Atlanta Home Inspection-certified member ICC,

Moisture Control

Water in your home can come from many sources. Water can enter your home by leaking or by seeping through basement floors. Showers or even cooking can add moisture to the air in your home. The amount of moisture that the air in your home can hold depends on the temperature of the air. As the temperature goes down, the air is able to hold less moisture. This is why, in cold weather, moisture condenses on cold surfaces (for example, drops of water form on the inside of a window). This moisture can encourage biological pollutants to grow.

There are many ways to control moisture in your home:

• Fix leaks and seepage. If water is entering the house from the outside, your options range from simple landscaping to extensive excavation and waterproofing. (The ground should slope away from the house.) Water in the basement can result from the lack of gutters or a water flow toward the house. Water leaks in pipes or around tubs and sinks can provide a place for biological pollutants to grow.
• Put a plastic cover over dirt in crawlspaces to prevent moisture from coming in from the ground. Be sure crawlspaces are well-ventilated.
• Use exhaust fans in bathrooms and kitchens to remove moisture to the outside (not into the attic). Vent your clothes dryer to the outside.
• Turn off certain appliances (such as humidifiers or kerosene heaters) if you notice moisture on windows and other surfaces.
• Use dehumidifiers and air conditioners, especially in hot, humid  climates, to reduce moisture in the air, but be sure that the appliances themselves don't become sources of biological pollutants.
• Raise the temperature of cold surfaces where moisture condenses. Use insulation or storm windows. (A storm window installed on the inside works better than one installed on the outside.) Open doors between rooms (especially doors to closets which may be colder than the rooms) to increase circulation. Circulation carries heat to the cold surfaces. Increase air circulation by using fans and by moving furniture from wall corners to promote air and heat circulation. Be sure that your house has a source of fresh air and can expel excessive moisture from the home.
• Pay special attention to carpet on concrete floors. Carpet can absorb moisture and serve as a place for biological pollutants to grow. Use area rugs which can be taken up and washed often. In certain climates, if carpet is to be installed over a concrete floor, it may be necessary to use a vapor barrier (plastic sheeting) over the concrete and cover that with sub-flooring (insulation covered with plywood) to prevent a moisture problem.
• Moisture problems and their solutions differ from one climate to another. The Northeast is cold and wet; the Southwest is hot and dry; the South is hot and wet; and the Western Mountain states are cold and dry. All of these regions can have moisture problems. For example, evaporative coolers used in the Southwest can encourage the growth of biological pollutants. In other hot regions, the use of air conditioners which cool the air too quickly may prevent the air conditioners from running long enough to remove excess moisture from the air. The types of construction and weatherization for the different climates can lead to different problems and solutions.

Moisture On Windows

Your humidistat is set too high if excessive moisture collects on windows and other cold surfaces. Excess humidity for a prolonged time can damage walls especially when outdoor air temperatures are very low. Excess moisture condenses on window glass because the glass is cold. Other sources of excess moisture besides overuse of a humidifier may be long showers, running water for other uses, boiling or steaming in cooking, plants, and drying clothes indoors. A tight, energy efficient house holds more moisture inside; you may need to run a kitchen or bath ventilating fan sometimes, or open a window briefly. Storm windows and caulking around windows keep the interior glass warmer and reduce condensation of moisture there.

Humidifiers are not recommended for use in buildings without proper vapor barriers because of potential damage from moisture buildup. Consult a building contractor to determine the adequacy of the vapor barrier in your house. Use a humidity indicator to measure the relative humidity in your house. The American Society of Heating and Air Conditioning Engineers (ASHRAE) recommends these maximum indoor humidity levels.

Outdoor Recommended Indoor Temperature Relative Humidity

+20o F.	35%
+10o F.	30%
0o F.	25%
-10o F.	20%
-20o F.	15%

Source:  Anne Field, Extension Specialist, Emeritus, with reference from the Association for Home Appliance Manufacturers (www.aham.org).

