Practical Science: Air Barriers

August 11, 2010

A little applied physics can improve building energy 
efficiency, durability and occupant satisfaction

Did you know your project leaks? Not water. Air. The building draws unconditioned air in, then leaks conditioned air back out through holes, cracks, gaps and other voids of varying size in the building envelope. It’s called “uncontrolled” air leakage, and it costs a lot of money. The United States Department of Energy estimates uncontrolled air leakage can account for up to 40 percent or more of a building’s heating and cooling costs, and contribute to problems with moisture  and occupant comfort.

Most industrial, commercial and institutional buildings in the United States leak air. So the good news you’re not alone with your leaky project and its high energy bills and drafts. Don’t blame the HVAC engineer. You can design the most efficient HVAC system imaginable, and the building will still draw in air. However, a simple understanding of basic physics and the resulting interaction between the HVAC and the building envelope can help you prevent future buildings from drawing air in and leaking air out with the inclusion of a continuous air barrier system. Here’s how it works…

We know air leakage occurs through cracks, gaps, holes, pores in materials and other openings in the building envelope. We also know air flow is the result of pressure differences. When air leaks, it takes with it heat, water vapor, smoke, pollutants, dust, odors, allergens and anything else it can find and carry. Physics dictates that energy moves from regions of high to regions of lesser potential: hot to cold, high pressure to low, and so on.

There are three major sources of pressure that cause air to leak: wind pressure, stack pressure and HVAC fan pressure. Of the three, wind is usually the greatest. When averaged out over the course of a year, it is about 10-15 mph (0.2-0.3psf or 10-14Pa) in most locations in North America . If it hits the building straight on, air enters the envelope on the windward side and exits on the other three sides and at the top, through the roof. If the wind hits at an angle, air exits the building on the two leeward sides and the roof.

Stack effect, also sometimes referred to as chimney effect, is caused by buoyancy or the simple physics lesson we all learned in grade school – hot air rises. The weight of the column of conditioned air inside the building compared with that outside creates a pressure difference across the building envelope. The taller the building is, the greater the stack pressure will be. Warm, conditioned air escapes through holes at the top of the building and at the roof. The resulting lower pressure at the bottom of the building draws in air from the surrounding environment.

The third pressure comes from the mechanical system itself. Mechanical engineers and on-site managers often choose to bring in makeup air to increase pressure and overcome the infiltration at the base of the building. Unfortunately, this increases pressure at the top, causing more exfiltration problems in that area.

How does an air barrier system increase energy efficiency?

When uncontrolled air leakage occurs, the HVAC system has to work harder to maintain the indoor environment. An effective air barrier system, quite simply, controls air movement into and out of the building. This allows the HVAC system to do its job uncompromised by having to make up for a disproportionately large amount of the air it is conditioning leaving the building.

And of course, increasing the operating efficiency of the HVAC system reduces energy consumption and, therefore, operating costs. In fact, the inclusion of an effective air barrier system may allow the HVAC system to be downsized at the design stage – in some cases by a substantial amount.

 “Despite common assumptions that envelope air leakage is not significant in office and other commercial buildings, measurements have shown that these buildings are subject to larger infiltration rates than commonly believed. Infiltration in commercial buildings can have many negative consequences, including reduced thermal comfort, interference with the proper operation of mechanical ventilation systems, degraded indoor air quality, moisture damage of building envelope components and increased energy consumption.”

         - Excerpt from the 2005 National Institute of Standards and Technology (NIST) report, Investigation of  
            the Impact of Commercial Building Envelope Airtightness on HVAC Energy Use.

According to the National Institute of Standards and Technology (NIST) report, Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy Use, the inclusion of an air barrier system in four sampled types and sizes of building reduced air leakage by up to 83 percent. This represents a large reduction in current and future energy consumption and operating costs: potential gas savings of greater than 40 percent, and electrical savings of greater than 25 percent.

The study evaluated the energy savings of an effective air barrier requirement for non-residential buildings in five cities representing different climate zones. The methodology included blended national average heating and cooling energy prices and cost effectiveness calculations matching the scalar ratio employed by ASHRAE SSPC 90.1. Energy simulations were performed using TRNSYS (Klein 2000). Simulations of annual energy use were run using TMY2 files (Marion and Urban 1995).

