All About Vapor Diffusion

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All About Vapor Diffusion

Even if a material has no holes, water can often move right through it

Posted on Jun 12 2015 by Martin Holladay

Building scientists talk about several different moisture transport mechanisms. Most of these mechanisms — for example, water entry due to a roof leak — are easy to understand. Other transport mechanisms, like vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. , aren't quite as intuitive.

First, some basic definitions. Water vapor is water in a gaseous state — that is, water that has evaporated. It is invisible.

Water vapor diffusion is the movement of water vapor through vapor-permeable materials. Vapor diffusion happens through a solid material even when the material has no holes.

A typical example of vapor diffusion happens when a material — for example, gypsum drywall installed on a wall — separates two zones. If the air on one side of the drywall is very damp, and the air on the other side of the drywall is very dry, moisture in the air will diffuse through the drywall.

An example of diffusion

To understand how this happens, imagine how drywall takes on water when it is damp. On a dry day in Arizona, drywall is crisp. When scored with a sharp knife, it can be easily snapped. However, if a sheet of drywall is left for a week lying flat on a damp basement slab in Vermont, it gets limp and noodly. Drywall absorbs moisture like a sponge from either face, and it also dries out readily from either face via evaporation.

If the drywall is screwed to a stud wall that separates a damp area from a dry area, the drywall absorbs moisture on its damp side. Moisture evaporates from its dry side. The moisture has moved through the drywall by diffusion.

Two possible driving forces for diffusion

When water vapor diffuses through a vapor-permeable material, the driving force is either a vapor pressure difference (in which case the water vapor moves from the zone of higher vapor pressure to the zone of lower vapor pressure) or a temperature difference (in which case the water vapor moves from the warm zone to the cold zone) — or both.

What does it mean to say that the air on one side of a partition has a “higher vapor pressure” than the air on the other side of the partition? It means that a cubic foot of air in the zone of higher vapor pressure has more molecules of water than the same volume of air in the zone of lower vapor pressure.

Vapor diffusion is distinguished from convective vapor transport — that is, water vapor that moves by piggybacking on moving air. With convective vapor transport, the water goes around materials; with vapor diffusion, the water goes through materials.

If we look at water movement through a building assembly over time, we realize that the direction of vapor transport frequently changes. Inside your walls and ceilings, a constant ballet is underway, as moisture condenses, evaporates, diffuses through materials, and diffuses back again the other way, all in the course of a single day.

The difference between permeance and permeability

The vapor permeance of materials varies widely. Some materials — for example, gypsum drywall — are vapor-permeable. Other materials — for example, glass — are vapor-impermeable. It’s often useful to know where a material falls in this permeance range.

Before discussing this range of permeance values, it’s important to distinguish between permeance and permeability.

The vapor permeance of a material is a value that varies with the thickness of the material. That means that it makes no sense to discuss a material’s permeance unless you specify the thickness of the material under discussion. If you measure the permeance of a 2-inch-thick piece of material, you will usually get a different value than when you measure the permeance of a 1-inch-thick piece of the same material.

In the U.S., the unit we use to measure permeance is the perm (grains of water vapor/hour∙square foot∙inch Hg). In other words, one perm is equal to the transmission of 1 grain of water vapor per hour per square foot of material under a vapor pressure difference of 1 inch of mercury.

Europe uses a different unit to measure permeance, namely ng/Pa∙s∙m² (nanograms per Pascal per second per meter squared). This unit measures the mass of water in nanograms that is transmitted per second through one square meter of material at a vapor pressure difference of 1 Pascal.

To convert metric permeance (ng/Pa∙s∙m²) to U.S. perms, divide by 57.452.

To convert U.S. perms to metric permeance (ng/Pa∙s∙m²), multiply by 57.452.

Water vapor will pass more readily through a material with a high permeance than it will pass through a material with a lower permeance.

For example, 3 inches of open-cell spray foam might have a vapor permeance of 16 perms. But if we look at a thicker sample — say, 5 inches — of the same material, its vapor permeance drops to 10 perms. So it’s harder for water vapor to pass through 5 inches of spray foam than through 3 inches of spray foam.

Permeability is not the same as permeance. The permeability of a material is a material property — one that doesn’t change with thickness. Permeability is the measure of the ease with which water vapor passes through a unit thickness (in the U.S., one inch) of the material. The unit used to measure permeability in the U.S. is the perm-inch; in Europe, it is ng/Pa∙s∙m.

