Green Building Library

Advanced Air Sealing:
Air Leakage in Homes

Everyone has been touched by air leakage at one time or another. If nothing else, you have probably felt a cold draft on a windy winter night. Of all the methods to reduce energy use in a house, air leakage control is one of the most cost effective. In a typical house with no special attention to air sealing, air leakage accounts for about one-third of the heating and cooling costs. Such big losses offer the best opportunity for big savings, so air sealing deserves attention.

Air Leakage Sites

When you think of air leakage, you probably think of all the small openings around windows, pipes and wires. These are some of the major sites, but there are others that most builders either overlook or ignore: the gap where the wall sets on the subfloor, the holes where pipes and wires go through wall plates, holes that lead into attics and crawlspaces and the gaps around built-in features, such as fireplaces and medicine cabinets. Every air leak deserves at least a moment of thought, even if you ultimately decide not to seal it.

Forces Affecting Air Leakage

Openings alone don’t cause air leakage. There must be a force to push air through the holes. (This is why a leaky houses can’t be counted on to ventilate themselves naturally or “breathe.”) Two natural forces cause air to move in and out of buildings: stack effect and wind.

Over the course of a year, the stack effect causes the most uncontrolled air leakage in the average house. As air becomes warmer it also becomes less dense. In wintertime, air inside is warmer and less dense than air outside. The difference in densities causes the warmer inside air to rise toward the ceiling where it escapes to the outside. At the same time, colder outside air enters near the floor.

Wind forces operate as you might think. On the side facing the wind (windward), positive pressure forces air into the building. On the other side (leeward), wind passing around the house creates negative pressure, which pulls air out of the building. Wind effects vary with local shielding and terrain conditions at the site. A building at an exposed site may have wind-induced air leakage three to four times as large a more protected building.

Effects of Forced Air Heating Systems

Leaks in System Components

Builders have no control over the outdoor temperature or the strength or direction of the wind -- the forces that drive air leakage. Therefore, to control air leakage, you have to minimize the “holes” that air flows through.

Forced air heating and cooling systems are another major source of air leakage. They affect air leakage rates in two ways: through leaks in the system components and by creating a pressure difference between parts of the home and the outside.

Recent studies indicate how severe the problem is:

  • In one study, homes with forced air distribution systems used 16 percent more energy than homes with zonal electric heat. All the homes had similar insulation levels.
  • Ducted air distribution losses cut heating and cooling efficiency by 25 to 40 percent.
  • In one study, the cracks and openings in ductwork represented 13 percent of the house leakage area. But when the furnace blower operated, ducts accounted for 70 percent of the air leakage.
  • Duct leakage commonly reaches 350 cubic feet per minute during blower operation.
  • During operation, air pressure inside ducts reaches 50 pascals (0.2 in. w.g.). That pressure can create 25 times more air leakage through a hole in the duct than the same size hole in the building shell. So, a one square inch hole in a duct is equivalent to a 25 square inch hole in a wall.
  • A building’s air leakage rate can triple when the furnace blower is turned on.

If the ductwork or the air handler is outside the heated space, air will leak through the joints, seams, filter slots, plenum connections and maintenance openings, unless they are properly sealed. The leakage is greatest when the system is on, because the blower creates higher pressure differences between the inside and outside of the duct.

But there is some leakage even when the fan is off. Ducts are commonly located in crawlspaces, basements, or attics, but the air in ducts is really inside air. Warm indoor air rises into return ductwork in the attic even with the blower off. This air can then leak into the attic, which contributes to the stack effect. Likewise, openings in supply ducts in the lower portion of the home allow air to enter from the crawlspace or unheated basement.

Differential Pressure

A forced air system works by creating a difference in pressure between the area where the supply registers are located and the area where the returns are located. A home with a typical duct layout has a positive net pressure around the perimeter of the home and a negative net pressure near the center. For example, bedrooms are usually pressurized and the hallway is depressurized.

Higher pressure inside the bedrooms compared to outdoors pushes conditioned inside air out through openings in exterior walls. Outside air is pulled into the central portions of the home where negative pressure dominates. Air commonly comes from the crawlspace, through openings in the floor for plumbing and through the ducts themselves.

