Apart from the obvious means of ingress/egress (i.e. windows, doors, ducts), air flows through building materials by means of pressure differentials acting from the outside of the building inward and visa versa. The three sources of pressure differential are:
- Chimney effect (stack pressure)
- Wind pressure
- Fan pressure
Chimney (or stack) effect occurs in all buildings, with the extremes of this effect occurring in areas that have hot summers and cold winters. In the winter, warm inside air rises and escapes though vents and windows, reducing the pressure at the base of the building. This creates a pressure differential on the wall’s surface. Because buildings are not completely sealed (there is usually a ground floor door present), the air can flow through cracks under the door or through unprotected cracks in the walls. At the same time, there is a higher pressure exerted on the interior walls of the higher floors compared to that outside. In the summertime, the effect is reversed. The cooler indoor air sinks and draws hotter air from the vents on the roof. There is more pressure exerted on the interior walls on the lower floors compared to the higher floors. These small but important pressure differentials are usually measured in either pounds per square foot (psf), or metric Pascal (Pa) units. The pressure differentials at these extremes are directly proportional to both the height of the building and the temperature difference between the inside and outside air.
By way of illustration, measurements have shown that a 200-foot tall building in the middle of summer (about 92°F outside, 72°F inside) will have a pressure difference of approximately 0.8 psf (38 Pa). As the building height increases, so too does the pressure difference exerted on the walls over the same temperature difference.
Wind moving into the side of a building infiltrates the building through cracks in the window seals or in the concrete or brick. Wind pressure causes the largest pressure differences on exterior walls, often 10 to 20 psf (500 to 1000 Pa), with wind gusts increasing the difference by as high as 2 to 2.5 times.
Fan pressure is caused by HVAC fans as they exhaust to introduce and circulate the air within a building. This pressure can be controlled positively, to meet the demands of stack effect buildings (especially in tall structures), or negatively, to keep moist air from entering through exterior walls or the roof. The pressure differences caused by fan pressure are low, but must be considered during building design.
These three sources of pressure difference create a means for air to flow. In most cases, the issue is not so much the air coming into the living or working space; it is what is carried with the air that causes the problems. Air pressure differentials are capable of carrying hundreds of times more water vapor through a block of concrete than it takes for the water vapor to naturally diffuse through the same concrete. The water vapor can carry microorganisms that are then deposited within the building. Also, pollutants and allergens can be carried into the building through cracks via air flow, which create problems for people with asthma or other respiratory issues.
Controlling Air Migration
Air barriers are designed to control the flow of air between a conditioned indoor space and an unconditioned outdoor space by:
- Providing a continuous coating (or covering) over the entire building or individual unit
- Acting as a smoke, gas and fire barrier between a garage or other fume source and conditioned air
- Resisting air pressure differentials that act upon the air barriers externally and internally (pressure from HVAC fans)
The Association of Air Barriers in America (ABAA) was incorporated in Massachusetts in 2001, and began requiring that air barriers be used in the state’s building code to help regulate air flow. Currently, over 35 states have regulations regulating air flow control to some degree.
ABAA has set the standard for air permeance, which is a measure of the volume of air that is permitted to pass through an area of substrate in a given time at a specified pressure difference across the coating. The units are normally given as either cubic feet per minute per square foot of surface (cfm/ft²), or metric liters per second per square meter of surface (L/s/m²). Permeance, in the context of air barriers, is generally defined in three levels:
- Air barrier materials must have a maximum permeance of 0.004 cfm/ft² (0.020 L/s/m²) at a pressure differential of 1.6 psf (75 Pa).
- Assemblies using air barrier materials cannot have more than 0.04 cfm/ft² (0.20 L/s/m²) at the same pressure differential; these are referred to as air barrier assemblies.
- Entire building enclosures cannot have more than 0.4 cfm/ft² (2.0 L/s/m²) over the same pressure differential; these are referred to as air barrier systems.
A balloon can be used to illustrate air permeance. Consider that an average 12-inch plastic balloon can hold about 0.5 cubic feet of air. Next consider a building with exposed walls composed of concrete block. Once dried, approximately 15 percent of the volume of concrete consists of empty space in the form of tiny pores, most with diameters smaller than a human hair. Compared to the size of the nitrogen and oxygen molecules found in air, however, the pores are enormous. Tests have shown that air permeance through unpainted concrete block (cinder block) can exceed 0.15 cfm/ft² (0.75 L/s/m²) at a sustained pressure differential of 1.6 psf (75 Pa). At this differential, enough air bleeds through a square foot section of concrete block in one hour to fill about eighteen Mylar® balloons. When an air barrier coating is applied to this same surface, the air permeance drops to less than 0.004 cfm/ft² (0.020 L/s/m²), only enough air to half-inflate one balloon.
Liquid air barriers are favored by contractors because they are easy to apply. They can be sprayed, forming a continuous seamless covering around the wall and up to the roof and wherever moisture barriers meet beneath ground level. VOC-compliant liquid air barriers have been formulated that can coalesce in cold environments at temperatures as low as 33°F, and which can adhere to damp substrates. Some even qualify for LEED® credits by optimizing energy performance, using recycled components, improving indoor environmental quality and using renewable raw materials. What’s more, air barrier coatings can be used in conjunction with other materials to create air barrier assemblies or air barrier systems.
In conclusion, with the emergence of new building codes requiring that air flow be controlled in new construction, the use of air barriers is an effective means of curtailing the effects of air transmission through the building envelope. With the air flow in check, moisture, pollutants, or allergens that would otherwise be carried into the building through unprotected surfaces stay outside where they belong, and the air can be used for more benign purposes, like, say, filling shiny red balloons.
Mylar® is a registered trademark of DuPont Tejjin Films.
LEED®, Leadership in Energy and Environmental Design®, and Green Building Rating System® are registered trademarks of The U.S. Green Building Council.