Dividing the permeability of a material by its thickness gives the material’s permeance (in perms).

Wet cup versus dry cup test

The permeance of a material is determined by a laboratory test, ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. E96. A vapor permeance test determines how much water vapor (measured in grains or nanograms) passes through a sample of a certain area (measured in square feet or square meters) in a defined amount of time (measured in seconds or hours) at a defined difference in vapor pressure (measured in inches of mercury or Pascals).

According to ASTM E96, permeance can be determined using either Method A (known as the desiccant method or the dry cup method) or Method B (known as the wet cup method).

Building codes that reference permeance values require that these values be determined by Method A, the dry cup method. Materials with very low permeance values (close to zero) are vapor barriers; materials with high permeance values are called “vapor-open” or “vapor-permeable” materials.

It’s fair to say that procedure A measures the vapor permeance of a material when it is dry, while procedure B measures the vapor permeance of a material when it is damp. The permeance of many materials (including asphalt felt, plywood, and OSB) is variable: when these materials are dry, they have a relatively low permeance; when they are damp, their permeance rises. (Some people refer to materials with a variable permeance as “smart retarders.”)

Permeance values are hard to pin down

If you try to create your own table of permeance values for building materials by collecting information from multiple sources, you will soon discover that different sources provide different values for common materials like #15 asphalt felt or 7/16" OSB. There are three reasons for this:

  • Materials produced by different manufacturers have different characteristics.
  • The permeance of some materials varies depending on moisture content, making permeance measurements difficult even when laboratory technicians try to use consistent test procedures.
  • The ASTM E96 procedure is difficult to perform and often produces inconsistent results.

William Rose, a research architect at the Building Research Council at the University of Illinois, elaborated on this last point in his landmark book, Water in Buildings. Rose wrote, "It is very difficult to conduct this test [the ASTM E96 test] well, and it is easy to wet the specimen in doing the wet-cup test. ... Unfortunately, results from ASTM E96 are not particularly dependable. Several round-robin test of water vapor permeance have been conducted, resulting in a wide range of values."

Multiple layers

How do you calculate the permeance of two layers of material? The formula is:

Total permeance = 1/ [1/(permeance of layer 1) + 1/(permeance of layer 2)]

A simple example is when a builder installs two layers of the same material — say, asphalt felt. In this simple example, the permeance of two layers of asphalt felt is half of the permeance of one layer of asphalt felt.

Building code definitions

These days, the International codes define three classes of vapor retarders:

  • Class I vapor retarders are rated at 0.1 perm or less. Examples include polyethylene sheeting, aluminum foil, and glass.
  • Class II vapor retarders have a rating greater than 0.1 perm but less than or equal to 1.0 perm. Examples include plywood, OSB, the kraft facing on fiberglass batts, 1-inch-thick XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. foam insulation, and one coat of vapor-retarder paint applied to drywall.
  • Class III vapor retarders have a rating greater than 1.0 perm but less than or equal to 10 perms. Examples include stucco, one or two coats of latex paint, and 1 inch of EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. foam insulation.

Permeance values for materials

As noted earlier, many materials have variable vapor permeance. These materials may have a relatively low permeance when dry, but a higher permeance when damp. Most vapor permeance tables (including the one provided below) list permeance values determined by the dry cup method — that is, the method used to determine permeance values in most building codes.

