Vapor Permeance: 7 Minutes of BS—Building Science w/a Beat

This episode features Jonathan Smegal, M.A. SC a Senior Project Manager with RDH Building Science Laboratories talking about how moisture moves through materials and what it means to walls and roofs.

What it is:

va·por per·me·ance | ˈvāpər pərmēəns — n (noun)

"Vapor permeance is the property of a material—and in our discussions this will be building materials—which permits the passage of water vapor through it."

—Jonathan Smegal, RDH Building Science Laboratories

How vapor permeance works

"Some materials have high vapor permeance which means they allow a lot of water vapor to pass and other materials have significantly less vapor permeance, so they block water vapor movement and these are called vapor barriers or vapor retarders."

Terms that most of us are familiar with, even if not completely clear on their differences.

"The amount of water vapor that passes through a material is dependent on the vapor permeance of that material and the amount of water vapor, also called vapor pressure, on each side of the material."

The permeance not only depends on the material itself, like drywall, but also on how wet the air is on each side of the wall.

"In simple terms, the vapor permeance can be determined in a lab by subjecting a known area of material to a known vapor pressure gradient or known RHs on both sides."

Vapor pressure gradient is how much ‘pull’ one side of the wall has compared to the other. Moisture is pulled from wet to dry, and how great the difference is determines the strength of the pull.

"The vapor pressure on each side of the test material can range but most commonly, or what we refer to as the wet cup test with 100% RH on one side ..."

Saturated air

" ... and 50% RH on the other side."

Pretty much regular air.

"The other test that is typical is called the dry cup test. With a 0% RH—or a desiccant—on one side of the test material, and 50% RH on the other."

Both of these tests are part of the ASTM E96 Standard, and whether you choose the wet cup test or the dry cup test depends on where the material is to live out its life: on the inside or on the outside of a building.

"For example, on the outside of a building in many climates it will be subjected to higher relative humidity, as you would expect during rain events, and different climatic conditions.

So the wet cup test is probably the more relevant test for building materials on the outside of the enclosure.

Inside, where the air is drier, the dry cup test would better indicate expected performance. Now, you are not supposed to do these tests on-site, they have labs—like Jonathan’s— to do that. All those lab results should work their way into standards and codes, but sometimes some of the codes and standards are a little unclear.

Jonathan tells me there are some anomalies in the building code that call for the dry cup test for exterior materials like sheathing. He also notes that air is a really big deal when considering vapor transport.

It's important to remember that vapor permeance and vapor movement by diffusion through the enclosure layers is independent of air movement."

Vapor diffusion is described by the Ideal Gas Law. It is basically water molecules in the air banging around into each other and into surfaces. The success of the vapor diffusion is dependent on how permeable the stuff they bang into is. A much quicker way into the wall is to ride an air current into a hole.

"Air infiltration, or exfiltration, into the enclosure can move orders of magnitude more water vapor. So like a hundred times more water vapor than by vapor diffusion alone."

Hence the recent emphasis in high performance building on air barriers over vapor barriers.

"There's often confusion between air barriers and vapor barriers, but that is a whole topic in itself."

For another show. In the meantime, some examples of highly permeable materials are some house wraps, like Tyvek,

"Latex paint is pretty highly vapor permeable."

Drywall is vapor open ...

"Mineral insulation or even fiberglass batt insulation both allow a lot of water vapor to pass through them."

Open cell spray foam insulation ...

"Ahh, see the confusion is there are lots of different foams now. Half-pound, open cell foam is quite vapor permeable and will not control the vapor movement."

So closed cell spray foam is a vapor barrier?

"At about two inches, closed cell, two-pound spray foam is considered a vapor barrier."

Other vapor barriers or vapor retarders include six mil polyethylene, kraft paper facing on fiberglass batts, there are many peel and stick membranes, which are complete vapor barriers.

"It is important to know the vapor permeance of the materials in the wall assembly, so water vapor is not inadvertently trapped at a location inside the wall enclosure."

How to use vapor permeance to your advantage

Ignoring for a minute whether a vapor barrier is a good idea,

"The general rule of thumb is to place any vapor barriers in the assembly on the warm side of the enclosure."

So, not saying you need one, but if one is specified, then it should be on the inside of the wall in cold places and the outside of the wall in hot places.

"If you were to put a vapor barrier on the interior of the enclosure on the drywall, in a hot or humid climate, such as vinyl wallpaper for example, it's likely that there will be moisture accumulation on the interior, trapped between the vinyl wallpaper and the drywall."

I’ve seen a lot of black mold behind wallpaper in Tennessee where I used to remodel houses.

"This is a generalized problem in hot humid climates, but we have done research that shows condensation on the interior polyethylene vapor barrier, even in climate zone five, with moisture storage cladding, such as directly adhered stone."

And we have a podcast about reservoir claddings, if you are interested.

"Enclosures should be designed to dry in at least one direction, either the interior or the exterior, depending on which climate zone you are in, and the material properties of the enclosure materials that have been chosen.

This brings up the importance of looking at the whole assembly when designing a high performance wall or roof.

It's important to remember the vapor diffusion through the enclosure is controlled by the least vapor permeable material. So, if you design a vapor open enclosure and put in one layer somewhere in there that is vapor impermeable, or a vapor barrier, it will stop all vapor from moving in or out of the enclosure at that layer."

Some scientists call such an analysis the vapor profile of the assembly, meaning which way the wall can dry from any given layer. If it cannot dry out or in from somewhere, it is a problem.

"So we talked about vapor open materials, and vapor barrier materials but there is also a category of materials that are often referred to as smart vapor barriers.

These types of materials have different vapor permeances at different ambient relative humidities, so in a drier environment with a low relative humidity, they will act as a vapor barrier.

But if the relative humidity were to increase, say as a result of a small amount of water leakage into the enclosure, in the window, then the vapor permeance of that smart vapor barrier would increase allowing more drying of the water that leaked into the assembly—decreasing the risk of moisture problem.

The most common smart vapor retarder is the kraft paper backing on many fiberglass batts. The paper is vapor closed unless the wall cavity becomes wet, at which point the paper becomes vapor open to allow drying.

There are other products on the market that are plastic films that will behave the same way and have a wider range of vapor permeance as well. 

MemBrain is probably the most common one in North America, but there are many others in existence, a lot of them are still just in Europe, but they are becoming increasingly more common in North America."

Another thing that is common in North America?

Breaking down scientific principles into Seven Minutes of BS.

Remember, you get paid for what you do and what you know. There’s only 24 hours in a day, but information is infinite.

We’d like to thank RDH Building Science for lending engineers to the cause, and for the technical corrections to my textual truncations.

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—7 Minutes of BS is a production of the SGC Horizon Media Network.