Insulation is Not Enough

Builders are under increasing pressure to produce beautiful, durable, sustainable, energy efficient homes. And while most strive to achieve these goals, many fall short on the fourth goal - energy efficiency – despite insulation over Code and high-efficiency HVAC designs. Why?

Because insulation (R-value) alone is not enough. It takes a properly designed, properly installed, high-performance air barrier system to make a structure truly energy efficient.

Uncontrolled air leakage can inflate the cost of heating and cooling a building by up to 40 percent, according to the United States Department of Energy (DOE). In addition to forcing homebuyers to pay bigger energy bills, uncontrolled air leakage also causes comfort problems like drafts, contributes to moisture, condensation and durability issues like ice damming, rot and spalling, and can even impact on occupant health and safety through mold and mildew infestations.

What causes air leakage?
In order to understand the significant role an air barrier plays in creating an energy efficient home, one must first understand the science behind what makes a house leak air in the first place.

Air leakage occurs through pores in materials, cracks, gaps, holes and other openings in the building envelope. 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. In colder climates, the air moves from inside the building to outdoors. In warmer climates, one sees the reverse effect.

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. If it hits the building straight-on, air enters the envelope on the windward side and edits 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 that 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 higher the stack pressure – something for multi-unit designers to pay particular attention to. Warm, conditioned air escapes through holes at the top of the building and at the roof and the resulting lower pressure at the bottom of the building draws in air.

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 air leakage cause moisture problems?
When air flows through the building envelope of the home, it takes water vapor with it from one location to another. Controlling the movement of moisture by air infiltration requires controlling either the moisture source (indoor, outdoor, or built-in moisture), the pathways (cracks and holes in the envelope) or the driving force (air pressure differentials created by stack effect, wind or mechanical systems).

The movement of water vapor by air flow is far more powerful than vapor diffusion effects in houses, as even fairly small air leaks can move large amounts of water vapor compared to vapor diffusion¹. In fact, some experts estimate that the ratio of moisture transported by air flow compared to vapor diffusion is 10 to 1, or even higher. ASHRAE 2001 Fundamentals recognizes that effectively controlling moisture in a home requires an effective air barrier, and states that “without effective control of airflow, vapor retarders are completely ineffective.”

And while the continuity of a vapor diffusion retarder is not crucial since its effectiveness is determined by the extent of coverage, continuity is critical for air barriers. Small gaps in air barrier systems can concentrate both air leakage and therefore moisture as condensation in localized areas, causing significant moisture damage².

How does an air barrier increase energy efficiency and improve moisture control?
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, allowing the HVAC system to do its job uncompromised by having to make up for the air it is conditioning leaving the building. Increasing the operating efficiency of the HVAC system reduces energy consumption and therefore monthly energy bills for the home owner. In fact, it has been shown, the inclusion of an effective air barrier system will allow the HVAC system to be downsized at the design stage.

When it comes to moisture control, an air barrier prevents water vapor in the air from entering the wall. The DOE estimates that in a 100-ft² wall, one cup of water can diffuse through drywall without a vapor barrier in a year, but 50 cups can enter through a ½-inch, round hole. In fact, “sealing air leaks is 10 to 100 times as important as installing a vapor barrier³ .”

What makes an air barrier?
Air barrier systems must be constructed of materials with an air permeance rating of less than 0.004 cfm/ft² (0.02 L/sm² at 75 Pa) and must be continuous throughout the building envelope with interconnected, flexible joints⁴ . The air barrier material and system 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.

Not all air barrier materials are created equal, however. Some can be difficult to install and require great attention to detail at joints, seams and penetrations, raising the labor cost during installation – especially for irregularly shaped or complicated designs. Almost all require additional insulation and vapor barrier materials to be included in the design and construction.

The only fully-tested, proven combination insulation and air barrier
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. It is the only fully-tested, proven combination insulation air barrier – a benefit made possible because 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 and condensing surfaces, while also increasing installation speed and reducing labor costs. It can also improve structural strength. Testing conducted by the National Association of Home Builders (NAHB) Research Center shows closed-cell SPF insulation between wood- and steel-stud wall panels increased rack and shear two to three times over standard stick-built components and glass fiber insulation when sprayed onto gypsum wallboard and vinyl siding, and increased racking strength by 50 percent when sprayed onto oriented strandboard (OSB)⁵ .

It should be noted, however, that only closed-cell formulations like COMFORT FOAM^© and WALLTITE^© insulation and air barrier offer these benefits. Most open-cell foams have not been tested for air permeance and therefore do not qualify as air barrier systems. One open-cell foam manufacturer’s product requires an application of 5.5 inches (its maximum allowable thickness) to pass the minimum requirements to qualify as an air barrier.

1. BUILDING MOISTURE AND DURABILITY PAST, PRESENT AND FUTURE WORK, U.S. Department of Housing and Urban Development, Office of Policy Development and Research
2. BUILDING MOISTURE AND DURABILITY PAST, PRESENT AND FUTURE WORK, U.S. Department of Housing and Urban Development, Office of Policy Development and Research
3. WALL INSULATION: Provide Moisture Control and Insulation in Wall Systems, Technology Fact Sheet, OFFICE OF BUILDING TECHNOLOGY, STATE AND COMMUNITY PROGRAMS, ENERGY EFFICIENCY AND RENEWABLE ENERGY • U.S. DEPARTMENT OF ENERGY
4. Commissioning the Air Barrier System, Anis, Wagdy, AIA, ASHRAE, ASHRAE Journal, March 2005
5. Evaluation Report 12932-R, Canadian Construction Materials Centre (CCMC), National Research Council (NRC) of Canada

COMFORT FOAM^©is a registered trademark of BASF Polyurethane Foam Enterprises LLC.
WALLTITE^© is a registered trademark of BASF Corporation.