   Water Vapor Permeance of Building Materials

Material  Vapor Permeance   Notes
  (in U.S. perms)  
Aluminum foil, 1 mil    0.00  
Polyethylene sheeting, 6 mil    0.06  
MemBrain smart retarder    Less than 1.0   Up to 20 when damp
Solitex Mento air barrier membrane    38.0  
Kraft paper facing    0.30 to 1.0   Up to 4.2 when damp
One coat of vapor-retarder primer    0.45 to 0.9  
Ordinary primer, one coat    6.3  
2 coats of oil-based paint on plaster    1.5 to 3.0  
2 coats of latex paint on drywall    5.0  
XPS, 1/2 inch (Foamular)    1.5  
XPS, 1 inch    1.1  
XPS, 2 inches    0.55  
EPS, 1 inch    2.0 to 6.0  
PolyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. with foil facing, 1 inch    0.05  
Polyiso without facing, 1 inch    26.0  
Mineral wool, 4 inches    29.0  
Cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection., 4 inches    29.0  
Fiberglass insulation, 4 inches    29.0  
Open-cell spray foam, 3 inches    16.0  
Open-cell spray foam, 5 inches    10.0  
IcyneneOpen-cell, low-density spray foam insulation that can be used in wall, floor, and roof assemblies. It has an R-value of about 3.6 per inch and a vapor permeability of about 10 perms at 5 inches thick. MDR-200 med. dens. SPF, 3"   1.3  
Closed-cell spray foam, 1 inch    1.9 to 2.5  
Closed-cell spray foam, 2.5 inches    0.8  
Softwood lumber, 1 inch    0.40 to 5.4  
CDX plywood, 1/2 inch    0.5   Up to 20 when damp
OSB, 7/16 inch    2.0   Up to 12 when damp
Structural fiberboard sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , 1/2"    5 to 28   Varies by brand
DensGlass Gold sheathing    23  
Huber Zip System sheathing    2.0 to 3.0  
Huber Zip-R sheathing    Less than 1.0  
Thermo-Ply sheathing    0.5 to 0.6  
Brick, 4 inches    0.8  
Concrete, 1 inch    3.2  
Concrete block, 8 inches    2.4  
#15 asphalt felt    0.56 to 6.0   Up to 60 when damp
Tri-Flex Xtreme roof underlayment    0.04  
Cosella-Dörken Vent S underlayment    120  
Plaster on wood or metal lath    11.0 to 15.0  
Gypsum drywall    50.0  
Tyvek housewrap    77.0  
Tyvek CommercialWrap    28.0  
Typar housewrap    11.7  

Martin Holladay’s previous blog: “Cost of Passivhaus Compliance Is Sometimes Hard to Justify.”

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  1. Building Science Corporation

Jun 13, 2015 8:06 PM ET


Martin - Thank you for a nice list of perm ratings. Mineral wool and cellulose are rated the same. Should dense pack have an "asterisk" due to its ability to hold moisture, or does that effect or property of cellulose not officially play into permeability?

Jun 14, 2015 5:42 AM ET

Response to Kevin Zorski
by Allison A. Bailes III, PhD

Great question, Kevin. You are correct that what you're asking about is different. Permeability/permeance is a measure of moisture transport. What you're asking about is moisture storage. If you read my article on the physics of water in porous materials, you'll learn about how storage works. (I muddied the waters a bit, though, by choosing poor labels for the sorption isotherm shown there. I need to go back and change it to read "surface adsorption" and "capillary filling" rather than "surface diffusion" and "capilary flow.")

In short, the sorption isotherms is where you'd see the difference you're asking about, not the permeance.

Jun 14, 2015 5:51 AM ET

Edited Jun 14, 2015 6:11 AM ET.

Response to Kevin Zorski
by Martin Holladay

The lab technician who performs the ASTM E96 dry cup test has some latitude in determining the thickness of the test specimen. In the case of cellulose insulation, some method of supporting the material must be used (since the material has to be installed on top of a cup filled with desiccant for the test). I'm not sure how lab technicians do this when testing cellulose, but I imagine that the test specimen is supported by something like insect screening.

For information on ASTM requirements for determining the sample thickness, see the image below. (The image came from ASTM E96; here is the link.)

Here is further information on the test procedure: "Two of the methods to test the water vapor transmission of a material are desiccant (dry cup) and water (wet cup). These two tests are similar in setup but the service conditions are different and the results are not comparable in any way. The dry cup is designed to simulate a heated dry building during a pouring rain, measuring the drive into the building. The water, or wet, cup measures the vapor drive moving in the opposite direction.

"For assembling these tests ..., the desiccant or water is placed in a dish leaving a small gap (0.25” to 0.75”) of air space between it and the material. The test chamber is maintained at a constant temperature of 23°C (73.4°F) and a relative humidity of 50 ± 2%. An initial weight is taken of the apparatus and during the course of the test the weight change of the complete test assembly is measured until the results become linear.

"The water method assembly measures weight loss due to water vapor from the cup transmitting through the material to the test atmosphere as well as the humidity of the test chamber.

"The desiccant method assembly measures weight gain due to water vapor from the chamber transmitting through the material due to the desiccant absorbing any moisture from the material sample and the humidity from the test chamber that is being absorbed by the material.

"Based on the equilibrium change of weight, a water vapor transmission value is calculated, which provides an average permeance for the tested material. The product’s permeability may be derived from this calculation as well as if a material is permeable or non-permeable."

