The question often arises as to what is the best relative humidity (RH) range for human health and comfort as well as for preventing moisture damage in the building. Building scientists agree that maintaining an indoor RH in the range of 35% to 55% is optimum.
High humidity in a house can lead to microbiological problems. Certain bacteria grow better when the moisture level is high. Molds and fungi thrive in a high moisture environment. In addition, moisture damage can lead to structural failure from disintegrating gypsum wallboard or rotting wood.
Low humidity (less than 30% for long periods of time) is just as unhealthy as high humidity, but for different reasons. Most of the human body’s defenses against upper respiratory illness depend on moist mucous membranes. Low RH levels in a building keep the mucous membranes too dry, which impairs their ability to transport disease microbes out of the body.
Relative Humidity and Pollutants
Research that has been conducted on what happens at different levels of relative humidity (RH). Note that humidity levels from 0% to 100% are shown on the horizontal line. If we go down the list on the vertical line, we can see that bacteria populations are high at low levels of RH, drop off at 30% RH, then begin to increase again at 60% RH.
Levels of viruses are high at low RH levels till about 50% RH, then begin to increase again at 70% RH. Fungi and dust mites need moisture to survive. We begin to see fungal growth at 60% RH and mites at 50% RH.
Research on respiratory infections and RH is limited, but suggests that more infections occur at lower levels of RH. Allergy problems and asthma occur at both low and high RH levels. Chemical interactions (here mainly formaldehyde emission) begin at about 30% RH and increase as RH increases. Ozone is a respiratory irritant that is produced by some appliances. Its levels are highest at low levels of RH.
There is an optimum zone of RH between 40% and 60%, where the presence of all these pollutants is minimized. However, be aware that in very cold weather, moisture levels in the home over about 50% RH can cause condensation problems, leading to the growth of mold and mildew.
Dew Point and Windows
Because windows are the most poorly insulated element of a building, and have the lowest R-value of insulation, they are often the first surfaces on which condensation occurs.
When the temperature is 70o F inside and 0o F outside, condensation will occur on the interior surface of a single-glazed window when the indoor relative humidity level is only 15%; on a double-glazed window at 42%; and on a low-emissivity window at 65%. By increasing the energy efficiency of the window, condensation is less likely because the interior glass surface does not get as cold.
Air Leakage is the Primary Moisture Transport Mechanism
In building science it is important to look at all of the openings and penetrations that normally occur in buildings. In construction or retrofit, attention must be given to sealing these holes to get control of air leakage. Without this control, no other efforts to improve the indoor air quality could succeed because ventilation will be ineffective if air leakage is too great.
In the past, people relied on air leakage to provide ventilation. However, leaky buildings do not guarantee good indoor air quality. Particularly in a cold climate, comfort and health problems result if the building is too leaky. A building leaking air during the coldest part of the winter will cause continuous drying of the indoor air and can lead to increased upper respiratory disease and discomfort.
Leaky buildings also enhance the transport of outdoor pollutants and help them enter the building. Leaky buildings allow for more insect entry as well.
High heating and cooling costs result if the building is too leaky, since the heated air leaks out so quickly.
In-House Moisture Generation
In 1990, the University of Illinois published a study of various activities of a family of four that contributed to in-house moisture generation. A family of four, just by breathing, can produce ½ pint of liquid water per hour, which is distributed as vapor in the air inside a house. Additionally, ½ pint can be produced by a shower or bath. Mopping a kitchen floor is a big source of moisture and can produce as much as 4½ pints in the house.
It doesn’t take much water evaporating in a building to raise its relative humidity to an unhealthy level. Four to 6 pints of water can raise the relative humidity of a 1,000 square foot house from 5% to 60%.
Biological Growth-Mold and Bacteria
Biological problems in buildings require three basic conditions.
First, there must be moisture of 20% by weight on most wood products (including the paper backing on gypsum wallboard), and the temperature range must be between about 40o F and 100o F.
Second, there must be fungal spores or bacteria available to initiate the growth of biologicals in the building. Fungal spores or bacteria are fairly widespread and almost always present in the air.
Third, there must be a nutrient source. Materials made of wood provide food for biological growth.
To prevent biological problems, particularly fungal growth, it is essential to keep the materials in the frame of the building dry.
Building Pressurization Due to Return Duct Leakage
A poor A/C duct system can cause many indoor air quality problems. Return duct leaks will draw in air from the space around the return ducts, which may include high levels of radon, moisture, odors, or other pollutants. This will not only impact A/C operating costs, but can result in mold growth within the warm and humid return duct system where it will not be visible to the homeowner.
Excessive moisture is a major cause of indoor air quality problems in warm climates. Moisture can be caused by high outdoor moisture brought in with ventilation air, poor dehumidification by oversized cooling units, high Sensible Heat Ratio (SHR) ratings on cooling equipment, unvented combustion heating systems in mild weather, or wet basements or crawlspaces that connect to the living space.
High Indoor Humidity and Indoor Air Quality Problems
While there are a number of health and indoor air quality problems that are caused by low humidity levels, the primary indoor air quality problem caused by high humidity levels results from the growth of mold. Many people exhibit allergy-like symptoms when exposed to high levels of mold spores. These symptoms are usually temporary and go away after leaving the space.
Other Causes of Moisture Problems
Leaky ductwork is not the only way that outdoor air with excessive moisture can enter a house. Many homes have a variety of exhaust appliances, such as clothes dryers, range hoods, and bath vents. Even power roof vents can depressurize the living space if there is an inadequate vent area in the attic.
High moisture levels can be made even worse when there are cool surfaces present, which permit mold to grow. Cool surfaces can result from cold walls below grade, or even from poor air circulation behind furniture or cabinets against outside walls.
Basements are a potential source of indoor air quality problems due to the possibility of radon, as well as high relative humidities present at the cool, below-grade walls. The humidity problem can be made worse by having carpeting or other materials on the floor or walls.
Letting outdoor air come into an unconditioned basement allows the basement humidity level to rise so mold can grow on the cool walls and floor, or in materials that cover them. To keep basement walls warmer during humid summer months and prevent mold growth, insulation should be applied to either the inside or the outside of below-grade walls. Pressurizing the basement with a small amount of air from the living space will help prevent the infiltration of both outdoor air and radon.
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University of Kentucky
College of Agriculture
October 1996, Revised October 1999
Reference: Alaska Housing Manual, published by the Alaska Housing Finance Corporation, 520 E. 34th Avenue, Anchorage, Alaska 9950
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