How to Identify the Cause of a Mold and Mildew Problem

Mold and mildew are commonly found on the exterior wall surfaces of corner rooms in heating climate locations.  An exposed corner room is likely to be significantly colder than adjoining rooms, so that it has a higher relative humidity (RH) than other rooms at the same water vapor pressure.  If mold and mildew growth are found in a corner room, then relative humidity next to the room surfaces is above 70%.  However, is the RH above 70% at the surfaces because the room is too cold or because there is too much moisture present (high water vapor pressure)?

The amount of moisture in the room can be estimated by measuring both temperature and RH at the same location and at the same time.  Suppose there are two cases.  In the first case, assume that the RH is 30% and the temperature is 70^oF in the middle of the room.  The low RH at that temperature indicates that the water vapor pressure (or absolute humidity) is low.  The high surface RH is probably due to room surfaces that are "too cold."  Temperature is the dominating factor, and control strategies should involve increasing the temperature at cold room surfaces.

In the second case, assume that the RH is 50% and the temperature is 70^oF in the middle of the room.  The higher RH at that temperature indicates that the water vapor pressure is high and there is a relatively large amount of moisture in the air.  The high surface RH is probably due to air that is "too moist."  Humidity is the dominating factor, and control strategies should involve decreasing the moisture content of the indoor air.

Should You Have the Air Ducts in Your Home Cleaned? - excerpt on duct cleaning and mold follows, please review the entire document for additional information on duct cleaning and mold.

You should consider having the air ducts in your home cleaned if:

There is substantial visible mold growth inside hard surface (e.g., sheet metal) ducts or on other components of your heating and cooling system. There are several important points to understand concerning mold detection in heating and cooling systems:

