Publications:  Mold In Buildings

Cause of Mold

The Grand Canyon clearly exemplifies the power of water and the concept of the path of least resistance, but the effects of moisture in a house, from structural degradation to its impact on homeowners, are much more subtle. Often entering below grade through cracks and fissures in concrete or block basement walls and floors, moisture can slowly and quietly render a house unlivable if not properly addressed and mitigated.

Perhaps no other issue in new housing is as critical as moisture and its potentially damaging effects on a home’s structural integrity and interior environment. So-called “tight-house” construction, developed to improve energy efficiency, has also heightened concerns about moisture being trapped inside a home’s structure—leading to new and better ways to accommodate both objectives.

When television celebrity Ed McMahon sued his insurance company for more than 20 million dollars and alleges, among other things, that toxic mold killed his dog, Muffin, you know the issue of mold is a cause celebre.

In June 2001, a Texas homeowner, Melinda Ballard, won a $32 million judgment (upheld on appeal) against Farmers Insurance Group (Farmers) for allegedly mishandling a water-leak claim, thereby allowing Stachybotrys to run rampant through her house, poisoning her family.

By the end of 2001, more than 8,000 mold-related claims were filed against Farmers in Texas. In 1999, only 12 cases were filed (Ref. A).

Molds are living organisms that require a source of moisture, organic material as a food source, and mold-friendly conditions (between 55° F to 90° F). If an orange is left in the back of the refrigerator or bread on a counter, mold will form. Depending on circumstances and the genus involved, mold can look fuzzy or powdery and can be green, white, black or other colors. The role of mold in nature is to break down organic material. The enzymes produced by molds literally digest the materials that they are growing upon and within. This digestion can give off organic gases, some of which can be foul or musty to our sense of smell. Mold reproduces by generating spores which are transported by air currents. Organic material can come from many sources such as paper backing on gypsum wallboard, wallpaper glue, wood, natural carpet materials, wood by-products, and even fuel oil.

Providing a mold-free building is largely a matter of controlling moisture infiltration. Moisture can be added intentionally to room air through the use of humidifiers or unintentionally through normal living activities. It is also added as ground moisture that migrates through foundation walls and basements or crawl spaces. Moisture for the growth of mold will usually occur above 65% relative humidity.

It is estimated that an average family of four (4) generates 7.4 to 12.7 quarts of water on an average day. This could increase to over 24.3 quarts on washdays. If the vent from a clothes dryer is not exhausted to the outside about 13 quarts of additional water will be introduced to the home each day. Moisture added during construction by concrete, plaster, and unseasoned wood will also be released during the initial heating season. Therefore, moisture problems tend to be most severe the first year after construction.

Sources of Moisture

There are numerous sources of moisture in a building, including:

  • Roof leaks
  • Parapet wall leaks from cracks caused by thermal changes, tears or openings in flashings
  • Wall leaks from top or face through openings
  • Condensation via contact between moisture-laden air and conditioned air (This could be in attics, underside of roofs, metal fasteners passing between different air temperatures. Air can be sucked into the building by negative air pressure.)
  • Gaps or cracks in caulked joints
  • Windows via lack of thermal break, poor weather-stripping
  • Breaks in plumbing and piping
  • Surface run-off and/or improper drainage

The major driving force of rain penetration is usually the difference in pressure between the outer wall face and the interior of the building.

Sealant failure is caused by improperly designed joints, improper spacing of joints, poor joint preparation, weak substrates, improper type of sealant and improper application.

Moisture vapor issues can occur under the following conditions:

  • Building is in an area with extremely cold or extremely hot or humid climate.
  • Moisture sensitive substrates or framing are used.
  • The inside of the building has a high humidity level such as a sauna or swimming pool.
  • An air barrier is omitted on the interior side of the exterior wall in an area with a cold climate.
  • A vapor barrier is placed on the inside of the exterior wall in a hot or humid climate.
  • Thin EIFS (exterior insulation and finish system) insulation layers are used with large amounts of batt insulation in the cavity.
  • A vapor barrier is omitted.

Water vapor can pass through a material or system without being restricted to the point that the water vapor turns into a liquid and forms condensation. The usual water vapor migration direction is from higher temperatures towards lower temperatures. Where the partial vapor pressure equals the saturated vapor pressure (dew point), condensation occurs. If this takes place in a component that is resistant to moisture, a problem generally does not develop if the moisture is allowed to flow out of the wall system. If the dew point falls within a less water tolerant component such as batt insulation, then prolonged exposure to the conditions that drive the dew point to this location will lead to eventual saturation and probable damage to the wall, as well as development of mold.

