Retrofit Existing Crawl Space Foundations with Cripple Walls to Increase Seismic Resistance and Thermal Performance

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    How to strengthen cripple walls with plywood panels.
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    When constructing or retrofitting homes with a crawlspace foundation with a “cripple” wall, connect the house walls to the foundation and connect the structural elements to each other to withstand seismic activity.

    • Connect the wall to the floor diaphragm.
    • Connect the floor diaphragm to the foundation with anchor bolts.
    • Install “cripple” wall bracing.
    • Install control layers to improve the energy efficiency and durability of the structure, including a rain control layer, an air control layer, a vapor control layer, and a thermal control layer.

    See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home programENERGY STAR Single-Family New Homes, and Indoor airPLUS.

    Description

    This guide describes options for providing seismic and thermal resistance for homes with a “cripple” wall crawlspace foundation. A cripple wall is a short wall that rests on the foundation and supports the floor and exterior walls (Figure 1). 

    How to strengthen cripple walls with plywood panels
    Figure 1. How to strengthen cripple walls with plywood panels (Source: IBHS 2001).

     

    Seismic Design Approach

    Earthquakes are a major threat to the structural integrity of homes. Most of the damage to a building during an earthquake is caused by lateral movements, which disconnect the house from its foundation. Uplift forces can also occur. These lateral loads (“shear”) and uplift forces due to ground movement need to be transferred to the ground to minimize the likelihood of the building being pulled apart. To withstand seismic stresses, the structural elements should be connected to each other and to the foundation.

    Although unrelated to earthquake resistance, when retrofitting a foundation to make it more earthquake resistant, the foundation should also be assessed to see if the insulation, air sealing, and moisture protection could be improved to make the home more thermally efficient and durable. 

    The following issues should be addressed to improve the ability of new and existing homes with crawlspace foundation cripple walls to resist seismic activity.

    Building-to-Foundation Connection

    Figure 2 illustrates a construction or retrofit approach for a vented crawlspace foundation with a cripple wall. In the retrofit of crawlspace foundations with cripple walls, the structural connection of the continuous exterior wood sheathing (plywood or oriented strand board [OSB]) to the foundation sill plate is necessary. Also, note that it is recommended that cripple wall retrofits have structural wood sheathing on both sides. Further note the foundation sill plate is connected to the concrete foundation with a foundation anchor bolt. In this manner, the wall and floor diaphragms are connected to each other and to the foundation. It may be necessary to add sill blocking if the interior faces of the existing studs are not flush with the interior face of the existing foundation sill.

    If there are interior posts and piers, additional bracing should be provided by installing cross bracing from the post to the floor beam on both sides of the posts. Floor framing should also have solid blocking of dimensional lumber installed between the floor joists. (The blocking is shown in Figure 3 as the white boxes with black outlines between the floor joists.) The solid wood blocking prevents the floor joists from racking or rolling sideways.

    Vented crawlspace cripple wall has seismic retrofits – plywood is added on interior of cripple walls and fastens to extra blocking added at sill plate and connected to foundation with new anchor bolts
    Figure 2. Vented crawlspace cripple wall has seismic retrofits – plywood is added on to the interior of cripple walls and fastens to extra blocking added at sill plate and connected to foundation with new anchor bolts (Source: Building Science Corporation).    

     

    For seismic retrofit of crawlspaces with posts and piers, add cross bracing to posts; add cross bracing and solid blocking between floor joists. (Blocking appears white in drawing; should be made of dimensional lumber.)
    Figure 3.  For seismic retrofit of crawlspaces with posts and piers, add cross bracing to posts; add cross bracing and solid blocking between floor joists. (Blocking appears white in drawing; should be made of dimensional lumber.)  (Source: Building Science Corporation).

     

    Thermal Efficiency

    When retrofitting the cripple wall, take a look at the thermal, air, and moisture control layers; these provide the environmental separation that is the basis for all thermal efficiency. Environmental separation requires a continuous water control layer (rain and groundwater), a continuous air control layer (air barrier), a vapor control layer, and a thermal control layer (insulation).

    Figure 2 illustrates one option for installing the environmental separation layers in a vented crawlspace foundation with a cripple wall. In this case, the exterior above-grade wall is covered with stucco.

    A continuous water control layer for controlling rainwater consisting of two layers of building paper is installed on the exterior of the wall structural sheathing and is connected to the continuous water control layer on the exterior of the concrete foundation stem wall. The outer layer of the building paper acts as a bond break to facilitate drainage. The bottom of the stucco assembly should have a weep screed that acts as a flashing. The below-grade water control layer, not shown in the figure, could be a fluid-applied waterproofing membrane or dimpled plastic drain mat.

