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.






Vapor Impermeable

Greater than or equal to 0.1 perm


Vapor Semi-Impermeable

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


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.

July 10, 2019
Design Wind Speed and Warranty Wind Speed

We’ve already discussed wind uplift design in a previous SpecTopics post (FM 1-90 vs. ASCE 7); this post will help you understand how those wind speed numbers relate to wind speed warranties.  To calculate wind uplift for a roofing project, you’ll need to determine the building type and local wind speed. In gathering this information, some designers look at the American Society of Civil Engineers’ ASCE 7 Wind Maps for their area, see a number like 90- or 120-mph, and think that is the wind speed their building will encounter. Therefore, they specify the same speed for their warranty (i.e. 120 mph local speed means I need a 120-mph wind speed warranty). Rest assured, this is not the case. ASCE 7 maps have contours with the local speeds in 10 mph increments. ASCE 7-2005 and ASCE 7-2010 were relatively straightforward; most of the U.S. was in a 90-mph zone. However, in 2016 ASCE deemed it necessary to have separate maps for each building risk category (Category I, II, III, and IV). This increased the wind speeds for most of the country, especially for projects with increased risk categories. Naturally, designers saw this increase and thought that since the local wind speed was increasing, they needed to ask for increased wind speed warranties. (i.e. 130 mph or more). Again, this is not the case. It’s true that warranted wind speed is the limit of 3-second peak gust recorded at the weather station nearest your building project, measured at 10 meters above the ground, during a weather event that affects your building project. But to achieve wind speeds over 90 mph, a cyclonic windstorm (tornado, hurricane, etc.) is generally necessary. If your building experiences a cyclonic windstorm, there will be flying debris, broken glazing, and other envelope breaches that could cause roof failure (over-pressurizing the building, detachment of decking from structural components, etc.). This would not be covered under a roofing warranty, regardless of the wind speed coverage. Keep in mind that a roofing warranty assumes that the building remains intact, the decking remains solid, the inside pressure of the building is generally equalized, and foot traffic is limited to maintenance and inspection of rooftop equipment. It is not building insurance. Like fires and vandalism, critical weather events such as tornadoes and hurricanes are covered by the building owner’s insurance carrier. Choose a warranted wind speed that makes sense for you and your client, but you don’t need to match that with your local wind speed. You’ll just be paying more for something you don’t need. Always verify your need for increased warranty wind speed before inquiring about matching your local wind speed with the warranty. Contact Craig Tyler at [email protected] with questions.

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June 26, 2019
Moisture in Concrete - Part 1

