CBP Magazine: Articles

Occupant comfort, a concept often overlooked in building designs of the past, has only recently become an area of focus for the design community. With the recent growth of the sustainable design movement, many factors aimed at improving the health and wellbeing of those working inside a building are gaining momentum. In addition to architects, building owners are beginning to realize that healthier and more comfortable building occupants are happier and more productive. As a result, many of today's sustainable building designs take the issue of occupant comfort into consideration.

In this final article on commercial building science, we will cover a variety of occupant comfort issues, including thermal comfort, indoor air quality (IAQ), acoustical comfort, and visual comfort.

There are a few standards for thermal comfort in buildings that have been set over the years by building and design industry organizations. Referenced most in the U.S. is the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), Atlanta, Standard 55, which provides guidelines and methods to measure the thermal comfort of building occupants. There are also two international standards that are very similar to the ASHRAE standard. Int'l Organization for Standardization (ISO) 7726 discusses instruments for measuring physical quantities, and ISO 7730 uses statistical means to determine comfort level.

These standards present detailed information on why building occupants complain about thermal conditions in a building. Air temperature and humidity are high on the list, but radiant temperature surfaces, such as walls and ceilings, also cause complaints when they are either too hot or too cold. Many people are sensitive to floor temperature.

Virtually any location in a building may be cause for thermal complaint. Vertical temperature differences and drafts, causing convective heat transfer, can also be added to the list. Secondary factors include daily and seasonal changes in temperature and humidity, the age of the occupant, and the occupant's adaptability to change.

There are six factors to consider when performing thermal comfort analysis:

Most people are comfortable in winter when the temperature ranges from 68 to 76 F and the relative humidity is between 60% and 30%. In summer, people feel comfortable in temperatures ranging between 74 and 80 F and when the relative humidity is between 60% and 30%.

Physical activity and corresponding metabolic rates definitely have an impact on comfort. Metabolic rate is measured in met units and the range can be from less than 1 met to 3 met, depending on activity level. Covered by ASHRAE Standard 55, the insulating values of clothing also have a unit of measurement-clo units.

IAQ requirements are addressed in ASHRAE Standard 62.1. This standard spells out minimum ventilation rates for new construction, as well as information on improving IAQ in existing buildings. It also provides lists of maximum contaminant levels for those spaces to maintain acceptable IAQ, which in turn minimizes the potential for adverse health effects on building occupants.

ASHRAE Standard 62.1 covers air classification and recirculation, the evaluation of contaminant concentration, sensory-irritation intensity, and odor offensiveness. There are four air quality classifications: 1, 2, and 3 for low, moderate, and significant levels of contaminants, and 4, for highly objectionable or harmful air quality. The chart at right shows just a few examples, ranging from a break room with air class 1, through a chemical storage area with air class 4. Air streams, air that flows through exhaust ducts, are classified separately. Exhaust from parking garages is a good example of an air stream that presents IAQ problems.

To maintain healthy IAQ, attached parking garages should be designed to limit vehicle exhaust from entering into other occupied spaces in the building. ASHRAE 62.1 recommends that air pressure in the garage be maintained at or below the pressure in adjacent space, so there is no pressure differential that draws exhaust gases into the building. Airtight vestibules should be used to separate the garage from the adjacent occupied space. Good design should isolate the garage to minimize any air exchange between it and the rest of the building.

Although it is often hard to detect, many buildings harbor potentially harmful elements that can contaminate interior air. Some of the most common airborne contaminants are:

There are guidelines for environmental emissions offered by the U.S. Environmental Protection Agency, Washington, especially on the subject of air pollution caused by a product's VOC emissions. The GREENGUARD Environmental Institute, Marietta, GA, certifies products for IAQ performance. Builders should look for their certification on components chosen for a building. GREENGUARD tests and evaluates products in accredited environmental chambers for formaldehyde and other aldehydes, VOCs, respirable particles, as well as ozone, carbon monoxide, nitrogen oxide, and other gases. ASHRAE 62.1 also addresses ways to remove such pollutants from the interior air.

Methods of airborne contaminant capture and removal include:

Humidity also has an impact on the design of HVAC systems for cooling. Sensible and latent loads must be calculated to properly size the air-conditioning system. Sensible loads account for air temperature changes only, without regard to moisture content. Latent loads take water vapor in the air into consideration. An important factor for designers to consider is the humidity ratio, the mass ratio of water vapor to dry air. It is also important to remember that geographic location determines the humidity ratio.

Miami has a hot and humid environment, with a lot of moisture that needs to be removed from occupied spaces. Phoenix and Miami can have similar air temperatures, but Phoenix has dry heat, with very little humidity. Even cities within the same state can have different climates. For example, in Texas, Amarillo has a similar climate to Phoenix, with the same dry heat. But Houston's hot and humid climate is the same as that of Miami. Therefore, HVAC designs must consider the specific climate, not just the region, into consideration.

