Building material moisture permeability or perm ratings.

Here we explain the movement of moisture or water molecules through building materials, we give definitions of perm, vapor barrier, vapor retarder, moisture barrier, and housewrap.

We provide tables of perm ratings for common building materials and for varieties of housewraps or sheathing wraps.

This article series discusses how to inspect, diagnose problems in, and install or repair building insulation & ventilation systems including heat loss, moisture, & interior stains.

Guide to Sources of Building Leaks & Moisture Problems: causes & cures

Article Contents

• PERM RATINGS of BUILDING MATERIALS
• DEFINITIONS & PHYSICS of MOISTURE MOVEMENT
• TABLE PERM RATINGS for BUILDING MATERIALS
• TABLE of PERM RATINGS for HOUSEWRAPS
• RESEARCH & MORE PERM TABLES

Definitions of Moisture Movement, Perms, Vapor Retarder, Moisture Barrier, Water Barrier

Basic physics of moisture movement

Because warm air can hold more moisture than cool or cold air, and because that will create greater water vapor pressure on the warm side than on the cooler side of a material, moisture will move from the warmer air or warmer side and higher humidity side of a building surface through the building material to the cooler side of the material.

Why does warm air hold more moisture than cold air?

Metaphorically we might say that warmer air has more space among the gas molecules comprising air. That provides space for water molecules. But that's not scientifically correct. Cold air is not really squeezing out water molecules.

More-accurately, a fall in temperature slows the speed of movement of water molecules in air, causing water molecules to stick together to form water droplets or condensation. Those heavier droplets then fall out of air to condense on a cool surface.

If you need to know the gases that make up air, see CONCENTRATIONS of GASES in AIR.

Water Vapor Transmission Rate

The rate of water vapor transmission through a material (WVT) can be calculated simply as

WVT = Area x Time x deltaP x PermRating

Area = the area of the material surface

Time = the time period of exposure in hours

deltaP = difference in water vapor pressure on the two sides of the material

PermRating = the moisture permeability rating of the material

Details and a nice example of this calculation are given in U. Alaska's

PERMEABILITY of COMMON BUILDING MATERIAL to WATER VAPOR [PDF] cited below. If you view this file, use your browser's "back" button to return to this article.

Definition of Perm & Perm Rating

Perm is a quantitative value that measures the relative ease with which moisture (water vapor) will pass through a material.

A perm-rating of 1.0 for any material is defined as follows: 1 grain of water vapor will pass through 1 square foot of the material when the moisture-vapor pressure difference between the colder side and the warmer side of the material is equal to 1 inch of mercury (1 in. Hg).

How to convert grains to pounds or pounds to grains: 1 grain = 1/7000 pounds or 0.00014286 pounds. 1 pound = 7000 grains.

Permeability & water vapor movement vs. Amount of Moisture, Humidity, Temperature & Material Thickness

The actual rate of movement of water vapor through a material is not a constant. It is the difference in water vapor pressure on two sides of a material that determines the rate of water vapor through the material. Conditions that affect the movement of water vapor (water molecules) through a material are:

• Water Vapor pressure: The greater the difference in moisture on the two sides of a material, the greater the rate of moment of moisture through the material.
• Relative humidity: An increase in RH on one side of a material increases the vapor pressure on the material.
  
  More about humidity and an explanation of relative humidity is
  
  at HUMIDITY MEASUREMENT INSTRUMENTS
  
  See DEW POINT TABLE - CONDENSATION POINT GUIDE for details about the dew point and how to measure or calculate it for a building area or surface.
• Temperature: an increase in temperature on one side of a material increases the vapor pressure on that side of the material. Similarly, a drop in temperature on one side of a material will decrease the water vapor pressure on that side.
• Material thickness: doubling the thickness of most materials cuts the material's permeability in half.
  
  Interestingly, adding a coating of moisture-barrier paint cuts the moisture permeability of the total paint coating by more than half.

Definition of Vapor Retarder or Vapor Barrier

A vapor retarder is any material that resists moisture movement at a level sufficient to comply with a specific building code or standard for moisture control.

The measurement of a vapor retarder's ability to resist the movement of moisture through the vapor retarder is given in perms.

The lower the perm number the greater is the material's resistance to moisture movement.

• Building materials with a perm rating of less than 1 are considered resistant to moisture movement through the material. Those make good vapor barriers.
• Building materials with a perm rating greater than 1 are considered to pass moisture more-easily from warm to cold sides. Those do not make good vapour barriers.

The black 15# felt shown on the exterior of 3 Willowbrook Hgts., a Poughkeepsie New York home under construction in the 1970's, has a perm rating of about 5.

A vapor barrier is, in practical terms, a synonym for vapor retarder. Some texts prefer the word retarder to barrier because barrier implies 100% blockage of water molecules or moisture.

