It’s in the details, it’s always in the details: Building Envelopes – Part 2

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It’s in the details, it’s always in the details: Building Envelopes – Part 2

by Christine Gries, PE, LEED AP BD+C

May 2016

In our previous article on building envelopes, we discussed how envelopes are changing more and more, becoming increasingly interesting and unique. From an engineering standpoint, the most significant aspect of the building envelope is not always the insulation factors (U-values, solar heat gain coefficients, etc.) but the infiltration. Infiltration is defined as the flow of outdoor air into a building through cracks and other unintentional openings. New envelope materials and transitions between different envelope constructions provide opportunities for gaps and cracks. Though materials and construction methods have improved and are focused on minimizing air infiltration, this is still a difficult task for designers. On top of difficulty in construction, as buildings age sealants and materials fail so attention to infiltration becomes especially important in older buildings.

Old buildings, new buildings, they all seem to be a challenge. The solution: A continuous air barrier. Kentucky has adopted the 2012 International Energy Conservation Code (IECC) which states “a continuous air barrier shall be provided throughout the building thermal envelope.” The International Code Council (ICC) recognizes that infiltration is a big cause of unnecessary energy usage and now requires infiltration limitations. There are three methods of compliance with the 2012 IECC: materials, assemblies or building test. The design team should select the method that makes the most sense for the building. For example, a single layer as the air barrier might be easier to verify continuity, but an assembly might be easier for construction, but a test will verify performance requirements have been met. Performance testing is becoming more prevalent in new code editions and may be required for envelope verification in the future; it eliminates guessing.

So, the ICC has determined that this is a problem, but how big of an energy drain is infiltration? How do we quantify the amount of infiltration that a building might have? As a recommendation from the ASHRAE Handbook, a building can be ranked as tight, average or leaky. Ranging from 0.1 cfm/ft2, 0.3 cfm/ft2, to 0.6 cfm/ft2, respectively. For example, in a large building with 150,000 sq. ft. of wall area, the amount of infiltration can range from 15,000 cfm to 90,000 cfm. Engineers and architects know that 90,000 cfm of unconditioned outside air in a building is problematic. This can account for an increased operating cost of up to $140,000 per year.

Not only can infiltration increase operating costs, but it also can cause issues with occupant comfort, mold and mildew. Loose windows can create unpleasant conditions on especially humid days, and can cause drafts on cold days. In addition to faulty windows, leaky and thermally insufficient walls can cause drafts. Finally, and most importantly, mold and mildew present the greatest risk to both the health of the occupants as well as to the long term integrity of walls, flooring and furniture. A well-sealed building is the best bet for a comfortable, energy efficient space. As stated in the previous article, there is a careful balance between innovation for aesthetic purposes and functionality. Attention to the details can close this gap, reducing the inefficiencies.



ASHRAE Handbook of Fundamentals, 2013

International Energy Conservation Code, 2012

Kentucky Building Code, 2013