Indoor Air Quality and Mold Prevention of the Building Envelope
Introduction
Microbiological organisms, such as fungi and bacteria, are important components of our ecosystem. These microorganisms break down dead material into its constituent components and as such are important participants in earth's continuing life cycle.

However, if these microorganisms proliferate in buildings, they can adversely impact indoor air quality (IAQ), create hazardous health conditions for the occupants and contribute to the deterioration of building components.

Moisture in envelope assemblies can cause numerous problems affecting the IAQ of a building and the longevity of building components. If elevated moisture levels persist on or inside a wall or roof assembly, these can lead to the growth of microorganisms such as mold and bacteria, as well as infestation by insects.

The metabolism of mold and bacteria can create microbiological volatile organic compounds (MVOCs) that adversely affect air quality inside the building. Musty smells in a damp building typically result from these MVOCs. Spores or cellular components are allergens and if released into the building environment can cause physical symptoms and health effects.

These organisms can also generate toxins that can cause health problems. Internal moisture degradation is a leading cause of premature failure of building envelopes. Persistent moisture can lead to rot, corrosion and other forms of deterioration. Moisture induced degradation could include reduced thermal resistance and decrease in the strength and/or stiffness of materials.

Moisture also supports insect infestation, ranging from mites, which are too small to be seen by the naked eye, to cockroaches and ants. Insect infestation can result in the release of fecal material and insect parts into the building environment. Moisture traveling through building components can cause corrosion of components and dissolve water soluble constituents damaging structures, i.e. gypsum drywall and mortar in masonry construction.

Building materials and construction methods have changed over time from labor intensive uninsulated assemblies of natural materials to labor efficient assemblies of engineered products that result in more comfortable and highly energy efficient buildings. Current construction is much more air tight and highly insulated than those of old. Unfortunately, in terms of resistance to moisture damage this higher performance comes with a price.

Current construction is less forgiving of shortfalls of design and construction than were those of the past. Current buildings are tighter and better insulated so they are able to maintain draft free environments with comfortable levels of temperature and humidity.

The ability to maintain comfortable levels of temperature and humidity inside buildings increases the psychrometric stresses driving moisture into the building envelope. Any defect in modern assemblies results in a large impact as present day construction has less moisture storage capacity than those of the past.

Also, the high performance engineered building materials in current use include hydrocarbon based materials such as paper and resins that are subject to attack by mold and bacteria.
Wall Construction - Eugene
Figure 1. Wall Construction from the 1800's, 1932 and Now
Drawing 1 shows bearing wall construction common in the 1800s, masonry curtain wall construction from the 1930s and current construction using metal studs. The construction of bygone eras included masses of masonry that could store moisture impacting the wall. Current construction provides an air tight insulated envelope, but has little moisture storage capacity.
Table 1: Water Storage Per Square Foot of Wall
  • Stone and Brick Bearing Wall  -  3.5 #/sq. ft.
  • Brick and Block Curtain Wall  -  2.1 #/sq. ft.
  • Brick and Insulated Metal Studs  -  0.5 #/sq. ft.
Table 1 gives the quantity of moisture that can be stored by each wall assembly before the surface moisture reaches the 70% that can result in mold and bacteria growth.

Moisture can move through building envelopes as liquid water, water vapor or be carried by air infiltration. The passage of water vapor is slowed by materials that are relatively impermeable. These materials such as sheet plastic, asphaltic coatings and foil are called vapor retarders.

Air barriers slow infiltration. Vapor retarders and air barriers must be carefully located within an assembly for an envelope to successfully deal with moisture migration. The way that moisture moves through a building assembly is complicated enough that the use of rules of thumb to determine the location of vapor retarders and air barriers is chancy.