While few materials provide a 100% barrier to moisture movement (glass would be an example), a number of materials have such low perm ratings as to serve effectively as a moisture barrier.

IBC International Building Code & IRC International Residential Code Vapor Retarder Specifications

Vapor retarder class shall be defined using the desiccant method* of ASTM E 96 as follows:

• Class I vapor retarder: 0.1 perm or less.
• Class II vapor retarder: 0.1 < perm <1.0 perm.
• Class III vapor retarder: 1.0 < perm <10 per m.

* The desiccant testing method specifies that testing is conducted at a 25% relative humidity or RH.

Watch out: builders and building diagnosticians note that indoor relative humidity levels are normally well above 25% RH. See HUMIDITY CONTROL & TARGETS INDOORS for details.

Definition of Moisture Barrier

In building science the term moisture barrier is a synonym for vapor barrier (or vapour barrier).

In our photograph a heavy plastic has been placed over the dirt floor and on the lower foundation walls of a crawl space as a moisture barrier (or vapor barrier) in order to keep moisture from wet soils out of the crawl area.

Definition of Water Barrier

If we note the distinction between water vapor (water molecules dispersed in air) and liquid water, a water barrier is a material specifically intended to keep water out of a building.

A water barrier keeps out water.

In our photograph above the heavy plastic is acting as a vapor barrier, moisture barrier, and water barrier combined.

Some water barriers block both water and water vapor, such as foundation waterproofing materials , while other water barriers block only water, but not water vapor (moisture) such as housewraps

See details at

FOUNDATION WATERPROOFING SEALANTS

HOUSEWRAP PRODUCT CHOICES

Is Housewrap a Water Barrier or a Moisture Barrier?

Housewrap is designed to block water but to pass water vapor. Thus housewrap is a water barrier but it is not a moisture barrier.

Housewrap, with a perm rating that may range from about 6.5 to 60 depending on the material, relies on the distinction between water barrier and moisture barrier to keep wind-blown water from entering the building walls from outside, while at the same time the house wrap permits water vapor or moisture within a wall cavity to escape or pass through to the building exterior.

Housewrap is also an air barrier, preventing wind or air currents from entering the building wall cavity.

HOUSEWRAP AIR & VAPOR BARRIERS gives detailed properties of this material.

Common Building Material Perm Ratings

Here are permeability numbers for some common building materials. Our references below include perm data for many other materials.

Notes to the table above:

Find below the technical bulletins, papers, standards used as sources for these perm ratings.

1. Grade D building papers must have a 10-minute rating under ASTM D779, commonly called the “boat test,” in which a piece of building paper is folded in the shape of a boat and floated in a dish of water until it soaks through and wets a powder on top. Some Grade D papers are rated as high as 60 minutes.

Permeability of Building Housewraps & Sheathing-Wraps

General performance characteristics of sheathing wraps are summarized the table below. Because of differences in data sources the perm ratings for some of these materials may vary a bit from those given in the general building materials permeability table given above.

The table above gives perm ratings for common building sheathing wrap products.

Notes to the table above

Adapted and expanded from Table 1-1, SHEATHING WRAP PERFORMANCE, [image file] found in Bliss, Steven. Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) : materials, finishes, and details. John Wiley & Sons, 2006.

• Ahuja, Deepak, B. Matthew Smith, and M. Arch. "Potential Pitfalls of a Green Building Material: A Case Study of Cellulose Insulation." In Forensic Engineering 2012@ sGateway to a Safer Tomorrow, pp. 239-249. ASCE, 2013.
• Gibson, Scott. "Air and Vapor Barriers." Fine Homebuilding 4, no. 94 (1994): 48-53.
• International Building Code (2006),
  Excerpts
  1404.2 Water-resistive barrier. A minimum of one layer of No. 15 asphalt felt, complying with ASTM D 226 for Type 1 felt, shall be attached to the studs or sheathing, with flashing as described in Section 1405.3, in such a manner as to provide a continuous water-resistive barrier behind the exterior wall veneer.
  
  2510.6 Water-resistive barriers. Waterresistive barriers shall be installed as required in Section 1404.2 and, where applied over wood-based sheathing, shall include a water-resistive vapor-permeable barrier with a performance at least equivalent to two layers of Grade D paper.
  
  Exception: Where the water-resistive barrier that is applied over wood-based sheathing has a water resistance equal to or greater than that of 60 minute Grade D paper and is separated from the stucco by an intervening, substantially non-water-absorbing layer or drainage space.
  
  R703.1 General. Exterior walls shall provide the building with a weather-resistant exterior wall envelope. The exterior wall envelope shall include flashing as described in Section
  
  R703.8. The exterior wall envelope shall be designed and constructed in such a manner as to prevent the accumulation of water within the wall assembly by providing a waterresistive barrier behind the exterior veneer as required by Section R703.2 and a means of draining water that enters the assembly to the exterior.
  
