Class I Vapor Retarders Not Installed in Above-Grade Walls in Warm-Humid Climate

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Scope

Don’t install polyethylene sheeting over batt insulation in warm humid climates; the sheeting can trap moisture in the wall providing a breeding ground for mold.
Don’t install polyethylene sheeting over batt insulation in warm humid climates; the sheeting can trap moisture in the wall providing a breeding ground for mold.

In warm-humid climates, do not install Class 1 vapor retarders on the interior side of air-permeable insulation in above-grade walls, except at shower and tub walls.

  • Don’t install materials with a perm rating of ≤0.1, for example, rubber membranes, polyethylene film, glass, aluminum foil, sheet metal, foil-faced insulating sheathings, and foil-faced non-insulating sheathings. 
  • Materials with a perm rating of ≤0.1 should note be installed on below-grade walls on the interior side of air-permeable insulation in any climate.
  • Class 1 vapor retarders (perm <0.1) can be used on the interior side of walls if air-permeable insulation is not present.
  • Class I vapor retarders, such as ceramic tile, may be used at shower and tub walls.
  • Class I vapor retarders, such as mirrors, may be used if they are mounted with clips or other spacers that allow air to circulate behind them.

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

Vapor retarders can be a crucial part of the building envelope and can help manage moisture issues to keep the home dry. However, before installing a vapor retarder (which slows or decreases the amount of water vapor movement), installers must be aware that in some instances vapor retarders can increase the likelihood of condensation issues.

In hot humid climates, condensation problems can occur in walls when vapor retarders are installed on the interior side of air-permeable insulation. Moisture flows from hot areas to cold areas and from dry areas to wet areas. So, if the interior air of the home is relatively cooler and drier than the exterior air, water vapor will naturally migrate through the walls from outside toward the inside of the home. If the wall insulation material is air permeable (such as mineral wool, fiberglass, or cellulose insulation), water vapor will pass through the insulation. If the wall contains a  vapor retarder on the interior side of the insulation, the water vapor will condense on this cool, impermeable surface. The vapor retarder could be a material attached to the insulation such as foil-faced paper, a traditional vapor barrier such as polyethylene film, or a vinyl wallpaper installed on the interior surface of the wall.

When this condensation occurs, the liquid water has no way to dry to the interior of the home and will accumulate in the wall. Over time, this dampness can lead to ruined insulation, mold, and structural rot of framing members.

You can reduce the risk of condensation in walls in hot, humid climates by understanding perm ratings for materials and constructing an appropriate wall assembly for the climate.

Mold growth

Figure 1 - Mold is growing on the inside surface of the wall because the plastic vapor barrier prevents the wall from drying to the inside.  Reference

Understanding Permeability Ratings on Vapor Retarders

Vapor permeability (commonly referred to as breathability) is a material’s ability to allow water vapor to pass through it. The moisture vapor transmission rate (MVTR) is the measurement referenced in building codes. The MVTR is measured in a lab using the American Society for Testing and Materials (ASTM) E-96. This test method measures how much moisture vapor is allowed to pass through a material in a 24-hour period (adjusted for vapor pressure across the sample). The resulting number is the moisture vapor permanence (MVP). The unit of measurement for MVP is perms. The higher the perm number, the more moisture vapor the material will allow to pass, and the better drying the material will allow. The water vapor permeability of a material is roughly inversely proportional to its thickness (i.e., doubling the thickness halves the permeability).

According to the IRC 2009 and 2012 definitions, vapor retarder classes are defined as follows:

  • Class I Vapor Retarder: 0.1 perm or less

  • Class II Vapor Retarder: 1.0 perm or less and greater than 0.1 perm

  • Class III Vapor Retarder: 10 perm or less and greater than 1.0 perm

Class 1 vapor retarders should not be used on the interior side of air-permeable insulation in above-grade exterior walls in warm-humid and hot-humid climates, or in below-grade exterior walls in any climate. The following materials are considered Class I vapor retarders:

  • rubber membranes

  • polyethylene film

  • glass

  • aluminum foil

  • sheet metal

  • foil-faced insulating sheathings

  • foil-faced non-insulating sheathings.

Note that this list is not comprehensive and other materials with a perm rating of 0.1 or less should not be used. Also, if manufacturer specifications for a specific product indicate a perm rating above 0.1, then the material may be used even if it is on this list.

Selecting the Appropriate Vapor Retarder with Air-Permeable Insulation
The main point to remember when using air-permeable insulation in warm-humid and hot-humid climates is that moisture passing through the wall must be allowed to dry to the inside of the building or it will become trapped and condense on the vapor retarder. It should also be noted that during summer months, when air conditioning is prevalent, condensation can also occur inside the home in localized areas where warm air comes in contact with cool surfaces.  Condensation can form on cold wall surfaces from outward diffusion of interior-born moisture and air exfiltration.