An improperly located vapor retarder can cause condensation within cavities. The best way of predicting the performance of a building envelope is with a computer simulation. One of the more highly regarded programs for this is WUFI. (WUFI is a hygrothermal modeling program distributed by Oak Ridge National Laboratory ORNL. See end references.) Year long WUFI simulations for Albany, NY were run for each of the walls in Drawing 1.
Table 2: Moisture Conditions for Albany, NY
Stored Water
Max Relative. Humidity
Location of Damp Surface

Stone and Brick Bearing Wall
1.63 lbs/ft³
65%
None

Brick and Block Curtain Wall
1.63 lbs/ft³
67%
None

Brick and Insulated Metal Studs
0.39 lbs/ft³
>95%
At face of sheathing

As can be seen from Table 2 construction of bygone eras did not develop moisture problems even without vapor retarders. However, sufficient moisture would collect inside a modern wall to support mold and bacteria just from diffusion of water vapor.

Additional investigation demonstrated that installation of a vapor retarder on the inside of this wall alleviates the problem. However, if this wall were built in a hot humid climate the vapor retarder would need to be on the outside of the wall.

In general a vapor retarder should only be placed on the warm side of the insulation layer. A vapor retarder on the cold side of the insulation (outside in cold climates, inside in hot humid climates) is likely to cause condensation within the envelope assembly.

The problem is that large portions of the country have both a heating and a cooling season. If a vapor retarder is to be incorporated into a wall in this situation it is best to perform a computer simulation to determine the correct location.

It is clear that it is critical that the design of modern envelope assemblies carefully consider the location of vapor retarders.
Mold

Mold requires a nutrient source, proper temperature and moisture to grow. Mold does not require light to grow. Mold is a saprobe meaning that it lives on dead organic materials.

It does not produce food, but instead adsorbs nutrients by breaking down hydrocarbons. As such it will grow on any organic building material such as paper, adhesives, resins, etc. It will even grow on the patina of dust that collects on surfaces.

The dust in office buildings consists primarily of paper dust and skin cells so that it provides an adequate nutrient source for mold growth. 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. The pervasive nature of nutrients and a temperature range suitable for mold growth leave control of moisture as the only practical way to control mold growth.
The Source of Mold - Eugene
Figure 2. Mold Needs a Nutrient Source, Proper Temperature and Water to Grow
Bacteria

Bacteria found in indoor environments typically come from human sources (skin and respiration) or from the outdoors. Like mold, most of the bacteria found in the air in buildings are saprobes meaning they grow on dead organic matter.

As far as building envelopes are concerned the primary concern is about bacteria colonies that may grow in damp areas.
Indoor Air Quality

In the process of digesting materials mold releases enzymes to break down complex hydrocarbons into glucose which it can absorb. During this process it produces metabolites which in turn produce MVOCs.

MVOCs from mold can produce a musty odor; those from bacteria can produce a putrid odor. In addition microbiological growth can introduce spores and cellular debris into the building environment.

As such, microbiological growth in buildings can adversely affect indoor air quality, lead to health complaints, and cause disease. This generally occurs when there is an abnormal amount of moisture in building components allowing the development of growing colonies of mold or bacteria. As mold and bacteria are ubiquitous in the environment this situation is referred to as amplification.
Mold gives off spores - Eugene
Figure 3. Mold Gives Off Spores, Cellular Debris and MVOCs

Microbiological Volatile Organic Compounds (VOCs): During metabolism mold and bacteria create microbiological volatile organic compounds (MVOCs). These are organic compounds that are in a gaseous state within the air of a building.

Because they are in this gaseous state they can quickly disseminate throughout a building, even through building components such as wall assemblies. Many of these compounds have a low odor threshold and produce a musty or putrid odor that is quickly detected by occupants.

MVOCs from mold and bacteria are obnoxious and can cause complaints about indoor air quality even in low concentrations. However, these appear to be mostly nuisance compounds and are not well linked to actual health effects.

Spores and Cellular Debris: Mold releases spores into the air as a means of reproduction. In addition cellular debris, such as cell wall material, can be released into the building environment from amplification sites.