• Many sections of your heating and cooling system may not be accessible for a visible inspection, so ask the service provider to show you any mold they say exists.
• You should be aware that although a substance may look like mold, a positive determination of whether it is mold or not can be made only by an expert and may require laboratory analysis for final confirmation.  For about $50, some microbiology laboratories can tell you whether a sample sent to them on a clear strip of sticky household tape is mold or simply a substance that resembles it.
• If you have insulated air ducts and the insulation gets wet or moldy it cannot be effectively cleaned and should be removed and replaced.
• If the conditions causing the mold growth in the first place are not corrected, mold growth will recur.
• What relative humidity should I have in my home? Seems like a simple enough question. However, the
  answer can sometimes be difficult to understand.
• Elevated relative humidity at a surface – 70 percent or higher - can lead to problems with mold, corrosion,
  decay and other moisture related deterioration. When relative humidity reaches 100 percent,
  condensation can occur on surfaces leading to a whole host of additional problems. An elevated relative
  humidity in carpet and within fabrics can lead to dust mite infestation and mildew (mildew is mold growing
  on fabrics).
• Low relative humidity can lead to discomfort, shrinkage of wood floors and wood furniture, cracking of
  paint on wood trim and static electricity discharges.
• The key is not to be too low and not to be too high. High enough to be comfortable, but low enough to
  avoid moisture problems associated with mold, corrosion, decay, and condensation.
• Unfortunately, determining the correct range depends on where the home is located (climate), how the
  home is constructed (the thermal resistance of surfaces determines surface temperatures), the time of
  year (the month or season determines surface temperatures), and the sensitivity of the occupants.
• Limits to Relative Humidity—Comfort and Health Aspects
• How low can you go? Comfort wise at least, the 2001 ASHRAE Fundamentals (8.12) tells us that at dew
  point temperatures of less than 32 degrees F, complaints of dry nose, throat, eyes, and skin occur. A
  dew point of 32 degrees converts to a relative humidity of 25 percent at 68 degrees.
• How high can you go? Again, using comfort as the criteria, the 2001 ASHRAE Fundamentals (8.12) tells
  us that a relative humidity of 60 percent should not be exceeded.
• This is consistent with ASHRAE Standard 62-2001 Ventilation for Acceptable Indoor Air Quality, which
  recommends that the lower boundary of the relative humidity range be limited to 25 percent and the
  upper boundary of the relative humidity range be limited to 60 percent (Figure 1).
• Now, it is important to consider the ASHRAE definition of comfort: “combinations of indoor space
  environment and personal factors that will produce thermal environmental conditions acceptable to 80
  percent or more of the occupants within a space.” Remember that you can’t please all of the people all of
  the time.
• The ranges cited above do not consider health, except indirectly. Some people love to live in desert
  climates, and some people love to live in the tropics. The upper limits from a health perspective are
  indirectly derived from a desire to control the growth of mold, bacteria, and other disease vectors.
  Similarly, for the lower limits, although the lower limits tend to be arguably “healthier” from a disease
  vector perspective. Dry conditions do not favor mold, most bacteria, and other disease vectors.
  However, some have argued that dry conditions dry out the mucus linings of the respiratory system and
  therefore make it more difficult for the body to fight off invaders. The other side of the argument is that
  there are fewer invaders to worry about.
• As can be expected, individual sensitivities and susceptibilities vary greatly, and it is typically very difficult
  to generalize with respect to relative humidity and health. Having said it is difficult to generalize, we will
  do so anyway. Keeping relative humidity in the 25 percent to 60 percent range tends to minimize most
  health issues – although opinions vary greatly.
• Incorrect recommendations in the popular press often lead occupants and homeowners to over humidify
  homes during the winter. The range of 40 percent to 60 percent relative humidity is commonly incorrectly
  recommended for health and comfort reasons. As we will see, there is a big difference between 25
  percent as a lower limit rather than 40 percent – particularly in very cold and cold climates.
• To complicate things further, most people are not capable of sensing relative humidity fluctuations within
  the range of 25 percent to 60 percent. If the relative humidity drops below 25 percent, most people can
  sense it. Similarly, if the relative humidity rises above 60 percent most people can sense it. In the range
  of 25 percent to 60 percent the majority of people cannot sense any difference. The range of 25 percent
  to 60 percent is typically defined as the comfort range for this reason. This is very different than people
  sensing temperature variations. Most people can sense a difference in temperature within a range of 1 to
  2 degrees. Less—below 0.1 degrees—if they are married (just kidding).
• Comfort is of course different than health. When relative humidity drops below 25 percent there have
  been some reports in the medical literature of eye irritation in office workers using computers. Breathing
  difficulties have been reported in some individuals when relative humidities drop below 15 percent due to