Poorly designed duct systems can contribute to the problem because the air flow between supply and return isn’t balanced. (The registers don’t supply the same volume of air that is drawn into the return grille.) Even well designed systems may have only one or two returns. So, closing doors between supplies and the return makes matters worse.

Benefits of Air Sealing

Uncontrolled air leakage in a home affects not only the annual heating and cooling bills, but also the effectiveness of fresh air ventilation systems and the long-term durability of the structure.

Air Leakage Rates

To discuss the impact of air leakage, it’s helpful to have a unit of measurement. One common unit is “air changes per hour” (ach), which refers to the number of times in an hour that a volume of air equal to the volume of the house will pass through the building. Here’s a simple example. The footprint of this house is 40 ft. by 45ft. (1800 sq. ft.), and the ceilings are 8 ft. high. (40 x 45 x 8 = 14,400 cu. ft.)

If the air leakage rate of this house is 0.5 ach, then half its volume (7,200 cu. ft.) of air would move through it in an hour. That’s 120 cu. ft. per minute. (7,200 cu. ft./hr. ÷ 60 min.)

Today, most newly-built homes have an average air leakage rate of about 0.5 ach. Sealing major openings in the building shell, such as around windows and doors, plumbing penetrations and wiring penetrations—can reduce the average leakage to around 0.3 to 0.4 ach. This could be considered “standard air sealing.” Installing a “continuous air barrier” can cut the leakage rate to 0.1 ach. This would be “advanced air sealing.”

Here’s another way to look at the difference that air sealing can make. Imagine that all the leaks were combined into a single hole in the wall. That typical 1800 sq. ft. house would have a hole about 120 sq. in., or 10 in. x 12 in. Standard air sealing would reduce the whole to 60 sq. in., while advanced air sealing would cut it to about 35 sq. in. For comparison, the area of this page is about 94 sq. in.

ach house

Energy Savings

The heating load due to air leakage can make up about a quarter to a third of a home’s total space heating requirement. Often in newer homes built with more efficient windows and doors and higher levels of insulation, little attention is paid to air sealing. Builders believe they construct “quality” homes and don’t believe that a little air leakage is “that big a deal, after all, a house has got to breathe.”

Of course, the size of the “deal” has a lot to do with the price of energy and the severity of the climate in which the home is built. Going from the “typical” house (0.5 ach) to one with advanced sealing (0.1 ach) will cut the energy loss due to air leakage by 80 percent. The dollar value of these savings depends on the climate. For example, in Duluth you might save $187 per year, in Chicago about $124 and in Atlanta around $55. (These estimates assume that typical 1800 sq. ft. house with gas heat in an average year. Savings for air conditioning are not included.)

Indoor Air Quality

Air sealing offers other benefits in addition to energy savings. If major pollution sources are not brought into the home, it’s possible for the indoor air quality of the low-leakage home to be better than that in the old one that was supposed to “breathe.” 

An airtight floor or basement can actually prevent some hazardous gases, such as radon, from entering the house. Even tightening a ceiling can reduce the entry of radon into a home by reducing the stack effect, which helps pull soil gases into the home.

When the outdoor temperature is mild and there is no appreciable wind, forces that drive air leakage are absent. Without a mechanical ventilation system there would be no air exchange in the home—even with the windows open. So automatically-controlled mechanical ventilation is absolutely necessary. In a tight house, it’s easier for a small mechanical ventilation system to provide enough fresh air when and where it’s needed. The motto for good indoor air quality should be “Build tight and ventilate right.” 

Moisture and the Building Shell

By reducing the amount of air leakage in homes, you also reduce the potential for wood decay (insects, fungi, etc.) supported by moisture. As inside air leaks through walls and ceilings, it carries with it large amounts of water vapor generated within the home. Air leakage carries much more moisture into building cavities than vapor diffusion through materials. Vapor retarders, such as the facing on insulation and 4-mil plastic sheets, block vapor diffusion, but they have little effect on air leakage.

If the wall or roof sheathing is at a temperature below the dew point, some of the water vapor will condense on these cool surfaces. If this continues for a long time, enough moisture can collect to support molds, fungi and insect pests.

If the drying potential of the wall or ceiling assembly is not sufficient to remove the accumulated moisture during yearly cycles, there is a high likelihood of structural damage over time. Reducing the air leakage and providing a mechanical ventilation system are key strategies in extending the life of wood structures.