Spores and cellular debris are micro-particulates and, as such, can remain airborne for considerable periods of time traveling freely through tiny cracks, crevices, and holes in building walls, and roofs. Spores, and cellular debris from mold, are allergens and result in reports of symptoms, i.e. itchy eyes, runny noses, headaches, and fatigue.

The responses to these allergens differ from person to person so that neighboring building occupants may report very different reactions to mold amplification. Cellular debris from bacteria contains endotoxins which can produce allergic symptoms.

At high levels, this material is an irritant and can produce flu-like symptoms. Spores and cellular debris can remain an indoor air quality problem even after active growth of an amplification site is corrected.

Toxins: Mold and bacteria both produce toxins as by products of metabolism. Some molds, under some circumstances, produce mycotoxins. Bacteria can produce Exotoxins, which are secreted in the environment and endotoxins, as part of cell walls. Some of these are powerful toxins.

In general, these compounds consist of large organic molecules that do not diffuse through the air in the same manner as MVOCs. They are found in the air when carried by contaminated dust, debris, spores, or cellular material. This means that exposure to these toxins are most likely when an amplification site is disturbed.

Adverse health reactions to mycotoxins from molds such as Stachybotrys Chartarum, have been suspected in buildings with extensive mold growth, but so far have not been documented as a health problem in buildings.
Deterioration of Building Components
Rot is the result of a fungal infection of wood and wood products. Wood consists of cellulose and a complex hydrocarbon called lignin. The rot fungus can attack either the cellulose or lignin.

If it attacks the cellulose the brown lignin is left and brown rot is the result. If the lignin is attacked the white cellulose is left producing white rot. In addition there is a rot process produced by a fungus Poria.

This fungus can start in moist soil and send rhizomes great distances through wood structures.
Wood Rot (white) - Eugene Wood Rot (brown) - Eugene
Figure 4. Wood Rot — (left) White Rot and (right) Brown Rot

Nutrients: Mold and bacteria live on hydrocarbon based, organic materials. In buildings there are abundant organic materials such as wood, wood products, paper, adhesives, and glues.

Even in the absence of organic building materials, dust and dirt, which collect on surfaces in buildings, are primarily organic and can, in the presence of water, readily support mold growth.

Specific building materials that can support this growth include:
The dust that collects in office buildings is primarily paper dust, skin flakes, and fibers from carpeting and clothing. This dust will support mold growth.

Basically, the food sources for mold are ubiquitous and attempts to control microbiological growth by limiting the food source is generally not successful.

Temperature: The temperatures routinely maintained in buildings and found in building envelopes are within the range that supports the growth of a wide range of molds and bacteria.

Most molds grow in the temperature range of 15° to 30° C. Most bacteria found in the air in buildings grow in a temperature range of 18° to 30° C for environmental bacteria and 35° to 44° C for human source bacteria.
Moisture Balance
Moisture constantly enters building envelope components as liquid water, water vapor and infiltrating moist air. In addition the envelope may contain water that has been built in during construction.

If designed properly, the envelope will also continually dry due to moisture transport out of the envelope. If the rate of introduced moisture exceeds the rate of drying the water content of envelope components increases.

Many building materials have the capacity to safely store water. For example, wood is not considered to be wet until its moisture content exceeds 17%. If a building component takes on enough moisture to exceed its storage capacity it becomes damp and microbiological growth and rotting can occur.
Moisture Balance - Eugene
Figure 5. Moisture Balance
Figure 6. Moisture Uptake Exceeds Dryings
Once the rate of moisture uptake exceeds drying the envelope component takes on water. If the storage capacity of the material is exceeded it becomes wet and mold growth and deterioration may occur.

The relatively small storage capacity of modern construction makes it important to restrict and control the amount of moisture that enters the envelope, and design envelopes so that they dry under the influence of natural forces.