  the mucus linings of the respiratory system desiccating. However, there is no medical consensus in this
  regard.
• Determining the Humidity Limits—the Debate
• Many people believe that 25 percent relative humidity as a lower level is still too high. The debate breaks
  predictably into several camps with the engineers (the aircraft people being the most vocal) arguing for no
  lower limit for health and only a discussion on comfort. Whereas the lung researchers and some MD’s
  argue that until there is definitive research, why not keep the level high from a prudent avoidance
  perspective. This of course terrifies the microbiologists and mold researchers since higher lower limits
  clearly lead to mold growth in buildings and are associated with microbial contamination in typical
  residential humidifiers.
• So on the lower limit there is no real consensus, but only a current compromise recommendation. It is
  pretty clear that the lower limit will not go up. The only question is how low it will end up. At present, 25
  percent relative humidity is the current compromise recommendation within ASHRAE.
• On the upper end, there is an emerging consensus. Interior relative humidity should be maintained so
  that a 70 percent relative humidity at a building surface is avoided in order to control mold growth and
  should never rise above 60 percent in any event.
• 
  Relative Humidity, Surface Humidity, and Condensation
• Consensus among microbiologists gives the critical relative humidity for adverse biological activity to
  occur on building envelope surfaces to be 70 percent. Where a relative humidity above 70 percent
  occurs at surfaces, mold growth, dust mite growth, decay, corrosion, etc. can occur. Therefore,
  conditions should be maintained within a building such that the critical 70 (or higher) percent relative
  humidity at a building envelope surface does not occur. Due to climate differences, interior conditions
  which must be maintained to avoid the critical relative humidity at a surface vary from region to region
  and time of year. They also vary based on the thermal resistance of the building envelope.
• This means in winter months in cold climates interior relative humidity should be kept as low as possible
  but within the comfort and health range (i.e. above 25 percent if you believe ASHRAE Standard 62-2001).
  In the summer months it means that interior relative humidity should never exceed 60 percent for both
  comfort and health reasons.
• There is a fundamental difference between relative humidity measured in the middle of a conditioned
  space, and the relative humidity found at surfaces due to the significant difference in temperature typically
  found between surfaces and the air in the middle of a conditioned space.
• For a given sample of air containing water, relative humidity goes up as the temperature goes down. If
  the air in the middle of a room is 70 degrees at a relative humidity of 40 percent, any surface below 45
  degrees will be able to condense water. Any surface below 54 degrees will have air adjacent it at a
  relative humidity of 70 percent – the mold limit.
• Whereas when air in the middle of the room is 70 degrees at a relative humidity of 25 percent, the
  temperature of a condensing surface drops to 32 degrees from 45 degrees. And a surface with a relative
  humidity adjacent to it of 70 percent drops to 40 degrees from 54 degrees.
• In other words, for condensation to occur with air at 70 degrees and a relative humidity of 25 percent,
  surfaces need to be colder than 32 degrees. For mold to grow, surfaces need to be colder than 40
  degrees. Of course, in a nice and happy coincidence, mold does not like to grow at surfaces below 40
  degrees, but will happily grow at 54 degrees. What does this tell us? Well, if surfaces are likely to be
  cold – say like in the winter - you are better off having a lower relative humidity.
• Where relative humidities near surfaces are maintained below 70 percent, mold and other biological
  growth can be controlled. Since relative humidities are dependant on both temperature and vapor
  pressure, mold control is dependant on controlling both the temperature and vapor pressure near
  surfaces.
• Surface Humidity and Building Assemblies: Applications in Heating Climates
• In heating climates, mold growth on interior surfaces occurs during the heating season because the
  interior surfaces of exterior walls are cool from heat loss and because moisture levels within the
  conditioned space are too high. Mold growth control is facilitated by preventing the interior surfaces of
  exterior wall and other building assemblies from becoming too cold and by limiting interior moisture
  levels. The key is to prevent relative humidities adjacent surfaces from rising above 70 percent. The
  thermal resistance of the building envelope and the local climate determine the interior surface
  temperatures of exterior walls and other building assemblies. Controlled ventilation and source control
  limit the interior moisture levels.
• Experience has shown, that where interior moisture levels in very cold climates during the heating
  season are limited to the 25 percent relative humidity at 70 degrees, relative humidities adjacent to the
  interior surfaces of exterior walls (of typical code minimum thermal resistance) fall below 70 percent and
  mold growth is controlled. The colder the climate (for the thermal resistance of any given building
  envelope) the lower the interior relative humidity necessary to prevent 70 percent relative humidities
  occurring adjacent interior surfaces of exterior walls. Building enclosures of similar thermal resistance
  (building code minimums) located in Minneapolis, MN and Cincinnati, OH should be limited to different