The above issues mean that what works for a building in Fairbanks, Alaska may not work in Houston, Texas. If it is hot and humid outside and air conditioned inside, like in Houston, then vapor may condense within the wall cavity as it gets close to the cooler inside surface of the wall. If the interior wall surface can breathe then diffusion can occur. A vinyl wall covering will prevent diffusion. In this instance, a vapor barrier should be installed near the exterior face of the wall.

The trapping of moisture in a wall is dependent on many factors such as temperature, humidity, vapor pressure and permeability of the primary and secondary vapor retarders. The primary is the vapor barrier in the wall; the secondary is the exterior surface. The difference in the humidity between interior and exterior is the driving force of the vapor pressure.

Masonry Walls

Evidence of distress in masonry walls from moisture consists of stained joints, efflorescence, cracked and spalled brick and mortar joints. Water or moisture can also enter through walls that have sustained damage or cracks through other causes. A number of factors cause masonry wall systems to move, including temperature changes, swelling from moisture absorption and material shrinkage. Often surface temperatures far exceed the range of surrounding air or ambient temperatures due to sun exposure on building faces. Interior temperatures and heat transfer between inside and outside climates can also affect thermal movement of the wall system. The following occurrences may also cause masonry movement:

  • Poor design
  • Water infiltration and extended exposure to the elements
  • Differential or extensive foundation movement
  • Carbonization of concrete and mortars
  • Crystallization of salts
  • Freeze/thaw cycles of water
  • Corrosion of embedded steel
  • Building or frame deformation from lateral forces
  • Insufficient or inappropriate jointing
  • Differential movement between masonry and its structural support
  • Creep (movement under a sustained load over time)

The use of architectural concrete masonry for exterior walls has become increasingly popular. Integral water repellants have played an important role in improving performance by minimizing penetration of water. Both brick and concrete masonry can be made less water permeable by the use of coatings such as acrylic based or a silicone-modified synthetic resin system.


We have come to rely too much on sealants to fill exterior joints and to provide air and water barriers in the building envelope. A failure to adequately provide for movement, or a failure of a sealant to maintain a continuous weather-seal across the joints as they move, invites the intrusion of moisture into the wall and into the building. Joint failures are a primary cause of moisture leakage in buildings.

Changes in Materials and Systems

Wall Systems

Exterior building walls may be classified as rain screen/cavity wall or barrier wall construction.

The exterior surface resists the intrusion of moisture and wind. In cavity wall construction the back-up system supports the exterior veneer and with curtain walls the floors provide the support.

In general construction in the past, multiple layers of masonry were used to create massive exterior walls forming a barrier to the weather. Any water that entered the exterior wall was conducted to the exterior by the use of weep holes and flashings. Concealed watertight membranes were used to prevent moisture from reaching the interior wall finishes.

The back-up system for cavity walls prior to the 1960’s was concrete masonry. In an effort to reduce costs, reduce construction time and reduce the weight of the structure light gage metal framing has been used and faced with a water-resistant sheathing board.

The exterior insulation and finish system (EIFS) has become a popular type of exterior cladding. Aesthetic flexibility, low first cost, and fast installations are among the characteristics of EIFS that are appealing. The impetus for the invention of EIFS was the energy crises of the 1970’s. The least costly method for improving the thermal efficiency of a wall was to add insulation on the exterior face. Expanded polystyrene (EPS) became available during the 1960’s and this led to the first use of EIFS in the USA in 1969 when Dryvit was imported. Its use was slow until the late 1980’s. The EPS is covered with a 1/8″ thick synthetic stucco finish which performs two functions. It was designed to provide a face-seal or barrier to seal out moisture and provide a decorative finish. This 1/8″ thick finish system consists of reinforcing mesh, latex fortified basecoat, and an aggregated, polymeric, textured finish.

Direct applied finish systems (DAFS) are essentially like EIFS without the insulation. It is directly applied to the wall sheathing but if the sheathing starts to move, then high stresses can be inducted into the DAFS, causing cracks, followed by water leaks.

The prime weakness to this concept is that there is only one line of defense against water intrusion. Once water enters the sealed wall, it remains trapped long enough to damage or rot any water-sensitive elements.

Around 1996, after numerous law suits involving EIFS had been filed, a water managed system evolved that utilized a weather resistant barrier and a water transport system incorporated into the back of the EPS. Flashing was also added at the base of the EIFS to direct water to the exterior.


