June 2011: Product Solution | Case Study | Technical Story
Moisture management has always been a key consideration in the design of the built environment. More recently, moisture vapor transmission rates (MVTR), permeability and permeance are performance measures frequently used to establish not only the need for an air barrier, but also the type of air barrier most suitable for use on a project. Other factors such as the local environment and structure end-use can dramatically impact air barrier selection as well. Various air barrier performance test methods and the interpretation of the attained results can also confuse or complicate the selection process. To that end, what do the numbers tell us, and what should one really be looking for when selecting an air barrier?
Most commonly, among the first questions encountered in the air barrier selection process is: "Should I install a permeable membrane or an impermeable membrane (vapor barrier)?"
From a design perspective, vapor barriers should always be installed on the warm side of the insulation. For some projects, where there is a predominance of either heating or cooling days, this may be relatively easy to identify. However, in many climate zones, the number of heating and cooling days may be nearly equal, making it difficult to identify the warm side of the building insulation. In these types of conditions, a permeable membrane is frequently specified.
ASHRAE classifies air barrier permeability based upon ASTM E96 results as follows:
The ASTM E96 test standard evaluates moisture vapor transmission, permeance and permeability of a material. These properties are measured at a specified set of carefully controlled temperature and humidity conditions. Mathematical relationships exist between the properties; however, their significance as it relates to specific conditions on a project is what must be established.
Moisture vapor transmission is the steady rate of water vapor flow over a specific time through a known area under specific temperature and humidity conditions.
Permeance is the rate of moisture vapor transmission through a known area over a specific time caused by a vapor pressure differential between the two sides, each under specific temperature and humidity conditions. The greater the vapor pressure differential, the stronger the vapor drive will be from one side to another.
Permeability is permeance as a function of material thickness. Permeability can be thought of as a normalized measurement, allowing for more accurate comparison of construction materials.
There are three different acceptable test procedures described in ASTM E96, each conducted at two temperature options: 74.3° F and 90° F. In each, a membrane at its required thickness is placed over the cup, then the lid is placed on top and tightened to create a seal. The cup is then placed in the environmental chamber at the indicated temperatures with 50% RH. The three methods can be briefly described as follows:
When evaluating a product and comparing its performance based on permeability versus the ASTM E96 test method, it is important that the permeability is expressed completely, including the test method, the scenario used and the material thickness. Evaluating the same material using all three test methods generates drastically varying results, potentially changing a material classification from one degree of permeability to another (ie: semi-impermeable to semi-permeable).
The geographic location and intended end-use of a facility can produce varying levels of vapor drive throughout the day, let alone throughout the year and eventually over the life of the facility. This information, along with complete material permeability data, allows design professionals to select the materials that best meet a project's specific needs. When specifications call for a specific permeance value, it is critical to get all the information that led to the evolution of the value so the intended performance of the product selected - and the project - are not jeopardized.
Performance of individual components is only part of the story. Suitability of the components for the application and how they come together and perform as a system is what ensures long-term success of the building enclosure.