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Water Management of Existing Crawlspace Floor

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

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 related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.

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 Crawlspaces and Conditioned Basements 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 Polyethylene Lapped Up Walls/Piers and Secured in the Ground and Concrete Slab over Polyethylene.
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. 

 

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

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. 

IECC Climate Zone Map
IECC Climate Zone Map

 

Training

Right and Wrong Images

Presentations

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Videos

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CAD Images

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Compliance

The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

200920122015, and 2018 IRC

R408.3 Unvented crawl space. Ventilation openings in under-floor spaces specified in Sections R408.1 and R408.2 shall not be required where:

1. Exposed earth is covered with a continuous Class I vapor retarder. Joints of the vapor retarder shall overlap by 6 inches (152 mm) and shall be sealed or taped. The edges of the vapor retarder shall extend at least 6 inches (152 mm) up the stem wall and shall be attached and sealed to the stem wall; and

2. One of the following is provided for the under-floor space:

2.1. Continuously operated mechanical exhaust ventilation at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7m2) of crawlspace floor area, including an air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.9 (N1102.2.10 in 2012 IRC, N1102.2.11 in 2015 and 2018 IRC);

2.2. Conditioned air supply sized to deliver at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7 m2) of under-floor area, including a return air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.9 (N1102.2.10 in 2012 IRC, N1102.2.11 in 2015 and 2018 IRC);

2.3. Plenum in existing structures complying with Section M1601.5, if under-floor space is used as a plenum.

[2018 IRC Only] 2.4. Dehumidification sized to provide 70 pints (33 liters) of moisture removal per day for every 1,000 square feet (93 m2) of crawl space floor area.

Retrofit: 200920122015, and 2018 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018 IRC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.

More Info.

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

Case Studies

None Available

References and Resources*

  1. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2009

    Code for residential buildings that creates minimum regulations for one- and two-family dwellings of three stories or less. It brings together all building, plumbing, mechanical, fuel gas, energy and electrical provisions for one- and two-family residences.

  2. Author(s): Lstiburek
    Organization(s): Building Science Corporation
    Publication Date: November, 2004

    Report outlining how conditioned crawlspaces perform better than vented crawlspaces in terms of safety, health, comfort, durability and energy consumption.

  3. Author(s): Building Science Corporation
    Organization(s): Building Science Corporation
    Publication Date: May, 2009

    Information sheet about crawlspace insulation, including installation details.

  4. Author(s): Dastur, Davis, Warren
    Organization(s): Advanced Energy
    Publication Date: February, 2012
    Guide about designing and installing closed crawlspaces.
  5. Author(s): Bianco, Wiehagen, Wood
    Organization(s): NAHB Research Center, National Renewable Energy Laboratory
    Publication Date: January, 2013

    Report outlining the steps of translating a new home construction scope of work (SOW) to SOW for retrofit.

  6. Author(s): Rudd
    Organization(s): Building Science Corporation
    Publication Date: September, 2014

    Report analyzing results in crawlspace retrofits in multifamily buildings.

  7. Author(s): Dickson
    Organization(s): IBACOS, National Renewable Energy Laboratory
    Publication Date: January, 2013

    Report discussing principles and approaches to closing and conditioning ventilated crawlspaces in existing homes. 

  8. Author(s): Karagiozis
    Organization(s): ORNL, Advanced Energy
    Publication Date: June, 2005
    Report outlining results from a 2005 hygrothermal pilot study in the South.

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

Last Updated: 02/09/2015