All About Vapor Diffusion
All About Vapor Diffusion
Even if a material has no holes, water can often move right through it
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."
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
|(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|
Martin Holladay’s previous blog: “Cost of Passivhaus Compliance Is Sometimes Hard to Justify.”
- Building Science Corporation
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