In warm-humid climates, any vapor retarder that is present in the wall assembly should be Class II or Class III (i.e., have a permeablility greater than 0.1 perm).  

ENERGY STAR permits materials considered Class I vapor retarders to be installed in the hot-humid climate in specific situations:

  1. Showers and tubs on exterior walls and tile, plastic, or fiberglass panels used for tub and shower surrounds

  2. Mirrors if mounted with clips or other spacers that allow air to circulate behind them.

How to Construct Walls with Appropriate Vapor Retarders Construction in Hot-Humid Climates
 

  1. Create a vented air gap to allow drainage behind the exterior siding. The vented air gap provides some relief for the water vapor passing through the backside of the siding. The minimum gap spacing depends on the siding material:
     a) For brick veneer or stone veneer, create a gap of 1 or 2 inches.
     b) For stucco siding, create a gap of 3/4 inch wide.
     c) For lap siding, create a gap of 1/6 inch wide (Baechler et al. 2011).

  2. Install materials on the inside surface that are Class II vapor retarders or Class III (greater than 0.1 perm), like gypsum board and latex paint, and allow the wall to dry to the interior. (Do not install Class I vapor retarders on the interior side of air-permeable insulation.)

  3. Ensure that no low-perm materials are attached to the inside of the wall as a finish (e.g., vinyl wallpaper or mirrors).

  4. Install 1 inch or more of plastic foam insulation as exterior sheathing to either replace or cover the OSB to further reduce the rate of water vapor passage into the wall and to provide additional insulation.

A Wall Assembly Example with Brick Veneer

Hot-humid and warm-humid climates present a unique challenge to controlling water vapor in and out of the building envelope. Consider for example a wall composed of brick veneer, which is prone to absorb water during heavy rain events, with high-perm building paper or wrap covering impregnated structural fiberboard sheathing over a fiberglass- or cellulose-insulated stud wall that has polyethylene film or vinyl wallpaper on the interior.  During the air-conditioning season, when the outdoor air is warm and humid, the vapor-drive is from outside to inside and the vapor retarder is in the wrong place, that is, on the cool side of the wall. This situation is made much worse after a rainstorm soaks the brick veneer. When the sun warms the wall, the vapor-drive towards the cooler wall components becomes many times greater as evaporating water in the pores of the brick increases the water vapor-drive towards the interior. However, if all the materials inboard of the brick have perm ratings greater than Class II (e.g., no vapor barrier, with painted gypsum board as the finish), the water vapor can pass through to the interior, thus avoiding moisture accumulation in the wall.

Figure 2 shows the appropriate construction of a brick veneer wall with a properly sized gap (or drained cavity) to halt or slow the migration of water vapor from the outside of the home (Lstiburek 2006). Bricks and other masonry absorb water from precipitation and irrigation. Solar energy will then drive this moisture in the form of vapor into the wall assembly. The 1-inch gap allows the vapor to dissipate before entering the wall cavity. The air space stops the capillary movement of moisture, discourages vapor diffusion, stops the contamination of the drainage plane via contact with the cladding, and allows air circulation for better drying. In some wall assemblies, ventilation openings to the exterior at both the top and bottom further encourage drying.

Wall assembly for a hot-humid climate

Figure 2 - Brick wall assembly for a hot-humid climate with no Class I vapor retarder and with an air gap (drained cavity) to dissipate vapor driven into the wall by the sun. Reference

Ensuring Success

In hot-humid climates, avoid Class I vapor retarders in the wall assembly. Construct walls with a vented air gap.

Climate

ENERGY STAR Version 3/3.1, (Rev. 08)

Water Management System Builder Requirements, Water-Managed Building Materials. 4.3 In Warm-Humid climates, Class 1 vapor retarders not installed on the interior side of air permeable insulation in above-grade walls, except at shower and tub walls. The 2009 IRC defines Class I vapor retarders as a material or assembly with a rating of ≤ 0.1 perm, as defined using the desiccant method with Procedure A of ASTM E 96. The following materials are typically rated at ≤ 0.1 perm and therefore shall not be used on the interior side of air permeable insulation in above-grade exterior walls in warm-humid climates or below-grade exterior walls in any climate: rubber membranes, polyethylene film, glass, aluminum foil, sheet metal, foil-faced insulating sheathings, and foil-faced non-insulating sheathings. These materials can be used on the interior side of walls if air permeable insulation is not present (e.g., foil-faced rigid foam board adjacent to a below-grade concrete foundation wall is permitted).  Note that this list is not comprehensive and other materials with a perm rating ≤ 0.1 also shall not be used. Also, if manufacturer specifications for a specific product indicate a perm rating above 0.1, then the material may be used, even if it is in this list. Also note that open-cell and closed-cell foam generally have perm ratings above this limit and may be used unless manufacturer specifications indicate a perm rating ≤ 0.1. Several exemptions to these requirements apply:

  • Class I vapor retarders, such as ceramic tile, may be used at shower and tub walls;
  • Class I vapor retarders, such as mirrors, may be used if they are mounted with clips or other spacers that allow air to circulate behind them.

climate zone map

International Energy Conservation Code (IECC) Climate Regions

Training

Right and Wrong Images

None Available

Presentations

None Available

Videos

  1. No Class I Vapor Retarders on Interior Side of Exterior Walls in Warm-Humid Climates
    Publication Date: July, 2015
    Courtesy Of: Train2Build

    Video describing vapor retarders used in Warm-Humid climates.

CAD Images

None Available

Compliance

ENERGY STAR Certified Homes

ENERGY STAR Certified Homes (Version 3/3.1, Revision 08), Water Management System Builder Requirements

4. Water-Managed Building Materials:

4.3 In Warm Humid climates, Class 1 vapor retarders not installed on the interior side of air permeable insulation in above-grade walls, except at shower and tub walls.7

Footnotes:

(7) The 2009 IRC defines Class I vapor retarders as a material or assembly with a rating of ≤ 0.1 perm, using the desiccant method with Proc. A of ASTM E 96. The following materials are typically ≤ 0.1 perm and shall not be used on the interior side of air permeable insulation in above-grade exterior walls in warm-humid climates or below grade exterior walls in any climate: rubber membranes, polyethylene film, glass, aluminum foil, sheet metal, and foil-faced insulating / non-insulating sheathings. These materials can be used on the interior side of walls if air permeable insulation is not present (e.g., foil-faced rigid foam board adjacent to a below-grade concrete foundation wall is permitted).

Note that this list is not comprehensive and other materials with a perm rating ≤ 0.1 also shall not be used. Also, if mfr. spec.’s for a product indicate a perm rating ≥ 0.1, then it may be used, even if it is in this list. Also note that open-cell and closed-cell foam generally have ratings above this limit and may be used unless mfr. spec.’s indicate a perm rating ≤ 0.1. Several exemptions to these requirements apply:

  • Class I vapor retarders, such as ceramic tile, may be used at shower and tub walls;
  • Class I vapor retarders, such as mirrors, may be used if mounted with clips or other spacers that allow air to circulate behind them.

Builders Responsibilities:  It is the exclusive responsibility of builders to ensure that each certified home is constructed to meet these requirements. While builders are not required to maintain documentation demonstrating compliance for each individual certified home, builders are required to develop a process to ensure compliance for each certified home (e.g., incorporate these requirements into the Scope of Work for relevant sub-contractors, require the site supervisor to inspect each home for these requirements, and / or sub contract the verification of these requirements to a Rater). In the event that the EPA determines that a certified home was constructed without meeting these requirements, the home may be decertified. 

ENERGY STAR Revision 08 requirements are required for homes permitted starting 07/01/2016.

DOE Zero Energy Ready Home

Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3.

More Info.

Contributors to this Guide

The following Building America Teams contributed to the content in this Guide.

University of Minnesota, lead for the NorthernSTAR, a DOE Building America Research Team
Building Science Corporation, lead for the Building Science Consortium (BSC), a DOE Building America Research Team
U.S. Environmental Protection Agency

Case Studies

None Available

References and Resources*

  1. Author(s): Baechler, Gilbride, Hefty, Cole, Adams, Noonan, Love
    Organization(s): PNNL, ORNL
    Publication Date: September, 2011

    Report describing measures that builders in mixed-humid climates can use to build homes that have whole-house energy savings of 40% over the Building America benchmark with no added overall costs for consumers.

  2. Author(s): BSC
    Organization(s): BSC
    Publication Date: May, 2009

    Brochure about creating an air barrier at tub, shower and fireplace walls.

  3. Author(s): DOE
    Organization(s): DOE
    Publication Date: May, 2015

    Standard requirements for DOE's Zero Energy Ready Home national program certification.

  4. Author(s): EPA
    Organization(s): EPA
    Publication Date: September, 2015

    Document outlining the program requirements for ENERGY STAR Certified Homes, Version 3 (Rev. 08).

  5. Author(s): EPA
    Organization(s): EPA
    Publication Date: October, 2015

    Website providing technical guidance to help home builders and their subcontractors, architects, and other housing professionals understand the intent and implementation of the specification requirements of the IAQ labeling program.

  6. Author(s): NorthernSTAR
    Organization(s): NorthernSTAR
  7. Author(s): EPA
    Organization(s): EPA
    Publication Date: February, 2011

    Guide describing details that serve as a visual reference for each of the line items in the Water Management System Builder Checklist.

Last Updated: 03/29/2016

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