Acoustic control is another method of enhancing occupant comfort that is currently acquiring attention in the design community. There are five goals to providing a superior acoustic environment:

Reverberation time is the time in seconds required for the sound-pressure level in a room to decrease or decay 60 decibels (dB). The shorter the reverberation time, the more absorptive the space.

The reduction of sound reverberation time is accomplished by employing sound-absorbing surfaces, such as acoustical ceilings, fabrics, and carpeting. The best plan is to configure those spaces to reduce, rather than amplify, the sound energy.

Long reverberation times, especially in the workplace, can have an effect on comfort. Sound that bounces off walls, floors, and ceilings can create an environment with poor communication and hard-to-understand speech. This can lead to increased stress levels, limited concentration, fatigue, and an increase in mistakes. So, it is wise to increase comfort by reducing reverberation times with the use of sound-absorbing materials. Ultimately it can make workers more efficient and help create a healthier working environment.

Limiting sound transmission through suspended ceilings usually means limiting crosstalk or possibly even machine noise. Builders can limit airborne noise transmission through ceilings by designing high ceiling attenuation class (CAC) assemblies. Partition heights can be extended through the ceiling plenum to provide additional noise isolation.

To limit the transmission of impact noise, design high-impact insulation class (IIC) assemblies. Isolate finished floors and ceilings by installing resilient underlayments, by using sound-absorptive floor coverings (carpets and carpet pads), and by using resilient ceiling suspension systems. The illustration in Figure 4 shows a method for acoustically isolating a floor.

Design HVAC systems to absorb energy and reduce background noise so airborne noise is not transmitted through the ductwork. Mechanical equipment should be isolated using vibration-dampening techniques and high sound-transmission-reduction enclosures. One other way that acoustical designers reduce the impact of background noise is to raise the level of background noise in the space with masking sound. Sound masking involves adding white noise or background music to the space.

Open plan office spaces-work areas that are not separated by ceiling-height partitions-should be designed for privacy and intelligibility. Speech intelligibility is mainly dependent on three factors: background noise, reverberation time, and the shape of the space. Speech privacy is the degree to which speech is unintelligible. Sometimes, speech should be unintelligible, especially when it's traveling from one office to the next. Sometimes, a high degree of intelligibility is important, such as in the classroom.

Sometimes, speech privacy is desirable, such as in an office setting where intruding speech should be made less intelligible, or in a doctor's office where voice transmission between examining rooms is undesirable.

Schools, in particular, have very specific acoustical performance requirements. The American National Standards Committee (ANSC) publishes criteria for mechanical-equipment noise control, HVAC systems, electrical systems, plumbing systems, and instructional equipment. They set a limit for background noise of 35 db. These criteria discuss sound-absorbing materials and give recommendations for noise isolation between interior spaces, in open-plan classrooms, outdoor-to-indoor, and other topics, such as impact sound and vibrating machinery.

Visual comfort is part practical and part aesthetic. It employs such strategies as artificial lighting, daylighting, and creating visually interesting environments.

There are a number of lighting strategies that can be employed to create a high-quality visual environment. Some recommendations are:

Whenever possible, integrate natural lighting into building plans. Studies show that natural lighting affects people's mood and comfort levels, so allow as much daylight as possible into a building. At the same time, it is important to avoid excessive heat loss and heat gain by specifying spectrally selective or low-e coatings on windows. Designing passive shading makes the interior spaces more thermally comfortable. Window glare can be controlled with window coverings, such as blinds or curtains.

Overlaying all of these principles is the need for aesthetics, or visual interest. Whenever possible, builders should provide a view of, and access to, the outdoors. They should balance the use of such elements as scale, color, texture, and pattern, as well as artwork and plants, to create visual interest.

These guidelines should impart a good understanding of how to design and build spaces that will increase the overall comfort and health of building occupants and help them to be happier and more productive. Occupant comfort makes a big difference in the success of a company.

Stanley D. Gatland II is the manager of building science technology for CertainTeed Corp.'s Valley Forge, PA, Insulation Group. He is responsible for generating and providing technical information to architects, engineers, builders, trade contractors, building-envelope consultants, building scientists, and building-code officials on the system performance of new and existing building-envelope materials, as well as building-science educational training. Stan has expertise in the areas of building science and architectural acoustics. He is a graduate of the Univ. of Massachusetts, Amherst, with BS and MS degrees in mechanical engineering. He is a member of ASHRAE, ASTM, ASME, and BETEC.

To learn more about designing for occupant comfort and other commercial building-science issues discussed in this column series, CertainTeed recommends its DVD, "Commercial Building Science: Concepts and Practices." For more information, or to obtain a copy, contact Stan Gatland at 800-233-8990.

Designing for Comfort Leads to Healthier Workers