Indoor Air Quality in Office Buildings: A Technical Guide

5. Detailed Assessment

5.2 Individual Source Evaluation

This section focuses on IAQ indicators and pollutants, discussing each in terms of characteristics, guidelines, ideal levels, and health effects. Also provided is a checklist for use in a walkthrough specific to each indicator or pollutant source and a presentation of the measurement methods. This is followed by a discussion of remediation strategy as applicable.

5.2.1 Temperature and Humidity

Temperature and relative humidity are two of several parameters that affect thermal comfort. Satisfaction with the thermal environment can also be influenced by such factors as radiant temperature, air velocity, occupant activity level, and clothing.

ASHRAE Standard 55-1992, Thermal Environmental Conditions for Human Occupancy, presents guidelines that are intended to achieve thermal conditions that at least 80% of the occupants would find acceptable or comfortable.

Relative humidity levels below 25% are associated with increased discomfort and drying of the mucous membranes and skin, which can lead to chapping and irritation. Low relative humidity also increases static electricity, which causes discomfort and can hinder the operation of computers and paper-processing equipment. High humidity levels can result in condensation within the building structure and on interior or exterior surfaces and the subsequent development of moulds and fungi. In most Canadian cities, ideal indoor relative humidity levels are 35% in the winter and 50% in the summer. ASHRAE specifies a range between 25 and 60%.

In large buildings, the air supply is humidified over the winter season, usually by a water spray or steam system. Water spray humidifiers require regular scheduled maintenance to control water quality. Steam humidifiers are cleaner and easier to maintain, but use more electrical power. In the summer, the air conditioning dehumidifies the outdoor air supply.

5.2.1.1 Checklist

During the preliminary walkthrough, comfort-related problems will have been identified from occupant complaints and from observation.

a. Temperature

• Check for any evidence of high or low temperatures. Are these due to occupant interference, such as installation of heaters or new equipment?
• Check for local sources of heating or cooling, such as uninsulated floors over a garage or overhang, solar load through windows, or cold window frames.
• Ensure that thermostats are functioning, calibrated, correctly located, and not obstructed or enclosed
• Check for evidence of thermal gradients; the floor-to-ceiling differ-ential should not exceed 3 °C.
• Check for a balanced air distribution network (even air circulation and air current). Do occupants use fans?
• Check for any obstruction of air circulation, such as high office partitions, taped diffusers, or perimeter units blocked by paper, books, or cabinets.
• Look for diffusers directly over occupants or close to return slots

b. Relative humidity

• Check humidifier operation, including excess scale or rust, blocked nozzles, broken pump, and areas of stagnant, dirty water.
• Look for a defective or poorly calibrated humidistat located in the return air duct.
• Look for condensation caused by excess humidity or by insufficient thermal insulation of the building shell.
• Check for chemical additives used for water treatment.

5.2.1.2 Measurement Methods and Equipment

There are several methods of measuring temperature and relative humidity, ranging from a simple thermometer for temperature and a wet and dry bulb thermometer (psychrometer) for humidity to sophisticated electronic instruments equipped with solid state sensors.

Avoid sampling locations that are near machinery or heated directly by the sun or other sources of radiation. If possible, the operator should stand facing the air current so that the instrument receives the air first.

a. Psychrometers

A psychrometer measures relative humidity using the temperature difference between two temperature sensors, one of which is moist and cooled by an airstream. An electric fan (in the case of the powered psychrometer) or simple manual whirling of the device (in the case of the sling psychrometer) is employed to produce the airstream.

Sling psychrometers are inexpensive and simple to use; however, the results are unreliable. The instrument should be calibrated frequently against a primary standard, and the wick must be kept moist and clean. Powered psychrometers are more expensive but provide a direct and more accurate readout of relative humidity.

b. Hygrometers

Hygrometers are small, compact electronic units with a digital display for spot measurements or continuous logging of relative humidity. Some units also measure temperature and air motion.

A hygrometer employs a sensor that changes its resistance or capacitance as the humidity varies. The sensor is usually a hygroscopic salt or a small, thin film capacitor that absorbs moisture, producing a proportional output. Hygrometers should be calibrated at least once a year. Kits are usually available from the manufacturer, or the unit can be sent to a laboratory for calibration.

5.2.1.3 Strategy for Remediation

When thermal comfort complaints occur, the HVAC system's capacity to provide adequate heating or cooling and to humidify or dehumidify the occupied zone should be verified. Actual requirements may be quite different from original design parameters, especially if changes have occurred in the use of space, fit-up, layout, and occupancy or if new equipment has been introduced. Large heat-generating equipment may have to be ventilated separately or removed if the existing system cannot cool the space where it is located.

The operating and maintenance condition of the HVAC system and the control system should be in good working order and properly balanced and calibrated. Although zone control is important, so that, for example, a south-facing location can be cooled while a north-facing one is heated, individual occupant control would ideally provide thermal comfort to all, irrespective of location, clothing, and activity level.

Other remedial measures may include the following:

• Hot or cold surfaces may be insulated to reduce radiant heat gain or loss, reducing temperature gradients, drafts, and condensation. Windows may be insulated by adding another pane, reflective surface, or covering.
• Infiltration and exfiltration of the building envelope may be reduced by sealing openings and gaps and by maintaining a proper pressure differential between floors and between the inside and outside of the building.
• Ventilation and air circulation may have to be increased. An additional air handling unit (AHU), diffuser, or heater may have to be added to the problem area.
• System hours of operation may have to be extended to maintain environmental control during occupancy periods. For example, the operator may have to run the air conditioning system throughout the evenings during a hot spell to ensure an acceptable indoor environment at the start of the work shift.