Permeability of Common Building Material to Water Vapor

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WHAT IS A PERM RATING?

If a material has a perm rating of 1.0, 1 grain of water vapor will pass through 1 square foot of the material, provided that the vapor pressure difference between the cold side and the warm side of the material is equal to 1 inch of mercury (1 inch Hg).

As temperature and RH go up, vapor pressure gets higher. The greater the vapor pressure differential across or through a material, the greater the tendency for water vapor to migrate from the high pressure side to the low pressure side.

EFFECT OF MATERIAL THICKNESS

The perm ratings given are for stated thicknesses of materials. Generally, doubling material thickness halves water vapor transmission: if 1 inch of a material has a perm rating of 2.0, then for 2 inches, the perm rating would be 1.0. With paints, however, adding a second coat more than halves the water vapor transmission.

ALASKAN VAPOR BARRIERS

Because of Alaska’s wintertime vapor pressure differentials and the lengths of the cold spells, the ideal vapor barrier has a perm rating approaching 0.0. The most widely used vapor barrier is 6-mil polyethylene, which has a perm rating of 0.06. Given the combination of high RH indoors and very cold weather outside, measurable amounts of water vapor will pass through 6-mil polyethylene. For high-moisture buildings, such as those housing swimming pools or Jacuzzis, 10-mil polyethylene is often specified. In practice, however, it is not usually the perm rating of the water vapor barrier which determines how much water will pass into the insulation, but the quality of the vapor barrier installation. A carefully installed, well-sealed 4-mil polyethylene vapor barrier is much preferred to a 6-mil vapor barrier with unsealed seams, gaps, tears at electrical boxes and unsealed attic scuttle openings.

QUALITY OF DATA

Perm ratings are established by testing. Several different test methods are used, and different tests on the same materials yield different results. Published perm ratings are a design guide. When the vapor barrier performance of a material is critical to an installation, it is best to rely on manufacturer’s specifications, or conduct an independent test.

A SAMPLE CALCULATION

For demonstration purposes, we shall now assume a set of conditions and make a calculation concerning the amount of water vapor that will migrate through a vapor barrier. The relevant formula is:

WVT = A x T x delta P x perms

WVT stands for water vapor transmission. Unit of measurement: grains. One pound equals 7,000 grains.

A means area. Unit of measurement: square feet (ft2). We will assume that this is a two-story house, 24 feet wide by 40 feet long by 17 feet high. The area of the vapor barrier, then, is about 3,900 ft2, allowing 10 percent of floor area for doors and windows.

T means time. Unit of measurement: hours. We will perform this calculation for the month of January, which has 744 hours.

Delta P means difference in vapor pressure between inside and outside. Unit of measurement: inches of mercury (inches Hg). In order to derive this number, we must assume a temperature and a relative humidity (RH) for both inside and out. Let the inside temperature be 70°, and the outside -10°, which is about average for January in Fairbanks. Let the inside RH be 40 percent and the outside be 70 percent. The table of vapor pressures gives figures for saturated air (100 percent RH); to get the vapor pressure at an RH less than 100 percent, you multiply by the percent RH. In this case, (.7392 x 40%) - (.022 x 70%) = .2803 inches Hg.

Perms means perm rating. Unit of measurement: grains of water vapor per hour per square foot per inch of mercury vapor pressure differential (gr/ft2/hr/inches Hg). We will assume that the average perm rating of the installation (including unpatched tears, nail holes, etc.) is 0.1 perms.

With these assumptions, then:

WVT = A x T x Delta P x Perms

3900 x 744 x .2803 x 0.1 = 81332

grains of water. This is 11.6 pounds, or about 1.4 gallons.

Notes:

1. Extrapolated from "Wrap Wars," Alex Wilson, New England Builder, August 1987.
2. Values in brackets, [ ], obtained from DuPont literature on Tyvek.
3. Values in brackets, { }, low because film installed backwards (worst-case situation).
4. All other values obtained from company literature or personal communication with company.
5. Perm: Vapor transmission rate of 1 grain of water vapor/ft2/hr/in. of mercury pressure difference.
6. The data are provided to permit comparison of material and selection of air-vapor barrier or weather barrier materials.
7. Exact values should be obtained from the manufacturer of material.
8. A continuous air-vapor barrier is usually placed on the warm side of the shell before the gypsum board (drywall) is placed.
9. The air-vapor barrier should have a permeance of less than 1 perm.
10. The weather barrier should have a high permeance, a low water resistance, a high air porosity, high tensile strength, and high tear resistance.
11. The compilation is from a number of sources; values from dry-cup and wet-cup methods were usually obtained from investigations using ASTM E96 and C355. Other values were obtained using techniques such as two-temperature, special cell, and air velocity.

Source: Axel R. Carlson, Professor Emeritus, University of Alaska Fairbanks, Fairbanks, Alaska, 9/3/87

Notes:

1. Extrapolated from "Moisture in Building Construction," ASHRAE, 1985.
2. Tables give water transmission rates of representative materials.
3. Perm: Vapor transmission rate of 1 grain of water vapor/ft2/hr/in. of mercury pressure difference.
4. Exact values should be obtained from the manufacturer of material.
5. The air-vapor barrier should have a permeance of less than 1 perm.
6. The air barrier should have high permeance, low water resistance, high air porosity, high tensile strength, and high tear resistance.
7. The compilation is from a number of sources; values from dry-cup and wet-cup methods were usually obtained from investigations using ASTM E96 and C355. Other values were obtained using techniques such as two-temperature, special cell, and air velocity.

Source: Axel R. Carlson, Professor Emeritus, University of Alaska Fairbanks, Fairbanks, Alaska, 9/3/87

Notes:

1. Extrapolated from, "Interior Wall Coverings For Moisture Control," James A. Lindley and Helen A. Lunde, Agricultural Engineering and Home Economics Departments, North Dakota State University, Fargo, North Dakota, 1987.

Source: Axel R. Carlson, Professor Emeritus, Cooperative Extension Service, University of Alaska Fairbanks, Fairbanks, Alaska, 9/21/87.

To simplify information, trade names of products have been used. No endorsement of named products by the University of Alaska Fairbanks Cooperative Extension Service is intended, nor is criticism implied of similar products that are not mentioned.

Art Nash, Extension Energy Specialist. Originally prepared by Richard Seifert, former Extension Energy and Housing Specialist.

Reviewed March 2021