  Protection against condensation in the exterior wall assembly shall be provided in accordance with Chapter 11 of this code.
  
  R703.2 Water-Resistive Barrier. One layer of No. 15 asphalt felt, free from holes and breaks, complying with ASTM D 226 for Type 1 felt or other approved waterresistive barrier shall be applied over studs or sheathing of all exterior walls.
  
  Such felt or material shall be applied horizontally, with the upper layer lapped over the lower layer not less than 2 inches (51 mm).
  
  Where joints occur, felt shall be lapped not less than 6 inches (152 mm). Such felt or other approved material shall be continuous to the top of walls and terminated at penetrations and building appendages in such a manner to meet the requirements of the exterior wall envelope as described in Section R703.1.
  
  Exception: Such water-resistive barrier is permitted to be omitted in the following situations:
  
  1. In detached accessory buildings.
  
  2. Under exterior wall finish materials as permitted in Table R703.4.
  
  3. Under paperbacked stucco lath, when the paper backing is an approved weather-resistive sheathing paper.
  
  R703.6.3 Water-resistive barriers. Waterresistive barriers shall be installed as required in Section 1404.2 and, where applied over wood-based sheathing, shall include a water-resistive vapor-permeable barrier with a performance at least equivalent to two layers of Grade D paper.
  
  Exception: Where the water-resistive barrier that is applied over wood-based sheathing has a water resistance equal to or greater than that of 60 minute Grade D paper and is separated from the stucco by an intervening, substantially non-waterabsorbing layer or drainage space.
• Kaellner, Bradford R., "Attachment# 14 Impact Of Title 24 Residential Leakage Reduction Credit On Water-resistive Barriers In California Homes", Architectural Energy Corporation, Boulder CO, USA, for California Energy Commission, Public Interest Energy Research Program, (2006).
• Listiburek, Joseph W., "Chubby CHecker and the "Fat Man" Do Permeance", ASHRAE Journal, June 2015 pp. 58-64

Research & More Tables of Building Material Permeability

• 2005 ASHRAE Handbook : Fundamentals [bookstore link] Inch-Pound Edition (2005 ASHRAE HANDBOOK : Fundamentals : I-P Edition) (Hardcover), Thomas H. Kuehn (Contributor), R. J. Couvillion (Contributor), John W. Coleman (Contributor), Narasipur Suryanarayana (Contributor), Zahid Ayub (Contributor), Robert Parsons (Author), ISBN-10: 1931862702 or ISBN-13: 978-1931862707
• 2004 ASHRAE Handbook : Heating, Ventilating, and Air-Conditioning [bookstore link] Systems and Equipment : Inch-Pound Edition (2004 ASHRAE Handbook : HVAC Systems and Equipment : I-P Edition) (Hardcover) by American Society of Heating, ISBN-10: 1931862478 or ISBN-13: 978-1931862479
• 1996 Ashrae Handbook Heating, Ventilating, and Air-Conditioning Systems and Equipment: Inch-Pound Edition [bookstore link] (Hardcover), ISBN-10: 1883413346 or ISBN-13: 978-188341334
• ASTM E 96 Standard Test Methods for Water Vapor Transmission of Materials, available from ASTM https://www.astm.org/Standards/E96
  Abstract:
  The purpose of these tests is to obtain, by means of simple apparatus, reliable values of water vapor transfer through permeable and semipermeable materials, expressed in suitable units.
  
  These values are for use in design, manufacture, and marketing. A permeance value obtained under one set of test conditions may not indicate the value under a different set of conditions. For this reason, the test conditions should be selected that most closely approach the conditions of use.
  
  While any set of conditions may be used and those conditions reported, standard conditions that have been useful are shown in Appendix X1.
  
  These test methods cover the determination of water vapor transmission (WVT) of materials through which the passage of water vapor may be of importance, such as paper, plastic films, other sheet materials, fiberboards, gypsum and plaster products, wood products, and plastics. The test methods are limited to specimens not over 11/4 in. [32 mm] in thickness except as provided in Section 9.
  
  Two basic methods, the Desiccant Method and the Water Method, are provided for the measurement of permeance, and two variations include service conditions with one side wetted and service conditions with low humidity on one side and high humidity on the other.
  
