of the crawlspace, thus creating a negative air pressure which drew conditioned air into the crawlspace through cracks in the floor.

 

Before sealing and insulating the crawlspace, the windows were sealed, the window wells backfilled, and sumps pumps were installed that discharged to the gutter downspouts.
Figure 3. As part of a crawlspace retrofit from a vented to a sealed, insulated crawlspace, the windows were sealed with water-proofing materials, the window wells were filled in to a height above grade, and sumps pumps were installed that discharged to the gutter downspouts.

 

Success
Ensuring Success

A period of post-retrofit monitoring should be conducted, preferably for one year, to track the crawlspace temperature and relative humidity conditions, and to test the moisture content of the rim and floor joists. If moisture accumulation is detected in the wood of the rim joists, mitigate by allowing air to reach the joists if necessary. For example if rigid foam has been installed in the rim joists and then moisture accumulation is detected in the wood, one-half inch of foam could be trimmed away around the joists to allow air to circulate around the wood (Rudd, 2014).

Climate
Climate

These recommendations for retrofitting a vented crawlspace to a sealed, insulated crawlspace apply primarily to climate zones 3 through 8. Crawlspaces are uncommon in climate zones 1 and 2, and the likelihood of termites in climate zones 1 and 2 discourages the use of below-grade foam insulation for unvented crawlspaces. 

The map in Figure 1 shows the climate zones for states that have adopted energy codes equivalent to the International Energy Conservation Code (IECC) 2009, 12, 15, and 18. The map in Figure 2 shows the climate zones for states that have adopted energy codes equivalent to the IECC 2021. Climate zone-specific requirements specified in the IECC are shown in the Compliance Tab of this guide. 

Figure 1. Climate Zone Map from IECC 2009, 12, 15, and 18.
Figure 1. Climate Zone Map from IECC 2009, 12, 15, and 18. (Source: 2012 IECC)
Climate Zone Map from IECC 2021.
Figure 2. Climate Zone Map from IECC 2021. (Source: 2021 IECC)

 

Training
Right and Wrong Images
Wrong – This vented crawlspace has standing water because steps were not taken to address site conditions that led to bulk water flow into the crawlspace
Wrong – This vented crawlspace has standing water because steps were not taken to address site conditions that led to bulk water flow into the crawlspace
Wrong – This vented crawlspace has standing water because steps were not taken to address site conditions that led to bulk water flow into the crawlspace
View Image Download
Right – Foil-faced insulation was added after the crawlspace was dried and sealed by diverting water runoff, sealing off crawlspace windows and vents, and adding sump pumps and exhaust fan ventilation
Right – Foil-faced insulation was added after the crawlspace was dried and sealed by diverting water runoff, sealing off crawlspace windows and vents, and adding sump pumps and exhaust fan ventilation
Right – Foil-faced insulation was added after the crawlspace was dried and sealed by diverting water runoff, sealing off crawlspace windows and vents, and adding sump pumps and exhaust fan ventilation
View Image Download
Right – A sump pump with a perforated sump pit was installed in the crawlspace slab to reduce water accumulation under the slab
Right – A sump pump with a perforated sump pit was installed in the crawlspace slab to reduce water accumulation under the slab
Right – A sump pump with a perforated sump pit was installed in the crawlspace slab to reduce water accumulation under the slab
View Image Download
Wrong – A poorly sealed window and window well allow bulk water to enter the crawlspace from ground that slopes toward the structure
Wrong – A poorly sealed window and window well allow bulk water to enter the crawlspace from ground that slopes toward the structure
Wrong – A poorly sealed window and window well allow bulk water to enter the crawlspace from ground that slopes toward the structure
View Image Download
Right – This crawlspace window was completely sealed off on the inside and outside with air barrier and water-proof materials and the window well was filled in to an above-grade height to prevent bulk water from entering the crawlspace
Right – This crawlspace window was completely sealed off on the inside and outside with air barrier and water-proof materials and the window well was filled in to an above-grade height to prevent bulk water from entering the crawlspace
Right – This crawlspace window was completely sealed off on the inside and outside with air barrier and water-proof materials and the window well was filled in to an above-grade height to prevent bulk water from entering the crawlspace
View Image Download
Right – Foundation drainage mat was installed over the waterproof membrane that covers the sealed crawlspace window
Right – Foundation drainage mat was installed over the waterproof membrane that covers the sealed crawlspace window
Right – Foundation drainage mat was installed over the waterproof membrane that covers the sealed crawlspace window
View Image Download
Wrong - A typical vented crawlspace in North Carolina exhibits water leakage, poor drainage, and a low-quality vapor retarder that does not cover all of the ground surface and is not sealed to the walls.
Wrong - A typical vented crawlspace in North Carolina exhibits water leakage, poor drainage, and a low-quality vapor retarder that does not cover all of the ground surface and is not sealed to the walls.
Wrong - A typical vented crawlspace in North Carolina exhibits water leakage, poor drainage, and a low-quality vapor retarder that does not cover all of the ground surface and is not sealed to the walls.
View Image Download
Wrong - The vapor retarder does not completely cover the pier block and is not sealed to the post; the support strapping pinches the flex duct.
Wrong - The vapor retarder does not completely cover the pier block and is not sealed to the post; the support strapping pinches the flex duct.
Wrong - The vapor retarder does not completely cover the pier block and is not sealed to the post; the support strapping pinches the flex duct.
View Image Download
Right - A technician wraps the foundation piers with a vapor retarder in preparation for laying vapor retarder over the floor of this crawlspace.
Right - A technician wraps the foundation piers with a vapor retarder in preparation for laying vapor retarder over the floor of this crawlspace.
Right - A technician wraps the foundation piers with a vapor retarder in preparation for laying vapor retarder over the floor of this crawlspace.
View Image Download
Right - Spray foam covers the walls and a sealed vapor retarder lines the floor of this unvented crawl space.
Right - Spray foam covers the walls and a sealed vapor retarder lines the floor of this unvented crawl space.
Right - Spray foam covers the walls and a sealed vapor retarder lines the floor of this unvented crawl space.
View Image Download
Wrong - The ground of the crawlspace should be covered with a vapor barrier that extends up the sides of the crawlspace.
Wrong - The ground of the crawlspace should be covered with a vapor barrier that extends up the sides of the crawlspace.
Wrong - The ground of the crawlspace should be covered with a vapor barrier that extends up the sides of the crawlspace.
View Image Download
Right - The posts and floor of this crawlspace are covered with a heavy sheet of vapor retarder that is sealed to the post, the walls, and at all seams.
Right - The posts and floor of this crawlspace are covered with a heavy sheet of vapor retarder that is sealed to the post, the walls, and at all seams.
Right - The posts and floor of this crawlspace are covered with a heavy sheet of vapor retarder that is sealed to the post, the walls, and at all seams.
View Image Download
Right - After wrapping the posts, a technician lays a vapor retarder over the floor of a crawlspace.
Right - After wrapping the posts, a technician lays a vapor retarder over the floor of a crawlspace.
Right - After wrapping the posts, a technician lays a vapor retarder over the floor of a crawlspace.
View Image Download
Wrong - The sump pump alone cannot address the water infiltration issues in this crawlspace.
Wrong - The sump pump alone cannot address the water infiltration issues in this crawlspace.
Wrong - The sump pump alone cannot address the water infiltration issues in this crawlspace.
View Image Download
Wrong - The sump pump pit should be covered with a gasketed lid.
Wrong - The sump pump pit should be covered with a gasketed lid.
Wrong - The sump pump pit should be covered with a gasketed lid.
View Image Download
A vapor barrier was installed on the floor of this crawlspace and extended up the walls then the foundation walls were covered with rigid foam.
A vapor barrier was installed on the floor of this crawlspace and extended up the walls then the foundation walls were covered with rigid foam.
A vapor barrier was installed on the floor of this crawlspace and extended up the walls then the foundation walls were covered with rigid foam.
View Image Download
Wrong - Siding is missing or broken and wood crawl space wall is unprotected.
Wrong - Siding is missing or broken and wood crawl space wall is unprotected.
Wrong - Siding is missing or broken and wood crawl space wall is unprotected.
View Image Download
Videos
Compliance

