FAQS: What is the Difference Between Permeable and Non-Permeable Air Barriers?
For this week’s blog post, we revisit our list of frequently asked questions. These are questions we receive on a regular basis from contractors, distributors, architects, consultants, etc. regarding our three product lines: architectural sealants, traffic coatings, and air, vapor, and water resistive barrier systems. This post will answer your burning questions about air, vapor, and water barriers. The question is, what is the difference between permeable and non-permeable air barriers?
In order to answer this question, we must first define and understand what an air barrier is, and how it performs. An air barrier is a material or group of materials that work alone or together to prevent air passage in excess of an established threshold through joints, seems, and holes in an exterior wall or roof assembly. An example of this is air leakage from inside to outside and vice-versa). The recognized air leakage rate threshold for an air barrier material and wall assemble are determined by performing the standard ASTM E2178 test method. Once determined, the value may be compared with threshold values given in energy codes and other standards.
The recognized air leakage rate threshold for an air barrier material is 0.004 cfm/sq. ft. @ 1.57 lbf/sq. ft. as determined by testing per ASTM E2178. The recognized air leakage rate threshold for a wall assembly is 0.04 cfm/sq. ft. @ 1.57 lbf/sq. ft. as determined by testing per ASTM E2357. Note they differ by a factor of 10.
Permeability is the flow rate of air or water-vapor as it passes through the molecular structure of a material of membrane made by a difference in air or vapor pressure on one side of the material or membrane. Permeable materials or membranes permit water-vapor to pass through above an agreed upon threshold value, encouraging circulation. Likewise, non-permeable materials or membranes do not allow water-vapor and air to pass through below an agreed upon threshold. Threshold rates include:
Air:004 cfm/sq. ft. @ 1.57 lbf/sq. ft. as determined by testing per ASTM E2178
Water-vapor: Generally, but not always 10 US Perm as determined by testing per ASTM E96. ABAA has always stated 10 US Perms, but some building codes, state 5 US Perms. The US Perm a sort of shorthand for a more complicated unit of: one grain of water-vapor per hour flowing through 1 square foot of a material or membrane induced by a vapor pressure difference of one inch of mercury between the inside and outside surfaces.
When deciding between a permeable or non-permeable air barrier, be sure to also consider the following factors: geography or climate zone, design of indoor air handling equipment, elevation, building code requirements, building occupancy, and overall design and configuration of façade skin and structural components. Understanding the difference between permeable and non-permeable and taking into account these factors will ultimately ensure the correct air barrier selected will protect the building’s envelope.
Pecora offers two vapor-permeable membrane products, XL-Perm ULTRA VP and ProPermVP, and a non-vapor-permeable membrane, XL-Perm ULTRA NP. XL-Perm ULTRA VP and XL-Perm ULTRA NP are both fluid-applied, STPU-based membranes that offer primerless application, can be sprayed or rolled in a single application, boast excellent weatherability, Low VOC compliant in all areas of the U.S., can be used on damp substrates and green concrete, and can be used as a durable UV and weather resistant barrier in rain screen applications. ProPerm VP is a fluid-applied vapor-permeable, water-based membrane. This product saves on labor costs because of its primerless adhesion and can be applied in one single coat by spray or roller, UV and water resistant once fully cured, cost effective, and also offers low VOC and is compliant in all areas of the U.S.