    The air control layer of the exterior wall is typically the building paper; in some assemblies, it is a fluid-applied air barrier or house wrap. The air control layer of the underside of the floor framing is the continuous rigid insulation. This rigid insulation should be fire rated and all joints and seams should be taped and sealed. The insulation is sealed to the interior of the perimeter cripple wall framing. The rigid insulation is also protected from animals, rodents, and insects with a protection board, such as fiber cement.    

    Vapor control is provided by the rigid insulation installed on the underside of the floor framing if foil-faced insulation is used. Additional vapor control is provided by installing a continuous polyethylene ground cover that is connected to the interior of the concrete foundation stem wall and by providing crawlspace ventilation. Figure 4 illustrates the continuity of the polyethylene ground cover at interior posts and piers.

    The polyethylene ground cover in the crawl space is lapped up sides of piers to posts to provide a continuous air and vapor barrier
    Figure 4. The polyethylene ground cover in the crawl space is lapped up sides of piers to posts to provide a continuous air and vapor barrier (Source: Building Science Corporation).

     

    Building code crawlspace ventilation requirements should be met in new and retrofit applications. Most building codes require a minimum net area of ventilation opening of 1 square foot for every1,500 square feet of under-floor area when a Class I vapor retarder is used. The continuous polyethylene ground cover is a Class I vapor retarder. Additionally, the required crawlspace ventilation openings should be placed to provide cross ventilation of the space. It is recommended that at least four ventilation openings be located around the perimeter of the building. Ventilation openings should not be located under low decks or porches.

    If a Class I vapor retarder ground cover is not installed, the net ventilation opening area needs to be increased to a minimum of 1 square foot for every 150 square feet of under-floor area. Note that the number of openings and vent area needs to be increased by an order of magnitude and this could adversely affect the ability of the retrofitted cripple wall to resist seismic activity.

    Thermal control is provided by the continuous rigid insulation and cavity insulation installed on the underside of the floor framing. 

    Ensuring Success

    Consult a licensed architect or engineer to develop a detailed design and approach for homes to withstand seismic activity. The seismic design can occur in conjunction with thermal efficiency where the approaches are complementary rather than incompatible.

    Climate

    Earthquake Areas

    The approaches to seismic control will work in all climates. However, check local building codes for specific requirements as seismic risk and requirements vary based on location; see map below. Insulation requirements for thermal efficiency are climate dependent; see the Compliance tab and consult local code for requirements. 

    The International Residential Code (IRC) takes a building’s seismic risk into account based on location. The IRC contours the United States into seismic design categories, from low risk to high risk as shown in Figure 1, which designates the categories by letter: A, B, C, D0, D1, D2, and E, with A designating the lowest risk and E designating areas with the highest risk. The IRC has design guidelines for categories A through D2 as well as scenarios for when a building in design category E can be reassigned to category D2. If a building located in design category E cannot be reassigned to category D2, then it must be designed using the International Building Code (IBC), not the IRC.

    Seismic map of the 2018 International Residential Code adapted by FEMA to show Seismic Design Categories in color
    Figure 1. Seismic map of the 2018 International Residential Code adapted by FEMA to show Seismic Design Categories in color (Source: FEMA 2020). 

     

    Right and Wrong Images
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    Possible failure scenarios due to house sitting on poorly braced and secured cripple wall
    Possible failure scenarios due to house sitting on poorly braced and secured cripple wall
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    How to strengthen cripple walls with plywood panels.
    How to strengthen cripple walls with plywood panels.
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    Wrong – The cripple wall beneath this house failed in an earthquake
    Wrong – The cripple wall beneath this house failed in an earthquake
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    The cripple walls in this home gave way in the 1994 Northridge Earthquake, causing the walls to partially collapse.
    The cripple walls in this home gave way in the 1994 Northridge Earthquake, causing the walls to partially collapse.
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    The cripple wall hiding the post-and-pier foundation of this wood framed house toppled when the house was shifted partially off its piers by an earthquake
    The cripple wall hiding the post-and-pier foundation of this wood framed house toppled when the house was shifted partially off its piers by an earthquake
    Image
    For seismic resistance in basement, crawlspace, and crawlspace “cripple” wall foundations, connect the plywood or OSB sheathing to the wall framing, rim joist, and sill plate and anchor bolt the sill plate to the foundation
    For seismic resistance in basement, crawlspace, and crawlspace “cripple” wall foundations, connect the plywood or OSB sheathing to the wall framing, rim joist, and sill plate and anchor bolt the sill plate to the foundation
    Image
    Right – Construction adhesive is applied to the rigid foam panels that will be installed on the interior basement walls.
    Right – Construction adhesive is applied to the rigid foam panels that will be installed on the interior basement walls.

    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.

