How Air Barriers and Low-Slope Roofing Can Work Together

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The International Energy Conservation Code (IECC) is a building code created by the International Code Council in 2000. It is a model code adopted by many states and local governments in the United States for the establishment of minimum design and construction requirements for energy efficiency.

In 2012, the IECC published air barrier requirements, which states, “Continuous air barrier shall be provided throughout the building envelope with the exception of climate zones 1, 2, and 3.” The purpose of an air barrier is straightforward:

  • Minimize the loss of conditioned air from within a building
  • Reduce energy loss — increase building energy efficiency

So, what does this mean for the commercial low-slope roof? Most people would assume that low-slope roof membranes are air barriers. However, that’s not the case and it’s very important to note that:

  • The air barrier is the system of materials that controls air leakage/convective heat flow through the building enclosure
  • The air barrier is not one material but instead is an integrated system of many different materials/components

Roofing membranes generally are very good at blocking airflow, but unless they are properly tied into the other parts of the building envelope, the building will still leak air. There are three ways of achieving compliance with air barrier requirements:

  • Materials (i.e. prescriptive) – the IECC published a list of materials they consider to be air barriers when installed the right way. Materials not on the list must be tested and shown to have an air permeance ≤ 0.004 cfm/ft2 under pressure differential of 0.3 in. water tested in accordance with ASTM E 2178.
  • Assemblies – assemblies of materials and components (sealants, tapes, etc.) that are to be used can be built and then tested. An average air permeance ≤ 0.04 cfm/ft2 under pressure differential of 0.3 in. w.g. tested in accordance with ASTM E 2357, 1677, or 283 is required.
  • Whole building testing  air leakage rate of completed building can be tested and confirmed to be ≤ 0.40 cfm/ft2 at a pressure differential of 0.3 inches water per ASTM E779 or equivalent method approved by a code official.

The IECC published a list of what are automatically considered to be air barriers. This is shown here:

  • Fully adhered roof systems, single ply, BUR, mod. bit.
  • Min. 3/8” plywood with taped, sealed joints
  • Min. 3/8” OSB with taped, sealed joints
  • Min. ½” extruded polystyrene with sealed joints
  • Foil-backed polyiso min. ½” thickness with sealed joints
  • Closed-cell SPF min. 1.5” and min. 1.5 pcf density
  • Open-cell SPF min. 0.4 – 1.5 pcf and min. 4.5” thickness
  • Min. ½” exterior or interior gypsum or cement board with sealed joints
  • Pre-cast or cast-in-place concrete
  • Fully grouted concrete block masonry with paint coating or sealer
  • Sheet steel or aluminum

The American Society of Heating, Refrigerating & Air Conditioning Engineers regularly update the ASHRAE 90.1 standard that provides minimum requirements for energyefficient designs for buildings except for low-rise residential buildings. They have included many of the IECC code requirements, but removed the words “fully adhered” from the roof system requirements.

Both ASHRAE and NRCA are working to get the “fully adhered” requirement removed from the IECC. However, mechanically attached systems can still be used provided that the final system complies with air-barrier requirements. That means either testing of a representative assembly or whole building testing after construction.

While the IECC is a model code and ASHRAE 90.1 is a standard, their respective requirements do get adopted into energy and building codes. Also, ASHRAE 90.1 is an industry standard referenced in the LEED® building certification program. It is frequently used as a baseline for comparison during energy retrofit projects or any project that employs building energy simulation.

As IECC and ASHRAE 90.1 get adopted and even surpassed, we can expect more attention to be focused on exactly how low-slope roofing membranes interface with air barriers within wall systems.  It’s important to remember that air barriers are the system, not individual materials. Building design professionals have an important role to play in helping to ensure compliance with these toughening regulations.

Related blog posts:

Do Reflective Roofs in the North Make Sense?
Are There Downsides to Reflective Roofs in the North?

There are 18 comments

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    • Thomas J Taylor, PhD

      Thank you. This is an evolving topic and a more recent blog covers some additional details. We have also published A Guide to Air Barriers in Commercial Low-Slope Roof Assemblies that can be found from a Google search on that title and GAF.

  1. Jake G.

    What a great article Thomas! While it’s quite technical in nature, your writing does address questions that some of our low slope roofing clients will ask from time to time.

    • Thomas J Taylor, PhD

      Jake – thanks for the compliment. We’ve recently improved the look and search-ability of our ProBlog site, check out the new Building Science section. We have some other articles in the works about vapor retarders and air barriers – so its worth checking back periodically.

    • Thomas J Taylor, PhD

      Jim – thanks for reading this. We have recently been giving presentations about air barriers versus vapor retarders after seeing that there’s lots of confusion out there. An easy way of looking at this is to think of air barriers as being part of the building envelope helping to minimize air leakage. This in turn can improve energy efficiency. Vapor retarders are used inside the building envelope and are used to control and manage moisture and condensation. We’ll have more to come on this topic on our blog.

  2. Cara

    This is great news and very interesting, as things are always changing in roofing! The International Energy Conservation Code (IECC) is a building code created by the International Code Council in 2000. It is a model code adopted by many states and municipal governments in the United States for the establishment of minimum design and construction requirements for energy efficiency. To select, detail, and specify the most appropriate roof system for a project; past experience with several of the available material options and an understanding of roof assembly materials and system options, and an understanding of roof design considerations is recommended

    • Thomas J Taylor, PhD

      Thanks for checking in with us. Yes, things are always changing and its important to keep up with the codes for each state. The roof stopped being just about weather protection some time ago once we recognized that cool roofs could lower energy costs. Insulation for thermal control has been part of roof systems for a very long time, but now we have to consider keeping conditioned air in the building. Air barriers are becoming increasingly important and how the roof ties into the rest of the building envelope has to be thought through carefully.

  3. Aaron

    Like Bob said: the times are a-changing! Roofing technology is no different. As a roofer I can’t believe I’m just now discovering this blog on GAF! Keep up the good work.

    • Thomas J Taylor, PhD

      Exactly – and the changes won’t stop, we need to go with it. All of these changes that we’ve seen over the years are opportunities for the knowledgeable contractor to add more value. Keep checking in with us – we’ll have more to say about air barriers in future posts.

  4. Hannah

    Insightful article into the exact does that have changed for commercial roofing, especially pointing out the incorrectness of assumptions from the past “Most people would assume that low-slope roof membranes are air barriers. However, that’s not the case and it’s very important to note that.” Keep up the good work!

    • Thomas J Taylor, PhD

      Thanks. Yes, expectations and assumptions about commercial roofing have changed and will continue to do so. It used to be that a roof was only expected to protect occupants from the weather. Insulation became important as a way of controlling heat into and out of a building. But now, the building envelope, including the roof system, is being asked to control air leakage. So, it’s protection from the weather AND reduction in energy use. Check out our related blog about net zero buildings.

    • Thomas J Taylor, PhD

      That’s unfortunate. At GAF we work hard to make sure that building design professionals and contractors have a wide range of material and installation options for most situations. While we can’t provide designs for individual jobs, we do try to provide as much general information as possible.

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