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.

May 15, 2019
FM 1-90 vs. ASCE 7

Wind uplift design for roofing can seem daunting to the uninitiated. But with a little help from online tools, it can be much more straightforward. Wind uplift is calculated for all buildings using formulas, tables, and wind maps developed by the American Society of Civil Engineers (ASCE) in their publication ASCE 7-2016. With a project’s location, building use/occupancy, building height, and roof plan, there are a number of online tools you can use to determine the wind uplift required for your building. The calculator used by the National Roofing Contractors Association (NRCA) can be found at http://www.roofwinddesigner.com/. Once the uplift pressures for your building are determined, you must choose a design for your building that meets these pressures. Roofing manufacturers list their system designs through the DORA Directory of Roof Assemblies https://www.dora-directory.com/ or through Factory Mutual Global’s RoofNav® https://www.roofnav.com/Account/Login. The DORA Directory lists roofing assemblies based on uplift testing that various manufacturers have received through third-party verification, while FM’s RoofNav lists roofing assemblies that have been tested through FM Global’s own testing facility. Roofing assemblies that meet the minimum uplift requirements per ASCE 7-16 will meet the International Building Code (IBC); however, FM Global ratings may require additional enhancements based on their own calculations. The more stringent guidelines are due to the fact that FM Global is an insurance company and they approve designs before they issue coverage for a particular building. While FM 1-90 is a rating used by FM Global-insured buildings as a standard for their insurance coverage, the calculation of wind load for a particular building using ASCE 7 calculations is the basis for designing a roof meeting the IBC for all buildings, whether or not they are insured by FM Global. Meeting the standard for FM 1-90 will result in higher pressures in the perimeter and corners than using the ASCE 7 method, thereby increasing the cost of the construction of the roof. Changing these requirements at a later date or finding out your project does not require FM ratings may cause confusion during the bidding process and could result in higher bids. Always verify your need for FM Global before proceeding with wind load design. Contact Craig Tyler at [email protected] with questions.

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April 17, 2019
Do Building Codes Require Structural Enhancement for Re-roofing Work?

Today’s re-roofing market is going strong, making up 62% of all roofing work versus 38% for new construction. While most specifiers and roofers know the requirements for re-roofing to meet current building and energy codes, there is always a level of uncertainty when it comes to the structure of the roof itself. Can I tear off the old roof and start my new roof application with the existing deck? Or is something more required? Re-roofing work consisting of a complete tear-off is considered an Alteration – Level 1 for the International Existing Building Code (IEBC) 2015 and 2018 editions. In Chapter 5, an Alteration – Level 1 is described as, “includes the removal and replacement or the covering of existing materials, elements, equipment, or fixtures using new materials, elements, equipment, or fixtures that serve the same purpose”. Descriptions of the code requirements for Alteration – Level 1 are in Chapter 7 and include Section 707 – Structural, which describes two additional structural requirements for roof replacement: 1.(707.3.1) Where the re-roofing work is more than 25% of the roof area, and the building is assigned a seismic design category of D, E, or F (Chapter 20 of ASCE 7), unreinforced masonry wall parapets must be braced according to 301.1.4.2 of the International Building Code (IBC). 2.(707.3.2) If the existing roofing system is removed and the deck is exposed for more than 50% of the roof area and the building is located where the ultimate design wind speed is greater than 115 mph OR the project is located in a special wind region, all structural roof connections must be evaluated for the wind uplift, and if unable to support 75% of the wind load, they must be strengthened or replaced as defined in Chapter 16 of the IBC. These requirements will not affect all buildings. Checking with a structural engineer to determine the existing building’s seismic design category or evaluating wind uplift potential of existing structural components will increase the project’s cost. Additionally, more costs could be added if structural remediation is required. Always check with the Authority Having Jurisdiction (AHJ) for local requirements before proceeding with re-roofing work. Contact Craig Tyler at [email protected] with questions.

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April 3, 2019
A New Trend in Building Envelope Specifications

With the ever-increasing emphasis on airtightness in commercial buildings, many of today’s building envelope projects are taking an approach similar to that of the roofing industry, where products are sourced from and warranted by a single company. Traditionally, building envelope projects have used products supplied by multiple manufacturers for use in below-grade waterproofing, walls, and roofing systems. This poses several concerns for architects and designers, including product compatibility, system performance, and liability. Similar concerns are what led the roofing industry to shift to a single-source, “system” approach in the late 1980s. Today, most roofing systems are single source. These systems utilize materials designed to work together from the start, allowing suppliers to offer extended and unique warranty coverage and eliminate finger-pointing in the event of a leak. One of the biggest advantages of a single-source building envelope is the ability to avoid product incompatibility at tie-in junctions, which can lead to air barrier breaches. With its NVELOP Building Envelope Solutions program and wide breadth of manufacturing capabilities, Carlisle Construction Materials (CCM) is leading the way in the movement toward single-source building envelope systems. CCM’s NVELOP is the industry’s most comprehensive single-source building envelope solution, featuring a variety of waterproofing, wall, and roof system materials. NVELOP's ability to ensure the compatibility of dissimilar materials eliminates the guesswork architects have conventionally dealt with when designing a building envelope system, while still allowing for design flexibility. The program’s tie-in detail suite provides vetted tie-in options that have been tested for durability, compatibility, and constructability. Additionally, NVELOP’s unique single-source tie-in warranty, available for up to 15 years, significantly limits architect and specifier liability and provides peace of mind to the building owner. For more information, visit the NVELOP website at www.carlislenvelop.com. If you have questions, please contact Chris Kann at [email protected]

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