The research team selected whole building airtightness levels that were judged to be readily achievable and used these as the whole building target used in the energy modeling. The baseline buildings used in the comparison were modeled with leakage levels based on a database of commercial building leakage measurements.

Let’s look at one of the building models used in the study: the office building.

The model was a two story office building with a total floor area of 2250 m2 (24,200 ft2) and a window-to-wall ratio of 0.2 with a floor-to-floor height of 3.66 m (12 ft), broken up between a 2.74 m (9 ft) occupied floor and a 0.92 m (3 ft) plenum per floor. The building also included a single elevator shaft.

The internal gains for the occupied spaces were divided into three parts: lighting, receptacle loads and occupants. The thermostats operated on a setpoint with setback/setup basis. The heating setpoint was 21.1 °C (70 °F) with a setback temperature of 12.8 °C (55 °F) and the cooling setpoint was 23.9 °C (75 °F) with a setup temperature of 32.2 °C (90 °F).

The HVAC system included water-source heat pumps (WSHPs) with a cooling tower and a boiler serving the common loop. Each zone had its own WSHP rejecting/extracting heat from the common loop. The outdoor air for each zone was supplied to each individual heat pump, and the heat pump blower was on at all times when the zone was occupied. When the location of the building required an economizer, the outdoor air controls were applied to the individual heat pump’s airflow. With this approach, different heat pumps could have a different percentage of outdoor air at the same time depending on the loads. Three of the modeled locations included economizers and two did not. Return airflow was specified to equal 95 percent of supply airflow.

The results showed that reducing the air leakage rate to the target level by including a continuous air barrier system resulted in an average reduction in infiltration of 83 percent. The economic impact is shown in Table 1 below:

Table 1 Energy cost savings for office building
3

City Gas Savings Electrical Savings Total Savings
Bismarck $1,854 (42%) $1,340 (26%) $3,195
Minneapolis $1,872 (43%) $1,811 (33%) $3,683
St. Louis $1,460 (57%) $1,555 (28%) $3,016
Phoenix $124 (77%) $620 (9%) $745
Miami $0 (0%) $769 (10%) $769

Note: The full report, Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy Use, is available for download from the NIST website at www.nist.gov
  

What makes an air barrier?
According to the Air Barrier Association of American (ABAA), air barrier systems must be constructed of materials with an air permeance rating of less than 0.004 cfm/ft2 (0.2 L/sm2 at 75 Pa) when tested at their intended-use thickness in accordance with ASTM E 2178. They must be continuous throughout the building envelope with interconnected, flexible joints . The air barrier must be able to withstand positive and negative air pressures without displacement and must be durable enough to last the life of the building.

Of course, all penetrations in the air barrier must be sealed or the assembly itself becomes leaky, which defeats the purpose of installing the system in the first place.

ABAA has published Master Specifications for several different air barrier materials and systems that meet the performance requirements of state and model Energy Codes on its website: www.airbarrier.org.

One of the most frequently specified air barrier materials is closed-cell spray-applied polyurethane foam. This is because in addition to providing an air permeance rating of less than 0.001 L/s/m2 at an application thickness of 1.5 inches, the material also offers an effective insulation R-value of over 6 per inch and in many states also qualifies as a vapor barrier. Spray-applied polyurethane foam is a two-component product manufactured on site but engineered in the molecular level to meet required performance criteria for every code and climate.

Spray-applied and seamless, it conforms to any shape, fully-adheres to the wall system and requires no fasteners, thereby eliminating thermal bridging, convection loss behind insulation boards, and condensing surfaces, while also increasing installation speed and reducing labor costs. It can also improve structural strength, according to testing conducted by the National Association of Home Builders (NAHB) Research Center .

Won’t it be ‘too tight for comfort?’
The first concern that comes to mind when discussing airtightness is indoor air quality. Occupants need fresh air to breathe, right? Yes, but far better to supply it in a controlled manner via mechanical ventilation. In fact, on the residential side of the coin, American Lung Association® Health House® guidelines require homes to be constructed more airtight to improve energy efficiency and prevent unplanned moisture movement and state:

“...Although many stories in the media attribute indoor air quality problems to houses being built too tightly, the reality is that homes need to be as tight as practical. Air leaking into and out of homes has created many of the problems. Moist air leaking out in cold weather can condense on wall and attic surfaces, creating mold growth and in some cases structural decay. This is a direct result of the home not being tight enough. Moist air leaking into a home in hot humid weather can have the same effect on finished surfaces of walls. Air leaking into a home from an attached garage has been shown to be a significant source of carbon monoxide in homes...”