Water in Building Materials: There are three (3) basic ways in which water is held in building materials.
Moisture Storage Capacity: Table 4 gives the safe moisture storage capacity of some typical building materials. This table has been extracted from a comprehensive database of materials included with the WUFI hygrothermal simulation software. The moisture storage capacity is defined as the amount of moisture a material can hold before the equilibrium relative humidity (ERH) at the surface reaches 70%.
Table 3. Moisture Uptake Exceeds Dryings

  Water Stored at 70% Relative Humidity (pounds/cubic foot)
  • Concrete (B45)  -  4.32
  • Concrete Block (Pumice aggregate)  -  0.96
  • Fiber Glass  -  0.00
  • Gypsum Board  -  0.38
  • Gypsum Plaster  -  0.09
  • Softwood  -  3.16
  • Solid Brick Masonry  -  0.61
  • Steel Studs  -  0.00
Construction Moisture: Moisture is incorporated into many materials during construction. For example concrete, masonry, drywall taping compound, and some paints all have water as a component. Construction moisture can equal or even exceed the safe moisture storage capacity of the material.

This water must be able to escape from an assembly or moisture problems such as mold growth can occur. For water based coatings this can be a simple matter of maintaining environmental conditions favorable for drying for a period after application.

In instances where a surface is to be covered with an impermeable covering or coating the substrate must be adequately dry prior to covering. Measurement of concrete moisture prior to application of resilient flooring is an example of this situation.

In situations where a moisture laden component such as concrete or masonry is built into an assembly it may be necessary to allow the materials to dry before covering with other construction. Otherwise, the assembly must be put together so that construction moisture can escape.
Moisture Entry into Envelopes: Moisture can enter envelope assemblies as liquid water through leaks and capillary action, and as water vapor through infiltration of humid air and water vapor diffusion.
Liquid Water: Control of liquid water entry into envelope assemblies are discussed in the sections on specific envelope components.

Given the inevitability of material failures and construction practice mistakes it is prudent to design building envelopes so that they are forgiving and dry out when water gets into them.
Water Vapor Diffusion: Water vapor in the air is a gas. As the temperature and relative humidity increases the vapor pressure of atmospheric water increases. This means that there will be a vapor drive from hot humid areas, where the vapor pressure is higher, toward areas that are cooler and dryer, where the vapor pressure is lower.

If the building envelope were completely transparent to the moisture this would not be a problem. The water vapor would simply pass through the assembly. However, envelope assemblies are built with multiple layers of materials with different permeability to water vapor. Problems arise when the passage of water vapor is retarded at a layer that has been chilled to a sufficiently low temperature to condense liquid water.

This can happen if a material, acting as a vapor retarder, that is located on the cool side of an assembly. For example, if vinyl wall covering is installed on the inside of a wall in an air conditioned building in a hot humid environment such as South Florida. This would constitute a misplaced vapor retarder, as it is on the cool side of the wall assembly.

Water vapor is driven through the wall from the hot humid outside until it comes in contact with the wall covering where water vapor transmission is essentially stopped. In this instance, moisture is likely to condense in the glue layer holding the vinyl wall covering in place. Mold and bacteria are then able to amplify in the moist glue.

The same thing can happen in a northern climate during the heating season if a vapor retarder is placed outside of the insulation under siding. As these examples illustrate it is very important to carefully consider the location of any materials that interrupt the smooth transmission of water vapor through building envelope assemblies…

When designing building envelopes (exterior walls and roofs) it is prudent to use a hygrothermal modeling program such as WUFI to assist in locating vapor retardant layers. (WUFI is a hygrothermal modeling program distributed by Oak Ridge National Laboratory ORNL. See end references.)
Vapor Diffusion through Walls - Eugene
Figure 7. Vapor Diffusion through Walls: The greater the temperature and humidity difference from outside to inside the greater the vapor drive though the wall.
When it is hot and humid outside a pressure differential is created as can be seen from the illustration above. This results in a vapor drive from outside toward the inside. If an impermeable finish such as vinyl wall covering were installed moisture would condense out on the back surface of the wall covering permitting mold growth in the adhesive holding the wall covering in place.

