By Sarfraz Panjwani
Managing Director
ALM Engineering and Instrumentation Pvt Ltd
Mold is a fungus. Outdoor mold breaks down dead trees and fallen leaves and hence plays an important role in nature. However, mold growing indoors should be avoided, because it can damage carpets and weaken floors and walls.
Mold requires a nutrient source, proper temperature and moisture to grow. Mold does not require light to grow. It does not produce food, but instead adsorbs nutrients by breaking down hydrocarbons. It will grow on any organic building material such as paper, adhesives, resins, etc. or even grow on the patina of dust that collects on surfaces. Nutrients to support mold growth are ubiquitous in the building environment. The temperatures required for mold growth are in the same range as indoor building environments. Most molds grow in the temperature range of 15°C to 30°C (59° to 87°F). Control of moisture is the only practical way to control mold growth.
Fig. 1: Mold development on the ceiling
Relative humidity reflects the relative amount of water that air can hold at a given temperature. Change the temperature and the humidity will change even if net amount of moisture in the air remains the same.
In fact a 1ºF change in room temperature can change the relative humidity by 2%. If outside air at 85ºF (29.5ºC) and 60% RH is cooled to 72ºF (22ºC) degrees without any moisture removal, the RH will increase to almost 90%. This temperature effect on relative humidity makes it useless as a moisture control parameter.
Relative Humidity is a confusing term especially when we are talking about controlling mold.
Pic. 2: RH changes widely and constantly with temperature
The question raised then is controlling relative Humidity the right way to control Mold?
First we have to understand that mold formation is always on a surface. Also, for the mold to develop the “moisture content” of the surface has to be in a certain range.
What is moisture content?
Moisture content is not relative humidity. Moisture Content (MC) is the mass of the moisture in a material relative to the dry mass of the material, expressed as a percent. For example if there is 17 grams of water in a piece of wood that weighs 100 grams when dry, then the wood has a Moisture Content (MC) of 17%.
That implies that the Rh and temperature of the space in which we are actually making all the measurements is giving us no indication of the surface conditions. Hence the water condensate on the surface of a material is a good indicator of mold growth, RH in the space cannot be used to predict or control the moisture content on mold vulnerable surfaces. This surface may be in the ductwork, in carpet, or on the inside of walls and at a very different temperature than what we would typically measure in the space.
We must therefore measure and control the temperature at which the water condenses on the surface, thereby controlling mold conditions. This temperature is called Dew point. Instruments that monitor dew point directly can be used for direct control, as usually the coldest surfaces in a building are known.
Pic. 3: Telaire NDIR DEW point sensor
Dew point instruments are even now considered complicated, expensive devices. Contrary to this belief, the new technologies have made these instruments very compact, user-friendly and maintenance free. Unlike the RH sensors which cannot be calibrated the Dew point sensors offer calibration possibility and an enhanced design life of over 15 years. These devises come in wall mount and duct mount designs.
Pic. 4: Protimeter Mini & Psyclone for measuring MC% and Dew Point(Picture Courtesy Amphenol Advanced Sensors)
Dew point measurement besides in mold control have proved to be a better tool for controlling all aspects of building operation from cooling, to economizer control to dehumidification control.
Alternatively one can also measure the surface Moisture Content % and absolute humidity or dew point as part of the preventive maintenance.
References:
1. Mold Prevention A Guide for Residents and Homeowners in Indian Country
2. Indoor Air Quality and Mold Prevention of the Building Envelope by Roger Morse, AIA and Don Acker, PE.