Compliance

ENERGY STAR New Homes and Apartments

DOE Zero Energy Ready Home

EPA Indoor airPLUS 

International Residential Code (IRC)

More

More Info.

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References and Resources*

Energy Efficient Crawlspace Foundation Retrofit: Mixed Humid Climate

Author(s)
Del Bianco Marie,
Wiehagen Joseph,
Wood Amber
Organization(s)
NAHB Research Center,
Home Innovation Research Labs,
HIRL,
National Renewable Energy Laboratory,
NREL
Publication Date
Report outlining the steps of translating a new home construction scope of work (SOW) to SOW for retrofit.

Conditioned Crawl Space Construction, Performance and Codes, BA-0401

Author(s)
Lstiburek Joseph W
Organization(s)
Building Science Corporation,
BSC
Publication Date
Report outlining how conditioned crawlspaces perform better than vented crawlspaces in terms of safety, health, comfort, durability and energy consumption.

Crawlspace Insulation

Author(s)
Southface Energy Institute,
Oak Ridge National Laboratory,
U.S. Department of Energy,
Office of Energy Efficiency and Renewable Energy
Organization(s)
ORNL,
SEI,
DOE
Publication Date
Information sheet about crawlspace insulation, including installation details.

Fixing Wet Basements and Crawlspaces

Author(s)
Todd Jason
Organization(s)
Home Energy Magazine
Publication Date
Article describing methods to manage moisture, rainwater, and vapor in basements and crawlspaces.
*For non-dated media, such as websites, the date listed is the date accessed.
Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Building Science Corporation, lead for the Building Science Consortium (BSC), a DOE Building America Research Team

Sales
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Building Science-to-Sales Translator

Water Managed Foundation = Foundation Water Barrier System

Technical Description

Ground water and rainwater can cause a lot of damage to a home. Building materials that are allowed to remain damp or saturated for long periods of time will eventually fail. Builders can take several steps to protect the home’s foundation. For example they can properly grade the site so water drains away from the home on all sides, install footing drains at the footing of the foundation walls that drain to daylight or to a French drain away from the home, build the foundation on a bed of aggregate rock, use a vapor barrier under slabs and on crawlspace floors, and damp-proof the exterior of foundation walls.

Alternate Terms

Dry-by-Design Foundation
Professionally-Installed Foundation Water Barrier
Foundation Water Barrier System
Sales Message

Foundation water barrier systems help drain water away from the slab, footing, and below-grade walls. What this means to you is peace-of-mind knowing your home has a comprehensive set of measures that minimize the risk of water damage in your basement. Wouldn’t you agree every home should have full water protection?

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