Plugging Leaks

Did you know your project leaks? Not water. Air. As a building draws in unconditioned air, it leaks conditioned air back out through holes, cracks, gaps, and other voids of varying size in the building envelope. In fact, most industrial, commercial, and institutional buildings in the United States leak. Called “uncontrolled” air leakage, the process actually costs a lot of money.

According to the U.S. Department of Energy (DOE) fact sheet, Air Sealing: Seal Air Leaks and Save Energy, uncontrolled air leakage can account for about 30 percent of a building’s heating and cooling costs and contribute to problems with moisture and occupant comfort.

But don’t blame the HVAC engineer for your leaky project or high energy bills and drafts. Even with the most efficient HVAC system imaginable, the building will still draw in and expel air. When uncontrolled air leakage occurs, the HVAC system must work harder to maintain the indoor environment.

Understanding the basic physics behind breathing buildings and the resulting interaction between the HVAC and the building envelope, however, can help prevent future projects from drawing in and leaking air. Add an effective air barrier system - which simply controls air movement into and out of the building - and the HVAC system will do its job, uncompromised by the need to replace disproportionately large amounts of conditioned air leaving the building. Increasing the HVAC system’s operating efficiency reduces energy consumption and, therefore, operating costs. In fact, the inclusion of an effective air barrier system may allow for a downsized HVAC system at the design stage - in some cases a substantially downsized system.

Physics Class

The air flow that occurs through cracks, gaps, holes, pores in materials, and other openings in the building envelope 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.

Three major sources of pressure can cause air to leak: wind pressure, stack pressure, and HVAC fan pressure. Of the three, wind is usually the greatest. When averaged over the course of a year, wind pressure is about 10 to15 mph (0.2 to 0.3psf or 10 to 14Pa) in most locations in North America, according to “Air Barrier Systems in Buildings,” an article in the National Institute of Building Science’s Whole Building Design Guide. If the wind hits the building straight on, air enters 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 - 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, the greater the stack pressure. 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.

An air barrier can change this.

One of the most frequently specified air barrier materials is closed-cell spray-applied polyurethane foam. 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 more than six 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 at 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 wall systems 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 also can improve structural strength, according to testing conducted by the National Association of Home Builders (NAHB) Research Center.

Individual System Specs

According to the Air Barrier Association of America (ABAA), air barrier systems must consist of materials with an air permeance rating of less than 0.004 cfm/s.f. (0.2 L/sm2 at 75 Pa) when tested at their intended-use thickness in accordance with ASTM E 2178. Wagdy Anis, AIA, notes in the March 2005 ASHRAE Journal that the systems must be continuous throughout the building envelope with interconnected, flexible joints. In addition, the air barrier must withstand positive and negative air pressures without displacement and must 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.

To view “master specifications” for several different air barrier materials and systems that meet the performance requirements of state and model Energy Codes, visit the ABAA Web site at www.airbarrier.org.

Serious Benefits

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 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 entire building airtightness levels judged readily achievable and used these as the entire building target used in the energy modeling. Baseline buildings used in the comparison were modeled with leakage levels based on a database of commercial building leakage measurements.

One model was a two-story office building with a total floor area of 2,250 square meters (24,200 square feet) and a window-to-wall ratio of 0.2 with a floor-to-floor height of 3.66 meters (12 feet), broken up between a 2.74 meter (9 feet) occupied floor and a 0.92 meter (3 feet) plenum per floor. The building also contained a single elevator shaft.

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.

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.

Depending on the location, the economic impact in gas and electric savings is tremendous. Average electrical savings was about 21 percent - but ranged from 9 percent to 33 percent, depending on the city. Gas savings averaged even more at nearly 44 percent, but this included a Miami location, with no appreciable gas savings. Among the locations that saw gas savings, the range was between 42 percent and 77 percent (see table).

Comfort Concerns

The first concern that comes to mind when discussing airtightness is indoor air quality. Occupants need fresh air to breathe, right? Yes, but it’s 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.

“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,” according to the guidelines.

For example, if a home is not tight enough, any outgoing moist air in cold temperatures “can condense on wall and attic surfaces, creating mold growth and in some cases structural decay,” the guidelines state. When weather is hot and humid, moist air leaking into a home will condense on finished walls. Attached garages present another hazard as carbon monoxide could seep into a home not properly sealed. Guidelines are available at www.healthhouse.org.

The same problems found in homes can arise in a leaky industrial, commercial, institutional, or multi-unit residential building but on a potentially larger scale, simply because of building sizes and populations.

Stopping uncontrolled air leakage also helps improve thermal comfort. As most occupants dislike drafts - the single-most telltale sign of uncontrolled air leakage - proper air sealing is the solution to the problem. The second leakage symptom is uneven or hard-to-control temperature and humidity levels, often with one side of the room hot while the other is cold. Or, one floor is humid, another dry. Prevention therefore is a most prudent plan, for everything from substantial energy savings to heading off clients’ comfort complaints.

Mandated Requirements

While 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 more than two decades. In recent years, Massachusetts, Michigan, and Wisconsin 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 Benchmark 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 this writing, this addendum had completed public review and was pending official publication.

With a better understanding of uncontrolled air leakage and how an air barrier system optimizes building energy efficiency and durability, as well as improve occupant comfort, health and safety, design professionals can improve building performance. To this end, the ABAA 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.

INFO: BASF (www.basf-pfe.com)