How Air Barriers and Low-Slope Roofing Can Work Together

By Thomas J Taylor PhD 06-23-2021
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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 energy–efficient 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.

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