In northern climates the vapor drive is from inside to outside during the cooling season. During periods of extreme cold this vapor drive can result in frost formation on the inside surface of plywood sheathing leading to wet insulation, mold growth and rotting.
Air Infiltration: It is physically impossible to build envelope assemblies so that they completely prevent air infiltration and water vapor diffusion. Air infiltrates envelope assemblies driven by the pressure differential created by wind and stack effect.
Wind creates a positive pressure - Eugene
Figure 8. Wind Creates a Positive Pressure on the Windward Side of a Building and a Negative Pressure on the Lee Side
The wind creates a positive pressure on the windward side of the building and a negative pressure on the lee side. This forces air into the building on the windward side (infiltration) and sucks it out on the lee side (exfiltration).
The Stack Effect - Eugene
Figure 9. Stack Effect
In the winter warm air inside a building is buoyant relative the more dense cold outside air. Just like hot smoke rising in a smoke stake the warm air tends to rise through vertical shafts and floors in a high rise building.

As the building is functioning like a smoke stack this is referred to as the stack effect. As the warm air rises it creates a negative pressure in the lower half of the building and cold outside air infiltrates in to replace the rising warm air.

The warm air creates a positive pressure in the top half of the building so that warm air exfiltrates out of the building. The same thing happens in reverse to high rise buildings during the cooling season in hot climates.
Air Leakage - Eugene
Figure 10. Air Leakage Through Envelope Assemblies: Air is driven through any small opening into building envelope assemblies.
Building materials, to a greater or lesser degree, are porous enough, or assemblies contain sufficient seams and joints, that air is forced into the envelope assembly by the pressure differential.

Air barriers limit the amount of air that is squeezed into the envelope assemblies, but cannot stop it completely. Air that enters the assembly from the outside in the summer or in hot humid climates carries moisture in the form of humidity.

During the heating season warm moist air inside the building will be forced into the envelope assemblies.
Air Infiltration vs. Water Vapor Diffusion: An opening in an envelope assembly will allow both air infiltration and water vapor diffusion to deliver moisture to the assembly. As can be seen from the diagrams below far more moisture is delivered into the assembly by air infiltration.
Air Infiltration During Heating - Eugene
Figure 11. Air Infiltration and Water Vapor Diffusion During a Heating Season: Far more moisture is carried by infiltrating air than is transported by vapor diffusion.
Air Infiltration During Cooling - Eugene
Figure 12. Air Infiltration and Water Vapor Diffusion During a Cooling Season: Far more moisture is carried by infiltrating air than is transported by vapor diffusion.
Application
Drying of Envelope Assemblies: Moisture inside building envelope assemblies will create a vapor pressure inside the assembly. If there is nothing to impede the transmission of water vapor on the cool, dry side of the assembly then water vapor will migrate from the assembly towards that environment.

In air conditioned buildings it becomes part of the cooling load, in heated building it passes outdoors. This allows water to escape from the assembly and allows the assembly to dry.
Design of Forgiving Envelope Assemblies: First, envelopes must be designed to prevent the entry of liquid water into the envelope assembly. The means for accomplishing this are discussed in other parts of this guide. The portions of exterior walls that are acting as a rain screen should be designed so that they are sufficiently well ventilated that they dry between rain events.

The building envelope components should be designed so that they are as airtight as possible. This requires an air barrier on both the inside and outside of the insulated portion of the wall.

Building envelope assemblies will tend to dry out and hence protect themselves from adverse affects of water incursions if water vapor transmission toward the cool dry side of the envelope is not impeded with a vapor retarder. In cold climates the envelope will dry toward the outside. In hot humid climates the envelope will dry to the air conditioned interior.

In mixed climates the envelope will dry to the outside during the heating season and to the inside during the cooling season. The dynamics of water vapor movement through envelope assemblies is complex, especially in parts of the country that have both a heating and cooling season.

For this reason it is prudent to verify the drying properties of an envelope construction by performing a hygorthermal simulation such as WUFI. (WUFI is a hygrothermal modeling program distributed by Oak Ridge National Laboratory ORNL)