This post focuses on the moisture phenomenon in concrete and the difference between lightweight and normal-weight structural concrete. Curing versus drying and the standards used to determine relative humidity levels are also addressed. Part II will address design recommendations and roof assembly selection. Moisture in Newly Poured Structural Concrete Roof Decks When investigating roofs for leaks, invariably, moisture is found beneath the roof membrane. However, the source of moisture is not always a roof leak. Newly poured structural concrete could be a contributor to the presence of moisture beneath a new or a replacement roof. Concrete is a mixture of several components that reaches its optimum strength through a chemical reaction induced by water. Concrete needs water to allow for flowability and workability, however, water also has adverse effects. Once the concrete has cured, the remaining water is considered “free water”, or moisture which is no longer consumed by the curing process. Rain and snow add moisture to exposed concrete roof decks and further prolong the drying. As an example, a 4” slab of structural concrete contains as much as 200 gallons of free water per 1,000 square feet. ​Structural Concrete Mix Ratio The ratio for both normal-weight and lightweight structural concrete (LWSC) is generally the same: •10-15% cement •60-75% aggregate (fine and coarse) •15-20% water The difference is in the aggregate; the lightweight aggregate is pre-saturated prior to mixing. The lightweight aggregate, which is made up of shale, slate slag, or clay, can absorb 5-25% of its mass. Normal-weight structural concrete, however, utilizes aggregates such as sand and stone, which are not as porous and do not need to be wetted before adding to the mix. The popularity of LWSC is increasing due to: •Lower building structural cost; •Lesser density for reduced dead loads; and •Environmental and sustainability claims. Drying Time To reach a 75% relative humidity for normal-weight structural concrete, it will take approximately three months. However, achieving the same 75% relative humidity for LWSC will take twice as long. According to the Portland Cement Association, the dry-down time for LWSC is more than normal-weight structural concrete. Standards for Moisture Testing For many years, the roofing industry has used a curing time of 28 days after the concrete is poured. However, there are test methods published by ASTM for determining the moisture content in concrete. Qualitative tests, such as the plastic sheet test and electrical resistance and/or impedance are good indicators of the presence of moisture in a given area but are not as accurate as quantitative tests. Quantitative tests, such as the moisture vapor emission rate test, surface humidity, or in-situ relative humidity tests demonstrate levels of moisture present in the concrete. The recommended quantitative test is the in-situ relative humidity test (ASTM F2170), in which a sleeved probe is placed in a drilled hole in the concrete and left in place for 24 hours. After the 24 hours, an electronic reader is attached, and the information is read directly from the sensor. The relative humidity reading should be less than 80% at a depth of approximately 40% of the thickness of the slab. The moisture values and test duration stated above have been slightly modified to better suit outdoor roof conditions.  Site Considerations The concrete pour schedule can affect moisture testing and provide inaccurate moisture values. Therefore, in phased construction, the field testing and the roofing installation should be aligned with the concrete pour schedule and ICRI-Certified Concrete Inspectors should be commissioned. For in-depth information, the International Concrete Repair Institute (ICRI) offers various resources that can aid with the proper steps required for testing and evaluation. Stay tuned for part II for recommendations on design and the selection of an appropriate roofing assembly.

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June 12, 2019
An Alternative to Metal Roofing: Special Color TPO with Contour Ribs

An Alternative to Metal Roofing: Special Color TPO with Contour Ribs Carlisle’s TPO Contour Rib Profile offers the look of a standing seam metal roof with the performance of a TPO single-ply membrane. Each section of Contour Rib Profile is manufactured from the same weather-resistant compound as TPO membrane and enhanced with fiberglass for added dimensional stability. The product’s rectangular shape provides exceptional shadow lines for aesthetic appeal.  TPO Contour Rib Profile Quick Facts  Available colors: White, Gray, Tan, Terra Cotta, Patina Green, Slate Gray, Rock Brown, and Medium Bronze. Size: Each section is 1 1/4" tall, 2 1/8" wide (including welding flanges), and 10’ long; the vertical profile is 3/8" thick with a 1/8" alignment hole and a 1/8" fiberglass reinforcing cord. Packaging: 20 pieces per carton, each carton includes 25 connecting pins. Special-Color TPO Program Quick Facts - Carlisle has the industry’s most comprehensive special-color TPO program, with membranes and accessories in Medium Bronze, Patina Green, Rock Brown, Terra Cotta, and Slate Gray. - There are no minimum order quantities. - The following TPO products are available in the five special colors. 1. 60-mil standard reinforced TPO membrane – 5’ x 100’ and 10’ x 100’ rolls.  • 10’ x 100’ rolls – limited quantities stocked in Mississippi; large orders may require a one- to    two-week lead time.  • 5’ x 100’ rolls – require a short lead time; customers must order an even number of rolls.  2. 24” Non-Reinforced TPO Flashing – limited quantities stocked in Mississippi; large orders may require a one- to two-week lead time. 3. TPO Contour Rib Profile – limited quantities stocked in Mississippi; large orders may require a one- to two-week lead time. 4. TPO Coated Metal – made to order; requires a one- to two-week lead time. 5. 115-mil 12’ x 100’ FleeceBACK® TPO – manufactured at the beginning of each calendar quarter (January, April, July, October) to fulfill all orders that are in-hand by the 15th of the previous month (March 15, June 15, September 15, December 15). You can combine Contour Ribs with special-color TPO to create a simulated metal roof in Medium Bronze, Patina Green, Rock Brown, Terra Cotta, or Slate Gray.  Contact Craig Tyler at [email protected] with questions.

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