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Table 1-1 Sheathing Wrap Peformance (C) Wiley Sons - S BlissMaterial Permeability Numbers
Building materials perm ratings, standards, codes

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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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.



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

Water trapped in poly vapor barrier (C) D FriedmanArticle Contents

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:

Definition of Vapor Retarder or Vapor Barrier

Typar house wrap example (C) Daniel FriedmanA 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.

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:

* 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

Plastic moisture barrier installed in a crawl space following mold remediation (C) Daniel FriedmanIn 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 (FOUNDATION WATERPROOFING SEALANTS), while other water barriers block only water, but not water vapor (moisture) such as housewraps (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.

Perm Ratings for Building Materials ordered by Perm Rating

Building Material Perm Rating Comments
Glass 0  
Aluminum Metal foil, 0.001" 0.01 ICA
Aluminum Metal foil, 1u / 0.00035" 0.05 Example: aluminum-foil-faced insulation or reflective vapor barriers, before considering air leaks at material edges.
Cork, Rubber, or Vinyl flooring 0.05 Estimate. NAHB
Polyethylene sheet, 2 mil 0.16 ICA
Polyethylene sheet, 4 mil    
Polyethylene sheet, 6 mil 0.06 Example: 6 mil poly vapor barrier used as ground cover in a crawl space,
     
Asphalt-coated paper backing on insulation 0.40

Note 1.

See FELT 15# ROOFING, as HOUSEWRAP/VAPOR BARRIER

Paint, Vapor Retarder Latex 0.0031" 0.45  
Insulation, Foil-Faced Kraft Laminate 0.5 ICA
Plywood, exterior, 1/4" 0.70 ICA
Insulation, Kraft-Faced 1.0 ICA

Materials below this line are not good vapor retarders

Plywood, 1/4", interior doug fir 1.9 ICA
OSB Sheathing 1/2" 1-7  
Drywall, 3/8" latex-painted, 2 coats 2.0 - 3.0  
Building paper, asphalt-saturated felt 15# 5

Note 1. FELT 15# ROOFING, as HOUSEWRAP/VAPOR BARRIER

Building paper, rosin paper 5 Note 1. FELT 15# ROOFING, as HOUSEWRAP/VAPOR BARRIER
Vapor-barrier paint, latex 5-9  
Drywall, un-painted, /12" 23.2 USG
Drywall, un-painted, 1/2"

20-90

Perm rating varies depending on ambient humidity levels ranging from 20 in very humid areas to 90 in very dry areas. Typical is a perm of 50.
Drywall, un-painted, 5/8" 26.7 USG
Insulation, mineral wool, un-faced, 4" 30 ICA
Housewrap, plastic, perforated 9-48  
Housewrap, non-perforated 48-59 Examples, Tyvek®, R-Wrap®, Amowrap® - HOUSEWRAP PRODUCT CHOICES
Drywall, un-painted, 3/8" 50  
     

Notes:

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.

Table of Permeability Ratings for Sheathing Wrap Products & for Building Sheathing Products

Material type Uses Advantages Disadvantages Perm
Rating
Comments & Recommendations
Asphalt felt, saturated felt, "tar paper" Sheathing wrap under siding & roofing Moderate resistance to liquid water. Absorbs & stores water. High permeability when wet Deteriorates if exposed to sunlight; prone to tearing; 5

Use min 15 lb. material, good option for all types of siding. Use with rain screen. Do not rely on for air-barrier

See FELT 15# ROOFING, as HOUSEWRAP/VAPOR BARRIER

See also RAIN SCREEN PRINCIPLES

Grade D building paper, red rosin paper Sheathing wrap under stucco Resists water from 20-60 minutes as per rating Deteriorates if saturated 5

Use two layers of rated material under stucco

See STUCCO WALL WEEP SCREED DRAINAGE

See also STUCCO WALL METHODS & INSTALLATION

Foil facing Insulation face, radiant barriers Waterproof

Traps water if any leaks into the building cavity

Easily torn

Close to zero

Used with or as part of insulating products, Not used as a housewrap without additional steps and in hot humid climates.

See SHEATHING, FOIL FACED - VENTS

See RADIANT BARRIERS

Housewrap: Tyvek & similar products Sheathing wrap under siding Resist water penetration, permit water vapor passage Water vapor permeability & perm ratings vary widely by product.

6.5-29 (Kaellner 2006)

48-59 (Bliss 2006), 77 (Gibson 1994)

See
Plastic housewrap, perforated Sheathing wrap under siding Good permeability Low resistance to liquid water 9 - 48 Use in relatively dry areas
Plastic housewrap, not perforated Sheathing wrap under siding Most have high permeability and high resistance to liquid water, good air barrier if taped Can trap liquid water from wall leaks 9 - 59

Good option for all types of siding. Use with rain screen. Can also serve as air-infiltration barrier if taped at all seams & edges.

 

Draining housewraps Sheathing wrap under siding, drainage plane. Provides drainage plane and air space for rain screen Effectiveness varies by construction details. Variable

Good option for all types of siding in areas subject to wind-blown rain.

See HOUSEWRAP PRODUCT CHOICES

Plywood, 1/2" CDX Wall & roof sheathing Structural material; absorbs & stores water depending on coating; May function as a buffer by absorbing and releasing moisture over time. 1-30

Not intended as sole vapor barrier in most construction.

Permeability varies depending on ambient relative humidity.

See also PLYWOOD Roof, Wall, Floor Decks & Sheathing

OSB, 1/2" Wall & roof sheathing Structural material; absorbs & stores water depending on coating Lower perm than 1/2" CDX plywood, less variability 1-7

Not intended as sole vapor barrier in most construction.

Permeability varies depending on ambient relative humidity.

See OSB - ORIENTED STRAND BOARD SHEATHING

Notes:

Adapted and expanded from Table 1-1, Sheathing Wrap Performance, [image file] found in Bliss, Steven. Best Practices Guide to Residential Construction: 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

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

Research & More Tables of Building Material Permeability

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Continue reading at MOISTURE CONTROL in BUILDINGS, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see INSULATION R-VALUES & PROPERTIES where we give perm ratings for various insulating materials

Or see these

Water Barrier & House Wrap Articles

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