According to another source, "The water or desiccant is placed on the bottom of the dish within 1/4" of the material being tested which is attached to the mouth of the dish. The test chamber is maintained at a constant temperature of 73.4°F and a relative humidity of 50 ± 2%. During the course of the test, the weight change of the test assembly (material, cup and contents) is measured. The water method assembly will lose weight due to water vapor from the cup transmitting through the material to the material absorbing moisture from the material sample as well as the humidity of the test chamber. The desiccant method assembly will gain weight due to water vapor from the chamber transmitting through the material due to the desiccant absorbing any moisture from the material sample and the humidity from the test chamber that is being absorbed by the material. Based on the change of weight, a water vapor transmission value is calculated for materials using both methods.

"The designer or specifier shall ensure that when comparing WVT values of multiple materials, they are always comparing the identical methods of this test (water or desiccant). The best practice for the industry is for manufacturers of a fluid-applied material to list the WVT thickness at their specified installation thickness as the thickness of the material will determine the WVT value obtained.

"When reviewing a test table of WVT of thickness and values of a material, there may be a correlation between the two but the results are not linear. Therefore, if a material was tested at 'X' thickness and the WVT is 'Y', a designer cannot calculate and/or assume that if a material was specified and applied at half the thickness the WVT will be double, or if the thickness the WVT is double the WVP will be halved."

From the information in these sources, I'm guessing that the thickness of the cellulose samples used in the ASTM E96 dry cup tests leading to the values in the table are relatively thin -- too thin for the hygric buffering characteristics of cellulose to affect the results.

The question of how long the test must be performed is complicated, and subject to some judgment by the technician. ASTM E96 notes, "For low low permeability materials, this method can be used to determine the results after 30 to 60 days..." so clearly the test can sometimes take quite a while. The idea is to calculate a rate, so a time measurement is a necessary part of the calculation. But the rate calculation isn't simple. ASTM E96 notes, "A mathematical least squares regression analysis of the weight, modified by the dummy specimen when used, as a function of time will give the rate of water vapor transmission."


ASTM E96.jpg

Jun 14, 2015 11:46 PM ET

Permeance of cellulose vs. mineral wool

Allison and Martin - Thank you for your attempted answers. I can now say,tongue in cheek, " Oh, that clears it up." I did reread Allison's article, and figure that cellulose is more like the wood, and mineral wool is (relatively) more like the brick on those graphs.

Jun 15, 2015 5:18 AM ET

Response to Kevin Zorski
by Martin Holladay

My point is that permeance (not permeability) is measured by a lab test, and the lab test procedure is set out in ASTM E96.

Once you know the permeance of a material -- for example, cellulose -- you have important information to determine how that material will perform in a building assembly. But knowing a material's permeance doesn't tell you everything.

It's useful to know a material's R-value per inch, for example. And (as you point out) it's also useful to know if a material acts as a hygric buffer, as cellulose does.

A good designer takes all of this information into account -- permeance, R-value, hygric buffering characteristics, installation requirements, cost, and availability of contractors able to install the materials -- when designing a wall assembly.

Jun 22, 2015 7:11 AM ET

Polyiso board - foil vs fiber faced
by Roman Stankus

Martin, combining a couple of your past article subjects - I am in the process of prepping for doing a retrofit insulation upgrade to a 100 year old wood framed home with clapboard siding in south Georgia that currently has no insulation. My plan is to use mineral wool batts between the studs with an interior layer of 1.5 inch polyiso board covered with airtight 1/2 inch gyp bd. I plan on using fiber faced polyiso board - which according to your chart has a very high permeance. Seems like this would be be a very forgiving wall and would dry well to both inside and outside. is there any downside to not using a foil faced polyiso bd in this situation - seems like the fiber faced would be a better choice? Any air barrier differences between the two products (foil vs fiber faced polyiso)?

Jun 22, 2015 9:40 AM ET

Response to Roman Stankus
by Martin Holladay

Although my table lists a permeance value for unfaced polyiso, it does not list a permeance value for the product you intend to use (fiber-faced polyiso). There are several types of "fiber facing," and I'm not prepared to generalize about the vapor permeance of these products. The best way to determine the vapor permeance of the insulation you'd like to use is to contact the manufacturer.

That said, I wouldn't worry too much about the vapor permeance of the the interior rigid foam that you intend to install -- as long as your wall assembly can dry to the exterior. For more information on this type of wall, see Walls With Interior Rigid Foam.

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