SECTION 4.505 INTERIOR MOISTURE CONTROL
The regulatory text for this section was amended in July 2011 and is effective July 1, 2012
4.505.2 Concrete slab foundations. Concrete slab foundations required to have a vapor retarder by the California Building Code, Chapter 19 or concrete slab-on-ground floors required to have a vapor retarder by the California Residential Code, Chapter 5, shall also comply with this section.
4.505.2.1 Capillary break. A capillary break shall be installed in compliance with at least one of the following:
1. A 4-inch (101.6 mm) thick base of ½ inch (12.7 mm) or larger clean aggregate shall be provided with a vapor retarder in direct contact with concrete and a concrete mix design, which will address bleeding, shrinkage, and curling shall be used. For additional information, see American Concrete Institute, ACI 302.2R-06.
2. Other equivalent methods approved by the enforcing agency.
3. A slab design specified by a licensed design professional.
COMMENTARY
Purpose:
These provisions reduce movement of moisture into the slab as well as into the building. The size of the base material is specified. The vapor retarder is also required to be in direct contact with the concrete. Equivalent alternate methods or designed systems are also permitted.
Examples of Acceptable Methods of Implementation and/or Compliance:
Follow prescriptive requirements in this section.
Use appropriate concrete mix design and cure periods for area.
Obtain approval from the enforcing agency for an alternate design.
Use the design specified by a licensed California architect or engineer.
Background:
Concrete is frequently subject to cracks due to shifting of substrate, uneven stresses, or exposure to temperature extremes or chemical or biological processes. Therefore, vapor retarders are commonly being used in both residential and commercial applications to retard moisture
migration from beneath the slab. When selecting a vapor retarder several important physical properties should be considered such as, a low moisture vapor transmission rate, high tensile
strength, high puncture resistance, and resistance to chemical or environmental attacks. Vapor
retarders can be located and purchased in several thicknesses; consult an engineer to determine which product is best for your application. Both the California Building Code and California Residential Code reference a minimum 6 mil thickness vapor retarder for vapor retardant purposes.
Moisture penetrating the building envelope is a major concern when protecting indoor air quality. This practice provides a method to address the growing concern of water intrusion through the slab and foundation walls. A capillary break will provide a discontinuity and prevent water wicking from the ground and being absorbed and transmitted through the concrete slab and foundation. This section mandates the installation of a vapor retarder in all concrete slabs as part of a
capillary break. The retarder must be placed in direct contact with the slab thereby separating the aggregate layer from the concrete. The vapor retarder must be overlapped by a recommended 6 inches to ensure continuity and taped with a water resistive tape product.
CALGreen Section 4.505.2 specifically references concrete slab foundations required to have vapor retarders pursuant to the California Building Code or California Residential Code.
Although not required, additional information on installation and selection of vapor retarders is available in the American Concrete Institute’s publication “Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials”, ACI 302.2R-06 and ASTM International’s “Standard Practice for Selection, Design, Installation, and Inspection of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under Concrete Slabs”, ASTM Standard E1643-10.
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