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3  FM quick reads on relative humidity

1. Keep Humidity Just Right For Good Indoor Air


Today's tip focuses on how relative humidity, both too high and too low, can have a negative effect on indoor air quality. In general, the sweet spot for humidity is somewhere between 30 and 60 percent relative humidity.

Don't operate your building at low relative humidity. One of the most irksome contributors to poor IAQ, especially in wintertime, is low relative humidity. Lowering the temperature in the occupied spaces is one practical way to improve this situation.

When the relative humidity in commercial and institutional buildings is low, occupant's mucous membranes in the nose, mouth and throat dry out. The result is they become much more sensitive to the ubiquitous pollutants found in an indoor environment. More cases of colds, allergies, and even nosebleeds are not uncommon.

The solution to this problem is difficult. Installing a central humidification system into the building's air handling system is often impractical and cost prohibitive. Individual space humidifiers can help, but they have stringent maintenance requirements which, if not adhered to, can present IAQ problems of their own.

But don't go overboard and make the mistake of operating your building at high relative humidity either. High humidity provides conditions for microbial growth. If high relative humidity conditions are a problem at your facility, consider using dehumidification equipment. You can also consider the use of high limit humidifier controls to prevent condensation on the inside surfaces of the downstream duct. Lastly, consider using unlined ductwork with external insulation directly downstream of the humidifiers or cooling coils to help prevent microbial contamination.

You can refer to ASHRAE Standard 55, "Thermal Environmental Conditions for Human Occupancy" for more detail on acceptable ranges for humidity.

2.  Data To Measure IAQ Is Complex Proposition

Today's tip is to understand that measuring IAQ is more complicated than a metric such as energy use. Measuring indoor air quality means amalgamating several different metrics to give a holistic IAQ picture.

"There is not just one measurement that can assess the dynamic relationship between the presence of air contaminants and the ventilation to effectively dilute and remove them," says David Bearg, president of Life Energy Associates. "Instead, assessing the healthfulness of an indoor environment is more a matter of measuring key parameters" such as effective ventilation rates, contaminant levels, absolute humidity, and even occupant satisfaction.

"The most important thing is measuring and maintaining airflow rates and exhaust," says John McFarland, director of operations, Working Buildings, Inc. Luckily, air flow is also the easiest IAQ-related metric to measure and benchmark, says McFarland, who is also the vice chair of the ASHRAE 62.1 committee.

ASHRAE 62.1 is generally thought of as a design standard, but facility managers can use its recommended ventilation rates toward IAQ. Determine the cubic feet per minute (CFM) of airflow per person, then use 62.1 to determine if the current airflow rates meet the minimum requirements for the space occupancy, especially if space functions have changed.

Also, a high percentage of completed preventive maintenance bodes well for good IAQ. It means you're regularly checking filters, examining dampers to make sure they're opening and closing properly, and making sure drain pans aren't full of water. Essentially, you're continuously commissioning your systems to make sure they're behaving as they should.

ASHRAE 62.1 also provides a standard for relative humidity (maximum of 65 percent in the 2007 edition), which is another part of the IAQ whole. "Extremes of moisture, either too dry or too humid, can adversely impact IAQ," Bearg says. If the air is too wet, mold can form. But air that is too dry (especially in the winter), makes occupants uncomfortable, says Bearg. So measuring the dew point temperature and humidity, and benchmarking that data, is one of the more important, yet overlooked, parts of getting a holistic IAQ view.

3.  Using ASHRAE 62.1 to Maintain Good IAQ

Facility managers can use ASHRAE 62.1's recommended ventilation rates to get an idea of whether the rates in their buildings are too high (and therefore not energy efficient) or too low (and therefore not conducive to good indoor air quality).

Now, no one will likely check whether a building is in ongoing compliance with ASHRAE 62.1 on an outside airflow basis, but if there's ever a problem, it's going to look bad if you don't know whether you're in compliance or not. It's good risk management best practice to have the data available.

Another aspect of compliance with 62.1 is preventive maintenance, which also fosters good indoor air quality. If you're measuring the percentage of prescribed preventive maintenance being completed, based on the maintenance manual that ASHRAE 62.1 requires must be provided when the building is handed over, a high percentage of completed preventive maintenance points to likely good indoor air quality. It means you're regularly checking filters, regularly examining dampers to make sure they're opening and closing properly, and making sure drain pans aren't full of water.

ASHRAE 62.1 also provides a standard (maximum of 65 percent in the 2007 edition) for relative humidity, which also plays into indoor air quality. Air that is too wet promotes mold growth. But air that is too dry (especially in the winter) can be just as bad for indoor air quality, causing occupant's mucous membranes to dry out and making them uncomfortable.

Measuring the dew point temperature and humidity, and benchmarking that data, is one of the more important, yet overlooked, parts of maintaining good indoor air quality.


relative humidity , indoor air quality , ASHRAE 55

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