Misconceptions About Permeance in Wall Air Barriers
May 1, 2019

In moderate climate regions, and especially in southern states, specifiers are often tasked with selecting an air barrier that is vapor permeable. In many cases, they are advised by product manufacturers’ reps that products with a higher perm rating will deliver better performance. Various manufacturers have used this tactic to drive the sale of their products and limit competition.

 

To counter this misleading marketing technique, it is imperative to understand permeance, how it relates to vapor retarder classification, and what it all means in terms of building performance. 

 

Permeance indicates the rate of water vapor transmission through a material and is dependent on the material’s thickness, much like R-value in heat transmission. Permeance is often abbreviated to “perm”, which is the unit of measure used for vapor retarder classifications. A material’s perm rating is also what is needed when comparing the water vapor transmission of different building products. 

 

The table below shows vapor retarder classification as accepted by the International Building Code (IBC). It is important to note that the less permeable a material is, the greater its resistance to water vapor transmission.

 

Classification

Definition

Permeance

I

Vapor Impermeable

Greater than or equal to 0.1 perm

II

Vapor Semi-Impermeable

Greater than 0.1 perm but less than or equal to 1.0 perm

III

Vapor Semi-Permeable

Greater than 1.0 perm but less than or equal to 10 perms

 

Vapor Permeable

Greater than 10 perms

 

 

As the table above illustrates, any material with a perm rating greater than 10 is classified as PERMEABLE. Selecting a product solely because it has a higher perm rating than the definition of permeable doesn’t add any meaningful benefits to the performance of the system.

 

The most important thing to consider when comparing perm ratings of various products is the test in which the perm rating was determined. ASTM E96 is the Standard Test Method for Water Vapor Transmission of Materials.

 

ASTM E96 contains two test methods to determine the perm rating of materials: Method A (the desiccant method) and Method B (the water method). Results from these two test methods vary considerably and cannot be compared in any way. Therefore, it is extremely important when comparing and choosing a vapor permeable or vapor impermeable air barrier that the results are from the same ASTM E96 test method. Method B is the most commonly used for classifying materials due to the higher results it yields, representing a worst-case situation with an excess presence of moisture.

 

Please contact Chris Kann at [email protected] with questions.

January 22, 2020
Wood Nailers for Roofing

Wood nailers are often overlooked, but they are a very important component of a successful roof assembly. A horizontal wood nailer is used to provide an effective substrate for some installation details and for other roof accessories. In addition, it is used to provide solid protection for the edge of the membrane underlayment. At a minimum, wood nailers must be thick enough that the top of the nailer is flush with the top of the membrane underlayment. General Criteria: The width of the nailers must exceed the width of the metal flange of edgings, scuppers, etc. When treated lumber is specified, it is recommended that only lumber that has been pressure treated with salt preservatives be specified. Lumber treated with any of the wood preservatives such as, Creosote, Pentachlorophenol, Copper Naphthenate, and Copper 8-quinolinolate will adversely affect the membrane when in direct contact and are, therefore, unacceptable. If non-treated lumber is to be specified, it must be stored to protect from moisture sources. A seal should be provided between the non-treated lumber and a concrete or gypsum substrate. Methods used to fasten the nailer vary with building conditions; however, it is essential that secure attachment of durable stock be accomplished. Factory Mutual Loss Prevention Data Bulletin 1-49 (Perimeter Flashing) contains options for the spacing and sizing of fasteners based on the project wind zone. Wood nailers that are anchored to steel, wood, or masonry decking should not be less than 2" x 6" nominal (minimum 1-1/2" x 5-1/2"). Wood nailers should be Douglas Fir, Southern Yellow Pine, or of wood having similar decay-resistant properties. The American Wood Protection Association (AWPA) publishes the AWPA Book of Standards and is the industry standard for wood treatment. U1 is the specification for treated wood and outlines wood species, preservatives, and specifications for their Use Category System. This is a great resource when you have questions of when and where you should treat wood and what are the recommendations for various treatments. Carlisle SynTec publishes some topics in the Design References portion of our product binder, which can also be accessed on the Carlisle SynTec website. For more information on wood nailer attachment, including some drawings, click here. Contact Craig Tyler at [email protected] with further questions.

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January 8, 2020
Air and Vapor Barriers for Roofs

In 2012, the International Energy Conservation Code (IECC) introduced the continuous air barrier requirement for new commercial construction. This meant that air and vapor barriers were now required for walls, and they must be tied to both the roofing assembly and the foundation. For years, many architects and designers only utilized an air and vapor barrier on the roof deck for high-humidity occupancies, such as swimming pools or food processing facilities. But the new requirement meant taking a hard look at the needs of all buildings and what a roof assembly could do for the building envelope. A single-ply membrane, as stated in the IECC and as tested utilizing the ASTM E2178 standard, qualifies as an air barrier and can satisfy the requirement for an air barrier on any given project. So why would you consider adding an additional air and vapor barrier to the roofing assembly? There are a couple of very simple reasons: Reason 1: Air Intrusion. While a properly installed roofing system will not allow air leakage (e.g., conditioned indoor air from exiting the building thermal envelope), it does allow air movement within the roof assembly. As the single-ply roof membrane is on the top of the assembly, indoor conditioned air can infiltrate into the roofing system and travel into the layers of insulation or cover boards. Why is this an issue? See Reason 2… Reason 2: Moisture Migration. Adding a deck-level air and vapor barrier is a great solution to prevent air intrusion and moisture migration. This also allows the wall air and vapor barrier to be tied together at the deck level, which allows the roof to be replaced more easily in the future. The contractor will not be modifying the continuous air barrier when re-roofing, as the roof is no longer that barrier. Carlisle SynTec provides many options for deck level air and vapor barriers: VapAir Seal MD for steel deck construction, direct to deck; VapAir Seal 725TR for Concrete Decks; VapAir Seal Flashing Foam for sealing around penetrations such as pipes; Go to the Air and Vapor Barriers Product Page on the Carlisle SynTec website for more information, specifications, and details. Contact Craig Tyler at [email protected] with further questions.

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December 18, 2019
Cold Weather Installation Tips Part 2 - Membranes and Insulation

As discussed in the previous SpecTopic, "Cold Weather Installation Tips Part 1 - Low-VOC Bonding Adhesives and Primers", specifying and handling of building envelope products is challenging during the colder winter months. Single-ply membranes and rigid insulation boards need some extra consideration, as they can be adversely affected by outside temperatures. For starters, all membranes will need time to "relax" after being unrolled from the original packaging; this applies to EPDM, TPO, PVC and KEE HP. It is also suggested that membrane widths be limited to a maximum of 10 feet for adhered roofing systems. Treat flashing products and accessories as you would adhesives and primers, by utilizing heated storage enclosures or "hot boxes". This practice is strongly recommended when ambient temperatures are expected to fall below 40°F for an extended period of time. In all applications, but especially in cold conditions, insulation and underlayments must be stored so that they are kept dry and protected from the elements. Insulations should be stored on a skid, covered with a breathable tarp, and weighted to prevent wind damage. In winter months, ice and frost can form on the membrane. This can be difficult to see and can remain on the roof well into the day, especially on white membranes. This can be especially hazardous when working near the edge of the roof. Additionally, frost on metal edges and copings can create a very slick surface and cause ladders to slide and shift. Never step onto a metal coping when it is frost- or snow-covered. So for your next cold weather specification for single-ply membranes and rigid board insulation, include some installation precautions as mentioned. Contact Craig Tyler at [email protected] with further questions.

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