Controlling airflow for a healthy building

by Katie Daniel | December 8, 2017 12:18 pm

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By Jason Smith
Balloons evoke childhood memories of a trip to the circus or a day spent at a local amusement park—the air that fills them is fundamental to life itself. Ironically, the same air that brings ‘life’ to a balloon (and us) causes headaches or worse for building owners, architects, and specifiers. In fact, air flow is the dominant force behind moisture transmission, spread of smoke, indoor air quality (IAQ) issues, and microbial deposition within building spaces.

Air barriers were developed to control airflow into and out of the building envelope. This article will help readers understand how controlling a building’s airflow through the use of coatings can keep the bad air out and the good air in.

Air quality impact of airflow leakage
Apart from the obvious means of ingress and egress (i.e. windows, doors, and ductworks), air flows through building materials by means of pressure differentials acting from the outside of the building inward and vice versa. The three sources of pressure differential are:

• chimney effect;
• wind pressure; and
• fan pressure. (This comes from North Carolina State University’s web article, “Infiltration [Also Called Air Leakage] Defined.” For more, visit energy.ces.ncsu.edu/infiltration-also-called-air-leakage-defined[2].)

The chimney effect, also known as stack pressure, occurs in all buildings, but especially those located in areas with hot summers and cold winters. In the winter, warm interior air rises and escapes through vents and windows, reducing the pressure at the base of the building. This creates a pressure differential on the wall’s surface. Even buildings without operable windows are never completely sealed—the air can flow through cracks under doors or through unprotected cracks in the walls. At the same time, higher pressure is exerted on the interior walls of the higher floors compared to outside. In the summer, the effect is reversed.

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The cooler indoor air sinks and draws hotter air from the vents on the roof. More pressure is exerted on the interior walls on the lower floors compared to the higher floors. These small but important pressure differentials are directly proportional to both the height of the building and the temperature difference between the inside and outside air.

The second source of pressure differential is wind, which can move into the building through cracks in the window seals or in the concrete or brick façade. Wind pressure causes the largest pressure differences on exterior walls, often 500 to 1000 Pa (about 10 to 20 psf), with gusts increasing the difference by as high as 2 to 2.5 times.

Fan pressure, the third major source, 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 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, but rather what that air carries with it. Air pressure differentials are capable of bringing hundreds of times more water vapour through a block of concrete than would otherwise naturally diffuse through. The vapour can carry micro-organisms that are then deposited within the building. Also, pollutants and allergens can be carried into the building through cracks via airflow, which create problems for people with asthma or other respiratory issues.

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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 sources and conditioned air; and
• resisting air pressure differentials that act on the air barriers externally and internally (i.e. pressure from HVAC fans).

To avoid confusion, this article is not going to cover vapour barriers, which are materials designed to prevent the diffusion of moisture through the surface on which it is applied. Vapour barriers and air barriers are not the same. An air barrier is designed to ‘breathe’ and therefore, is permeable. An air barrier sheet, for example, can be an air barrier and vapour-permeable. Other sheets can be an air barrier and vapour- impermeable. Use of these materials will depend on specific construction and building code requirements that are beyond the scope of this article. (For additional information on vapour barriers, Joseph Lstiburek’s “Understanding Vapour Barriers” is a useful resource. Visit buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers[5].)

The National Air Barrier Association (NABA) was founded in 1995 to bring the air barrier industry together to encourage proper research and development, standards and specifications, manufacturer and contractor licensing, installer training and certification, documentation and reporting, and a third-party audit process. The group’s membership includes contractors, manufacturers, design professionals, testing agencies, consultants, and utilities.

NABA has set the Canadian standard for air permeance—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 litres per second per square metre of surface (L/s•m²), or in imperial units of cubic feet per minute per square foot of surface (cfm/sf). Permeance, in the context of air barriers, is generally defined in three levels:

1. Air barrier materials must have a maximum permeance of 0.02 L/s•m² (0.004 cfm/sf) at a pressure differential of 75 Pa (1.6 psf). Examples include liquid coatings or manufactured sheets.

2. Assemblies using air barrier materials cannot have more than 0.2 L/s•m² (0.04 cfm/sf) at the same pressure differential. Referred to as an ‘air barrier assembly,’ one example would be a wall composed of concrete masonry units (CMUs) coated with an air barrier material and perhaps rigid foam insulation to mimic real-world conditions.

3. Building enclosures cannot have more than 2 L/s•m² (0.4 cfm/sf) over the same pressure differential. Referred to as ‘air barrier systems,’ they include the whole enclosure—from the latex interior wall paint to the exterior façade around the entire building. (For more, see NABA’s January 2013 Technical Bulletin, “Air Barrier Requirements in Canadian Building Codes and Standards.”)