     

    ENERGY STAR Single-Family New Homes, Version 3/3.1 (Rev. 11)

    ENERGY STAR Single-Family New Homes requires that ceiling, wall, floor, and slab insulation levels meet or exceed those specified in the International Energy Conservation Code (IECC) (2009 IECC for Version 3.0 and 2012 IECC for Version 3.1), with some alternatives and exceptions, and achieve Grade 1 installation per RESNET Standards. Builders must also meet or exceed the locally mandated requirements. Visit the U.S. DOE Building Energy Codes Program to see what code has been adopted in each state.

    Please see the ENERGY STAR Single-Family New Homes Implementation Timeline for the program version and revision currently applicable in your state.

    DOE Zero Energy Ready Home (Revision 07)

    Exhibit 1 Mandatory Requirements.
    Exhibit 1, Item 1) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.
    Exhibit 1, Item 2) Ceiling, wall, floor, and slab insulation shall meet or exceed 2015 IECC levels and achieve Grade 1 installation, per RESNET standards.

    2009201220152018, and 2021 International Residential Code (IRC)

    Section 301.2.2 Seismic Provisions. Discusses determination of seismic design categories based on building location. See the seismic map on the Climate tab.

    Section R403 provides details on footings including reinforcement requirements for footings based on Seismic Design Categories A-C and D0, D1, and D2.

    Section R403.1.6 discusses foundation anchorage

    [Seismic Categories A and B]

    Wood sole plates at all exterior walls on monolithic slabs, wood sole plates of braced wall panels at building interiors on monolithic slabs, and all wood sill plates shall be anchored to the foundation with minimum ½-inch-diameter anchor bolts spaced not greater than 6 feet on center or approved anchors or anchor straps spaced as required to provide equivalent anchorage to ½-inch-diameter anchor bolts. Bolts shall extend at least 7 inches into the concrete or grouted cells of concrete masonry units. The bolts shall be located in the middle third of the width of the plate. A nut and washer shall be tightened on each anchor bolt. There shall be at least two bolts per plate section with one bolt located no more than 12 inches and no less than seven bolt diameters (≥3.5 inches) from each end of the plate section… (see code for additional details).

    Section R403.1.6.1 Foundation Anchorage in Seismic Design Categories C. D0, D1, and D2

    In addition to the requirements of Section R403.1.6, the following requirements shall apply to wood light-frame structures in Seismic Design Categories D0, D1, and D2, and wood light-frame townhouses in Seismic Design Category C.

    1. Plate washers conforming to Section R602.11.1 shall be provided for all anchor bolts over the full length of required braced wall lines except where approved anchor straps are used. Properly sized cut washers shall be permitted for anchor bolts in wall lines not containing braced wall panels.

    (See code for additional details.)

    R602.9 discusses cripple walls.

    Table R602.10.3(3) gives bracing requirements based on seismic design category.

    Section R602.10.10 discusses cripple wall bracing.

    Section R602.11 discusses wall anchorage.

    2009201220152018, and 2021 International Energy Conservation Code (IECC)

    Minimum Insulation Requirements: The minimum insulation requirements for ceilings, walls, floors, and foundations in new homes, as listed in the 2009, 12, 15, 18, and 21 IECC and IRC, can be found in this table

    Retrofit: 2009201220152018, and 2021 IECC

    Section R501.1.1 in 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.)

    Existing Homes

    The approach to seismic retrofit is the same as the approach to new design for seismic resistance. The basis of all earthquake resistance is to control and transfer lateral loads (“sheer”) caused by ground movement to foundations and the ground. The Earthquake Overview Guide in the Building America Solution Center provides extensive guidance on retrofitting measures to strengthen cripple walls.

    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.

    References and Resources*
    Author(s)
    Collaborative for Disaster Mitigation,
    San Jose State University
    Organization(s)
    California Seismic Safety Commission
    Publication Date
    Description
    Guidance on the design and retrofit of earthquake resistant housing.
    Author(s)
    Joseph Lstiburek
    Organization(s)
    Building Science Corporation,
    BSC
    Publication Date
    Description
    Information sheet with methods for constructing or retrofitting basements to reduce moisture issues.
    Author(s)
    Lstiburek
    Organization(s)
    Building Science Corporation,
    BSC
    Publication Date
    Description
    Article discussing whether or not to seal and insulate crawlspaces.
    Author(s)
    Building Seismic Safety Council of the National Institute of Building Sciences
    Organization(s)
    FEMA,
    NEHRP
    Publication Date
    Description
    This document presents the minimum recommended requirements necessary for the design and construction of new buildings to resist earthquake ground motions; These provisions, which have been adopted into ASCE/SEI 7, were developed by the National Earthquake Hazards Reduction Program (NEHRP), a...
    Author(s)
    Federal Emergency Management Agency
    Organization(s)
    FEMA
    Publication Date
    Description
    Guide provides homeowners ways to prepare for an earthquake and protect a home against earthquake damage.
    *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

    Pacific Northwest National Laboratory

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