The same problems can arise in a leaky industrial, commercial, institutional or multi-unit residential building. Just on a potentially larger scale because of the opportunity to impact more people.

Stopping uncontrolled air leakage also helps improve thermal comfort. Most occupants dislike drafts, after all, and what are drafts except the single-most telltale sign of uncontrolled air leakage? The second symptom is uneven or hard-to-control temperature and humidity levels. One side of the room is hot, the other cold. One floor is humid, another dry. If you’ve ever been on the receiving end of a client’s comfort complaints after they took occupancy, you know prevention is probably a prudent plan.

For the future
Although the idea of mandating air barrier systems for new commercial construction is a relatively new phenomenon in the United States, Canada has included air barriers in its National Building Code for over two decades. In recent years, Massachusetts, Wisconsin and Michigan have begun mandating air barrier systems as part of their Commercial Energy Codes. Although air barrier systems are now required by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE’s) Advanced Energy Design Guide: Small Office Buildings, and the New Building Institute’s BenchmarkTM for Advanced Buildings, for the first time, continuous air barrier systems are slated to become a requirement by ASHRAE under Addendum z to Standard 90.1-2004, Energy Standard for Buildings Except Low-Rise Residential Buildings; at the time of writing, this Addendum has completed public review and was pending official publication.

Design professionals can improve the performance of their buildings by understanding the impact of uncontrolled air leakage and the role of the air barrier system in optimizing building energy efficiency and durability, as well as occupant comfort, health and safety. To this end, the Air Barrier Association of America provides training programs for architects, specifiers and engineers, as well as a certified installer program with third-party quality control inspections to ensure correct installation of the systems.

SIDEBAR:
Additional Resources on the Air Barrier:
The air barrier concept is still relatively new in the United States. Those looking to learn more about the impact of air barriers on building performance, commissioning the building envelope or specifying air barrier systems and materials can find a wealth of information on the Internet from proven, scientific sources.

The National Institute of Standards and Technology (NIST) www.nist.gov
The Air Barrier Association of America www.airbarrier.org
Whole Building Design Guide www.wbdg.org
Oak Ridge National Laboratory www.ornl.gov
The United States Department of Energy www.energy.gov
American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) www.ashrae.org
The National Institute of Building Sciences (NIBS), Whole Building Design Guide www.wbdg.org
National Association of Home Builders (NAHB) www.nahb.org
Canada Mortgage and Housing Corporation (CHMC) www.cmhc-schl.gc.ca
National Research Council of Canada www.nrc-cnrc.gc.ca








About the Author – Jim Andersen
With more than 35 years of industry experience as a contractor, distributor and currently a manufacturer of materials, Jim Andersen is considered a leading expert on spray-applied polyurethane foam materials and applications in North America. As Manager of Applications and Training for BASF Polyurethane Foam Enterprises LLC, Jim is active in the National Roofing Contractors Association. He has also participated on projects with the National Roofing Foundation. Jim has served on many committees of the Spray Polyurethane Foam Alliance, which honored him with its Lifetime Achievement Award in 2000. Jim received his Business Administration and Economics degree from the Wisconsin State University of Whitewater in 1971.

                                                         

 1Air Sealing Fact Sheet, www.eere.energy.gov/buildings/info/homes/sealingair.html
 2Air Barrier Systems in Buildings, Anis, Wagdy AIA, Whole Building Design Guide, www.wbdg.org , a program of the National Institute of Building Sciences (NIBS)
 3Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy Use, Emmerich, Steven J. (Building and Fire Research Laboratory, NIST), McDowell, Timothy P. (TESS Inc.) and Anis, Wagdy AIA (Shepley Bulfinch Richardson and Abbott), 
 4Commissioning the Air Barrier System, Anis, Wagdy, AIA, ASHRAE Journal, March 2005.
 5Evaluation Report 12932-R, Canadian Construction Materials Centre (CCMC), National Research Council (NRC) of Canada.







         
 

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