The balloon example can again be used to dramatically illustrate air permeance. An average 305-mm (12-in.) latex balloon can hold about 14 L (0.5 cf) of air. For a building with exposed walls comprising CMUs, approximately 15 per cent of the volume of concrete, once cured, 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 air permeance through unpainted concrete block can exceed 0.75 L/s•m² (0.15 cfm/sf) at a sustained pressure differential of 72 Pa (1.5 psf). (This comes from “Building Airtightness Requirements—Code Requirements for Commercial and Multifamily Building Airtightness Testing, City of Fort Collins.” For more information, visit www.fcgov.com/utilities/img/site_specific/uploads/2015-03-25-FtCollins-MFProtocol-ABT-PiePresentation.pdf[6].) If this pressure differential is sustained for more than an hour, enough air bleeds through 1-m² (10.7-sf) section of concrete to fill about 193 latex balloons. When an air barrier coating is applied to this same surface area, the air permeance drops to less than 0.02 L/s•m² (0.004 cfm/sf), which, after an hour at that sustained pressure difference, would only fill about five balloons—a reduction of more than 97 per cent.

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Coatings
Liquid air barriers are often favoured by contractors because they are easy to apply and can be either sprayed, rolled, or, in the case of a mastic, trowelled onto the surface. Usually not exceeding about 1 mm (40 mils) in thickness, fluid-applied air barriers form a continuous seamless covering around the wall and up to the roof even in more complex architectural structures containing protrusions or tight spaces as well as beneath ground level.

Liquid coatings are usually formulated with resilient water-based polymers that may or may not contain an emulsified asphalt to impart additional moisture resistance. Rheology modifiers are added to impart sag resistance to the coating.

There are products compliant with volatile organic compound (VOC) restrictions that can coalesce in cold environments at temperatures as low as 0.6 C (33 F), and can adhere to damp substrates.

Wraps and sheets
Air barrier membranes can be made by casting a butyl polymer (which is very sticky) on a high-density polypropylene, then rolled into a wide sheet that is nominally 1 mm (40 mils) thick. The butyl side is adhered to the wall panel. These can be cut to size and adhered to a wall surface with relative ease and provide suitable air infiltration resistance. The butyl has fantastic flexibility, even at low temperatures.

Sheet air barriers bridge gaps better than air barrier coatings, and no special equipment is needed to install one. Unlike liquid-applied barriers, applying sheets creates many seams that need to be covered with additional butyl tape or sealant to prevent air infiltration at those sites. Sheets applied around windows or protruding pipes must also be addressed in order to ensure a tight seal is achieved at the cuts in the sheet.

Air barrier systems
Air barrier coatings and sheets can be used in conjunction with other materials to create air barrier assemblies or air barrier systems. (To read Joseph Lstiburek’s 2006 article on the topic, “Understanding Air Barriers,” visit buildingscience.com/documents/digests/bsd-104-understanding-air-barriers[8].) For example, an exterior wall may consist of a painted gypsum board wall placed over a polyethylene air barrier and vapour barrier. This sheet is placed over the stud and cavity insulation that may comprise a polyurethane foam or fibreglass insulation. OSB covers the other side of the stud on the exterior side of the wall, followed by vapour-permeable building paper or house wrap and, finally, the siding. The latex paint, gypsum, and air barrier sheet is an air barrier system.

Air barrier manufacturer websites are useful starting points for determining the right product or system for the building owner. (For additional information, training, and technical literature on air barrier types, access NABA’s website at www.naba.ca[9].)

Conclusion
With the emergence of more stringent building codes requiring airflow 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 airflow in check, moisture, pollutants, or allergens that would otherwise be carried into the building through unprotected surfaces stay outside where they belong. This way, the air can be used for more benign purposes, like, say, filling balloons.

Jason Smith is the senior research and development chemist for The Garland Company in Cleveland, Ohio. He has been awarded numerous U.S. patents for roofing and has regularly published articles related to roofing science, coatings applications, and solvent regulations. Smith is a member of the Roof Coatings Manufacturers Association (RCMA) where he serves on the board of directors; he also co-chairs its Technical Committee. He received his undergraduate degree in chemistry from the University of Pittsburgh and his master’s degree in polymer chemistry and coatings from DePaul University in Chicago. Smith can be reached via e-mail at jasonsmith@garlandind.com[10].

Source URL: https://www.constructioncanada.net/controlling-airflow-healthy-building/

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