  Agreement should not be expected between results obtained by different methods. The method should be selected that more nearly approaches the conditions of use.
• ASTM Technical Publication MNL40, Moisture Analysis and Condensation Control in Building Envelopes (2001)
• Best Practices Guide to Residential Construction [bookstore link] by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Amazon.com and also Wiley.com. See our book review of this publication.
• ICA, TECHNICAL BULLETIN No. 6, USE OF VAPOR RETARDERS (2004) [PDF] Insulfoam (Carlisle), - retrieved 2018/03/17, original source: https://www.insulate.org/tb6.pdf
• Insulfoam, EPS vs XPS DATA SHEET [PDF] (2021) Insulfoam Co., Carlisle, Tel: (800) 248-5995 Web: www.Insulfoam.com - retrived 2022/07/15 original source: https://www.insulfoam.com/wp-content/uploads/2021/07/IF-14188-EPS-vs-XPS-Sell-Sheet_07-07-21.pdf
• Kaellner, Bradford R., "Attachment# 14 Impact Of Title 24 Residential Leakage Reduction Credit On Water-resistive Barriers In California Homes", Architectural Energy Corporation, Boulder CO, USA, for California Energy Commission, Public Interest Energy Research Program, (2006).
• NAHB, ICC & ICC Evaluation Service, FLOORS ABOVE CRAWL SPACES: Reducing the Risk of Moisture Accumulation with Wood Floor Assemblies for Residential Buildings in Hot-Humid Climates [PDF], (2014) retrieved 2018/03/17, original source: https://www.homeinnovation.com/~/ media/Files/Reports/ TechNotes-Floors-above-Crawl-Spaces-June-2014.pdf
• Zbyšek Pavlík, Robert Černý. HYGROTHERMAL PERFORMANCE STUDY of an Innovative Interior Thermal Insulation System [PDF] Applied Thermal Engineering, Elsevier, 2010, 29 (10), pp.1941.
  ff10.1016/j.applthermaleng.2008.09.013ff. ffhal-00608407f - retrieved 2021/02/19, original source: https://hal.archives-ouvertes.fr/hal-00608407/document
  
  Abstract:
  An innovative interior thermal insulation system on the basis of hydrophilic mineral wool which can serve as an alternative solution to the commonly used systems with water vapor barrier is presented in the paper. At first, the process of materials design is described.
  
  Then, the hygrothermal performance of the designed insulation system is tested in the difference climate conditions that correspond to the winter climate in Middle Europe.
  
  In the experiment, the profiles of temperature, relative humidity and liquid moisture content are monitored. Measured temperature profiles demonstrate the proper thermal insulation function of the system.
  
  The hygric function can also be considered very good as no water condensation during the whole testing period of five months appears in the insulation layer. Therefore, the basic requirements for the successful application of the system in building practice are met.
• PERMEABILITY of COMMON BUILDING MATERIAL to WATER VAPOR [PDF], EEM-00259, University of Alaska, Fairbanks, Cooperative Extension Service, retrieved 2018/03/18, original source: https://www.uaf.edu/files/ces/publications-db/catalog/eeh/EEM-00259.pdf
• Toman, Jan, Alena Vimmrova, and Robert Černý. "Long-term on-site assessment of hygrothermal performance of interior thermal insulation system without water vapour barrier." Energy and Buildings 41, no. 1 (2009): 51-55.
  Abstract
  
  On-site assessment of hygric and thermal performance of an interior thermal insulation system on the basis of hydrophilic mineral wool is presented in the paper. The system is applied during the summer period on a brick-built house from the end of 19th century.
  
  Hygrothermal testing is done during the time period of 4 years. The reconstructed building envelope exhibits very good hygrothermal performance.
  
  The thermal resistance increases approximately two times.
  
  Water condensation is never observed inside the envelope during the whole testing period.
  
  A comparison with the previous experiment in semi-scale conditions which was performed for similar insulation system constructed during the winter season shows that the precedence should be given to the application of the system in summer period as it is more considerate to the old masonry.
• U.S. DOE Website: www.eere.energy.gov)
• USG, Sheetrock® GYPSUM SHEATHING SUBMITTAL SHEET 09250 [PDF] U.S. Gypsum Corporation, retrieved 2018/03/17, original source: https://www.usg.com/content/dam/USG_ Marketing_Communications/ united_ states/product_ promotional_ materials/finished_assets/ sheetrock-gypsum-sheathing-submittal-WB2380.pdf
• Weston, PhD., Theresa, VAPOR PERMEABILITY of MATERIALS & ASSEMBLIES - Determining the When and Where of Vapor Retarders, [PDF] (2016), DuPont Protective Solutions, ICC Annual Conference Education Program, Kansas City MO, ICC, retrieved 2018/03/17, original source: http://media.iccsafe.org/ Annual/2016/Vapor-Permeability-of-Materials-and -Assemblies-Determining-the- When-and-Where-of-Vapor-Retarders.pdf

...

Continue reading at INSULATION R-VALUES & PROPERTIES where we give perm ratings for various insulating materials, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

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• MOISTURE in BUILDING WALLS, EFFECTS
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