Understanding High Permeability vs. Low Permeability
What does it mean and how does it affect your building practices? The intent of this paper is to explain this property thoroughly so that a better understanding of this building science principle can be gained.
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The first step in understanding high perm vs. low perm housewraps is to break down the physical attributes of what a high quality housewrap can offer. A good weather resistive barrier (WRB) housewrap has 4 equally important functions:
- It MUST have a high level of air resistance - to help prevent drafts, reduce energy bills and resist the flow of moisture laden air though wall cavities.
- It MUST have a high level of water resistance - to help protect the wall cavity from water that gets behind the cladding.
- It MUST have moderate to high vapor permeability - to promote drying in wall systems.
- It MUST be durable -to withstand the rigors of the construction site and continue to perform once construction is completed.
While all 4 properties are important, it is permeability that is probably the most ignored and the least understood function, yet it can have the greatest impact on how a wall system performs. So let’s take a look at this critical property.
WHAT IS VAPOR PERMEABILITY?
Vapor permeability (commonly referred to as breathability) is a material’s ability to allow water “vapor” to pass. This is often confused with the concept of holding out bulk water. So what’s the difference? Bulk water is moisture in liquid form; water vapor is a gas. Also, the amount of moisture that moves and the method of movement are significantly different. A single drop of water consists of thousands of molecules. In general this can be compared to a crowd of people who do everything together. This is the way they like to travel. Now just imagine this crowd of people trying to go through a door all at the same time. They won’t fit so no one gets through. But just suppose everyone in the crowd spreads out, and in an orderly fashion and one-by-one, they attempted to exit. Some of them would get out.
In just the same way, when thousands of molecules of water get behind cladding, they can’t fit through the small pores in the housewrap. When you have moisture vapor in your wall, one molecule can break away from the pack and is able to escape by diffusion. When a material allows these individual molecules to pass we call it vapor permeable. Building codes require that WRB’s be vapor permeable and establish the minimum allowable level to be around 5 perms. This value is based on traditional building practices and does not necessarily address the requirements of modern energy efficient construction, new cladding systems and materials. DuPont Building Scientists believe that 5 perms are not enough to ensure the kind of consistent performance you need from a WRB. DuPont suggests that a WRB have a moderate to high vapor permeability.
So why is vapor permeability important? Because wall cavities do get wet, roofs leak, condensation occurs, plumbing leaks, construction materials are installed wet and internal moisture loads can be very high. However it happens, walls get wet and require a way to dry out. When a wall can’t dry out, it becomes vulnerable to moisture-induced damage including mold and rot. DuPont Building Scientists believe in designing and building forgiving wall systems. All WRBs produced by DuPont, are engineered to allow maximum drying while still maintaining the other critical properties.
Comparison of Tyvek® and other housewrap products in Moisture Vapor Transmission Rate (MVTR) and Oriented Strand Board (OSB) drying performance
DuPont™ Tyvek® is very different from every housewrap on the market!
DuPont™ Tyvek® weather resistive barriers help provide the right type of protection you need and require for your wall systems.
What is duPont™ Tyvek®?
DuPont™ Tyvek® (Figure B) is a continuous non-woven, non-perforated sheet made by spinning extremely fine continuous high-density polyethylene (HDPE) fibers that are fused together to form a strong uniform web. (Imagine thousands of spider webs being laid down to form one large sheet.) This tough and unique structure is not susceptible to property losses inflicted by stresses in any direction. The fibrous structure is engineered to create millions of extremely small pores that resist bulk water and air penetration while allowing water vapor to pass through. No thin fragile films or punched holes are needed with DuPont™ Tyvek® to permit water vapor permeability. This is the uniqueness of Tyvek® - a true miracle of science.
How do You Measure Permeability?
There are two standard laboratory measurement tests, which are described below. Standard lab tests are good measuring tools, but what happens in a “real world” environment? Since DuPont is a world leader in scientific Research and Design, we have also included a description and the results of a “real world” field evaluation.
The Laboratory Test – ASTM E96
Moisture vapor transmission rate (MVTR) is the measurement referenced in building codes. This is measured in a lab using ASTM E96. The test method measures how much moisture vapor is allowed to pass in a 24-hour period. This measurement can be impacted by vapor pressure, so when you want to compare materials you adjust the measurement for vapor pressure across the sample to get the moisture vapor permeance (MVP). The unit of measurement for MVP is perms. We can use ASTM E96 to give materials a relative rating to show how resistant a material is to allowing moisture vapor to pass. The higher the number, the more moisture vapor the material will allow to pass, and the better drying the material allows. For comparison, the following chart (figure C) shows the relative vapor permeance of several weather resistive membranes.