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 to cold areas and from dry 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. If the wall insulation material is air permeable (such as mineral wool, fiberglass, or cellulose insulation) water vapor will pass through the insulation and condense on the vapor retarder. This retarder can include material attached to the insulation, such as foil-faced paper, traditional vapor barriers such as polyethylene film, or vinyl wallpaper 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. Over time, this dampness can lead to ruined insulation, mold, and structural rot of framing members.
To reduce the risk of condensation in walls in hot, humid climates, you must:
- Understand perm ratings and how to select the right vapor retarder for walls in hot and humid climates to reduce condensation risks.
- Construct an appropriate assembly for the inside of a wall that promotes the proper water management strategy for the home.
Figure 1 - Mold growth. Water passing through this wall is unable to dry to the interior because of a vapor retarder, resulting in mold growth and compromised indoor air quality.
Understanding Permeability Ratings on Vapor Retarders
If water vapor is allowed to pass through the wall and insulation, it must be allowed to dry inward in order to avoid condensation on the vapor retarder. To do this, the wall covering on the interior of the framing must have a relatively high vapor permeability rating.
The 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
The following materials are typically rated as Class 1 vapor retarders and therefore 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:
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 order to protect the home, the steps to remember are as follows:
Select a vapor retarder that is Class II or higher (greater than 0.1 perm) for the interior of a wall with air permeable insulation.
Install a Class III vapor retarder in areas of excessive moisture, which will allow for the greatest amount of drying and is often the best choice.
Several exemptions to these requirements apply:
In some situations there are exemptions to the perm rating rule when it comes to vapor retarders used in above-grade walls in a hot-humid climate. These exemptions include:
Showers and tubs on exterior walls: Class I vapor retarders, such as ceramic tile, may be used in shower and tub walls. The insulation behind the tub or shower should be equivalent to the insulation in the rest of the exterior walls and should be covered with an air barrier of cement backer board, rigid foam insulation, or non-paper-faced drywall that is sealed at the edges and seams to provide a continuous air seal. The recommended backer for exterior walls behind showers and tubs (in both above- and below-ground situations) is cement board. Note that cement board is not waterproof and must be coated with fluid-applied waterproofing or a water-resistive barrier that allows drainage (BSC 2009
). The low-perm vapor retarder is allowed due to the high moisture content created by the shower or tub, and is used to protect the framing and insulation from excessive moisture. Another alternative would be to find tile backer specifically designed for use behind showers and tubs.
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. Unlike other wall coverings, such as ceramic tile or vinyl wall paper, mirrors can be installed with enough space behind them to allow moisture to migrate through the air permeable insulation and dry to the interior. If the mirror is directly fastened to the wall, it will act as a vapor barrier, and condensation will form on the foundation side of the mirror.
Overall Steps for Appropriate Wall Construction in Hot-Humid Climates
To successfully construct a wall in a hot and humid climate, observe the following steps:
Create a vented air gap to allow drainage behind the exterior siding (EPA 2012
). The vented air gap provides some relief for the water vapor passing through the backside of the siding. These gaps vary depending 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
Install materials on the inside surface that are Class II vapor retarders or higher (greater than 0.1 perm), like gypsum board, and allow the wall to dry to the interior. (Do not install Class I vapor retarders on the interior side of air permeable insulation.)
Ensure that no low-perm materials are attached to the inside of the wall as a finish (e.g., vinyl wallpaper) or a furnishing (e.g., a mirror affixed to the wall) because these might accidentally create a vapor barrier in the wrong place.
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 movement in and out of the building envelope. Consider for example a wall composed of a 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 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 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 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.
Figure 2 - Wall assembly for a hot-humid climate. This image shows the correct construction of a brick wall in a hot-humid climate. Note the gap (drained cavity) behind the brick and not that as designed, there is no Class I vapor retarder.