What You Should Know About Humidity
Lew Harriman serves as director of research at Mason Grant, Portsmouth, N.H., and is the current chair of ASHRAE technical committee 1.12 — Moisture Management in Buildings. He has served as a mold consultant, forensic investigator, author, and trainer for the last 25 years.
Harriman believes if there’s one thing contractors need to know about mold, it’s that mold doesn’t care about what’s going on in the air; it only cares about how much moisture ends up in its food source.
“When diagnosing a problem, I advise contractors to obtain a moisture meter and test the moisture content of the materials rather than assuming the relative humidity of the air is the main issue,” he said. “If wall board and ceiling tile — both great sources of cellulose, which is easy mold food — have a moisture content above 16 percent, as measured by a wood moisture meter, risk is increasing. If the moisture content of wall board is over 19 percent, there’s a very high risk of mold growth. Below 13 percent, there’s hardly any risk.”
Harriman served as chair on a committee of engineers and public health officials who revised ASHRAE’s official position document, “Limiting Indoor Mold & Dampness in Buildings.” The document gained approval from the ASHRAE board of directors in June, and is scheduled to appear shortly on the ASHRAE website, http://bit.ly/PklV12. The document observes there are 18 HVAC decisions that can either increase or reduce the risk of indoor moisture accumulation and mold growth. Factors observed to reduce risks include: Ensuring that all ventilation air is dried to a dew point below the dew point maintained inside the building; ensuring that all condensation inside HVAC components and air distribution ductwork is drained away to an appropriate sanitary drain or condensate collection system; and ensuring that indoor surfaces of both occupied and unoccupied spaces are not cooled to temperatures so low as to create an average surface relative humidity of over 80 percent which lasts for more than 30 days, or surface cold enough to allow visible condensation.
Other factors that also reduce risks include keeping the indoor dew point low enough to ensure that there is no condensation on the exposed surfaces of cool HVAC ductwork components; ensuring that humidifiers are sized, installed, and controlled so they do not overload the air with humidity; and ensuring that cold HVAC and plumbing components are sufficiently insulated to keep the temperature of all surfaces at least 10˚F above the dew point of the surrounding air.
Factors that have been observed to increase the risk of moisture accumulation include failing to keep the indoor dew point low enough to prevent condensation indoors, or failing to keep surface RH below 80 percent in occupied spaces or inside hidden building assemblies; over chilling a building’s surfaces during humid weather; redistributing microbial air contaminants from a contaminated space into occupied areas; and failing to make air distribution components, joints in return plenums, and supply and exhaust ducts airtight.
Other factors include failing to keep the long-term average indoor air pressure positive with respect to the outdoor temperature when the outdoor dew point is higher than indoor surface temperatures; failing to prevent dirt and dust accumulation on cooling coils, duct surfaces, and sound lining downstream of cooling coils; failing to keep the air velocity through cooling coils low enough to prevent droplet carryover into downstream ductwork and filters; failing to install condensate drain traps deep enough to allow free-flowing drainage of normal cooling coil condensate; and more.
“The moral of the story is that you need airtight buildings to limit humid air infiltration, especially during unoccupied hours. And don’t oversize the cooling equipment so it creates a meat locker instead of a comfortable environment,” said Harriman. “More tons do not produce more dehumidification. In fact, it’s the reverse. Less cooling tonnage means longer compressor run times, and longer run times produce more dehumidification. Good target ranges are 75 to 78˚F along with a dew point no higher than 55˚F.”
The Air Conditioning Contractors of America (ACCA) approach air moisture issues with a proper design followed by installation, maintenance, and then proper remediation practices, if there is a failure.
ACCA offers a series of manuals to enable contractors to properly design, install, operate, and maintain HVAC systems. Load calculation is defined in Residential Manual J; equipment selection, Residential Manual S and Commercial Manual CS; duct system design, Residential Manual D and Commercial Manual Q; air distribution, Residential Manual T and Commercial Manual Q; and test, adjust, and balance, Manual B.
“These manuals are also excellent resources for instructors to use as they teach those same subjects. Each element in the process is interrelated; therefore successful execution of one is dependent of proper completion of earlier elements,” said Wes Davis, director, Quality Assured Programs, ACCA. “The process starts with the design manuals addressing quality elements of the HVAC system through its operating life cycle. The standards specify minimum industry-developed requirements that provide contractors with nationally recognized references for system installation, maintenance, and when there is a big problem, with restoring the cleanliness of HVAC systems. In addition, there is a standard to comprehensively evaluate attributes of the home. These attributes should be considered to reduce the size of the HVAC system, identify safety issues, and to ensure that the home’s components will perform as a system.”
Certified Expert Advice
Phillip Fry, a certified environmental hygienist, mold inspector, mold remediator, and author of five mold advice e-books, believes mold is present in a large number of residential, commercial, and public buildings.
“One of the major causes of HVAC mold is that the air conditioning cooling process condenses humidity moisture in the treated air and turns it into water inside the air conditioners, air conditioning equipment, and air ducts to drive significant mold growth,” he said. Another major air conditioning-related mold problem in offices, stores, and other commercial buildings happens when air conditioning systems are turned off after business hours or on the weekend. “Such non-operation of the air conditioning can allow indoor building humidity to build up to cause widespread mold growth in both the HVAC system and the building itself,” said Fry. “In general, when air conditioning is operating, it usually reduces indoor humidity below the mold-causing 70 percent level that allows mold to easily grow.”
Fry encourages HVAC contractors to, at the very least, be able to recognize what mold growth looks like (mold comes in many colors, forms, and appearances); test outward HVAC airflow for the possibility of elevated levels of airborne mold spores; and inspect and test inside HVAC equipment and ducts for mold infestation.
Other necessary skills include cleaning and removing mold growth and accumulated dirt/dust from the inside of HVAC equipment and ducts; killing any remaining mold with a high ozone generator and the spraying and fogging of EPA-registered fungicides that are specifically EPA-authorized for use inside HVAC systems; and treating HVAC equipment and ducts with EPA-registered-for-HVAC mold preventive coating.
Publication date: 11/5/2012