  interior moisture levels during the heating season. A 25 percent interior relative humidity at 70 degrees
  would be appropriate for Minneapolis. Whereas interior relative humidities up to 35 percent at 70
  degrees would be appropriate for Cincinnati – which is located in a cold climate rather than a very cold
  climate like Minneapolis. Correspondingly, the higher the desired interior relative humidity, the higher the
  thermal resistance necessary to control relative humidities adjacent to interior surfaces.
• In a mixed climate, during the heating season, interior moisture levels should be limited to 45 percent
  relative humidity at 70 degrees. This limits the relative humidity adjacent to the interior surface of exterior
  walls to below 70 percent for the typical thermal resistance found in most building assemblies in this
  climate zones.
  In cooling climates, interior mold growth also occurs because interior surfaces are typically cold and then
  exposed to moisture levels that are too high. The cold surfaces in cooling climates arise from the air
  conditioning of enclosures. When exterior hot air is cooled, its relative humidity increases. If the exterior
  hot air is also humid, cooling this air will typically raise its relative humidity above the point at which mold
  growth can occur (70 percent).
  Where air conditioned "cold" air is supplied to a room, and this air can be "blown" against an interior
  surface due to poor diffuser design, diffuser location, or diffuser performance, creating cold spots on the
  interior gypsum board surfaces. Although this cold air is typically dehumidified before it is supplied to the
  conditioned space, it can create a mold problem on room surfaces as a result of high levels of airborne
  moisture within the room contacting the cooled surface. This typically leads to a rise in relative humidity
  near the surface and a corresponding mold problem.
• If exterior humid air comes in contact with the interstitial cavity side of cooled interior gypsum board mold
  and other biological growth can occur. Cooling this exterior hot, humid air by air conditioning or contact
  with cool surfaces will raise its relative humidity above 70 percent. When nutrients are present mold and
  other growth occurs. This is exacerbated with the use of impermeable wall coverings such as vinyl
  wallpaper that can trap moisture between the interior finish and the gypsum board. When these interior
  finishes are coupled with cold spots (from poor diffuser placement and/or overcooling) and exterior
  moisture, mold and other growth can occur.
• Accordingly, one of the most practical solutions in controlling mold and other biological growth in cooling
  climates is the prevention of hot, humid exterior air, or other forms of moisture transport, from contacting
  the interior cold (air conditioned) gypsum board surfaces (controlling the vapor pressure at the surface).
  This is most commonly facilitated by maintaining the conditioned space at a positive air pressure to the
  exterior and the installation of an exterior vapor diffusion retarder. Pressurization of building assemblies
  is expedited by airtight construction.
• Interior moisture levels within the conditioned space should also be limited to 60 percent relative humidity
  at 75 degrees by dehumidification and source control to prevent mold growth on the interior surfaces
  within the conditioned space.
• 
  Many people are concerned about wood floors and wood furniture being damaged if humidifiers are not
  installed. More often than not, people tend to over humidify their homes in an attempt to protect their
  wood floors and wood furniture. They need not do so if relative humidities are maintained in the range of
  25 to 60 percent between winter and summer.
• Let us examine the effect of varying humidity inside of a home between a low of 25 percent and a high of
  60 percent on wood. Wood moisture content changes directly with exposure to varying relative humidity.
  The relationship is extremely well understood by generations of wood workers and furniture makers
  (Figure 2 from Hoadley). The moisture content of wood will vary from 5 percent moisture content by
  weight at 25 percent relative humidity to 11 percent moisture content by weight at 60 percent relative
  humidity. This results in a maximum change in dimension of approximately 2 percent tangential to the
  grain (Figure 3 from Hoadley). If the wood in question is oak, and the board is 4 inches wide, the
  maximum movement is 0.08 inches.
• If we have a wood floor installed with 4 inch wide wood boards initially conditioned to the mid range of
  expected moisture content, i.e. 8 percent moisture content by weight, the range in movement is plus and
  minus 0.04 inches or approximately the thickness of a credit card. This is not an aesthetically displeasing
  or unacceptable range of movement. Of course if the wood is not initially conditioned to the mid range of
  the expected moisture content, then the movement can be two credit card thicknesses.
• The amount of bound water in wood is determined by the relative humidity (RH) of the
  surrounding atmosphere; the amount of bound water changes (albeit slowly) as the
  relative humidity changes. The moisture content of wood, when a balance is established
  at a given relative humidity, is its equilibrium moisture content (EMC). The solid line
  represents the curve for white spruce, a typical species with fiber saturation point (FSP)
  around 30% EMC. For species with a high extractive content, such as mahogany, FSP is
  around 24%, and for those with low extractive content, such as birch, FSP may be as high
  as 35%. Although a precise curve cannot be drawn for each species, most will fall within
  the color band.