A survey performed in 1983 indicated that about two-thirds (2/3) of the owners of large new buildings said that they had major roof leaks within the first three years and 50% of distressed structures where law suits occurred involved roof leaks. Suspect roof locations for moisture intrusion are at flashings and roof penetrations by vents and flues.

The purpose of a roof on a building is to prevent water from entering the building and to shed water. To perform this function, roof surfaces must slope down to drains or gutters. If heavy snow loads occur, the roof deck may deflect and ponding may occur. Ponding of water can increase the risk of water entry into a low-sloped roofing system. Frost may build up on the underside of a sloping roof when attics are not properly ventilated.

Up until the mid 1970’s, most low sloped commercial and industrial roofs were constructed using bituminous built-up membranes. Since then, the use of single ply membranes has increased and presently accounts for about 65% of the waterproofing membranes being installed. Deterioration of built-up membranes due to mechanisms such as blistering, splitting, wrinkling, and ridging may be aggravated by moisture in the system. Some seams in single ply membranes may be adversely affected by prolonged exposure to moisture.

There are four categories of single ply membranes:

  • Vulcanized Elastomers—resilient, rubber-like that can return to their original shape after stretching. They are thermosetting materials and sealed at the seams with contact adhesive. (EPDM, Neoprene)
  • Non-Vulcanized Elastomers—not resilient when installed but cure in-place to a rubber-like substance. They are field-seamed using heat or solvent welding). (CPE, CPSE, PIB, NBP)
  • Thermoplastics—remain somewhat deformed after stretching. They soften when heated or exposed to solvents. They are field-seamed using heat or chemicals. (PVC, EIP)
  • Modified Bitumens—are composed of a polyester or fiberglass mat sandwiched between two layers of polymer-modified bitumen. The roofing may be applied and seamed with hot asphalt or mastics. It can also be installed by melting a portion of the bitumen with a torch.

The most common application methods for single-ply membranes are ballasted, fully adhered, partially adhered and mechanically fastened.

Many other materials have been used in the past 25 to 30 years as roofing systems with various degrees of success, including: sprayed-in-place polyurethane foam with elastomeric coatings; standing seam preformed metal roofing; and mechanically attached single ply roofing system—non-penetrating. However, failures of these systems have allowed water to enter buildings.

Severe winters where major snowstorms have occurred, followed by partial melting and then refreezing, forming ice, have led to ice dams. This is the blockage of rainwater gutters and downspouts. Ice build-up at the eaves creates a dam that continually grows, backing water up the roof. Most roofs are designed only to shed water and are water-resistant, not waterproof. One condition causing this is heat from inside the building which rises through the roof insulation and into the snow. The water from the melted snow will flow through seams or nail holes in the roofing and into the building.

Concluding Discussion

The previous sections of this report addressed the following:

  • Cause of mold
  • Source of moisture
  • Changes in material and systems

This section addresses the question, “Why is mold in buildings a hot issue?” A brief history of the evolution in the construction of exterior walls helps to shed some light on the question.

Mold can be a hazard to our health and well being as noted by the numerous law suits that have been filed. One might ask the question, “Why has this occurred in recent years?” and “Why was it not a problem in previous years?”

With the knowledge that mold is ubiquitous in most buildings, assuming that the structure is erected when temperatures are mold-friendly and rain or high humidity has occurred, “What has changed?”

Prior to the 17th and 18th Centuries the dominant building material was wood. After the Great Fire of London in 1666, rebuilding was always done largely in brick. This tradition of using brick masonry continued in the United States. The exterior walls were masonry for both a weather barrier and as a support for floor and roof loads. In some instances, cut stone such as marble and granite were used as the exterior finish. The exterior wall was called solid wall bearing masonry and was a barrier type wall. This construction continued until about 1950. It should also be noted that insulation was not being used with this type of wall. During this period the resistance to air infiltration was minimal. Buildings were not air-conditioned, windows were single glazed and not very tight. Mold was not noticed and was not a health problem.

In an effort to reduce time and cost for construction, the exterior walls became thinner and lighter by the use of curtain walls. The sudden escalation of fuel costs in the late 1960’s and 1970’s was the impetus for making buildings more energy efficient. This required the use of more building insulation and resulted in less air infiltration which in turn created tighter buildings. The requirement for developing weather resistance of the building envelope also led to the development of more responsive elastomeric systems and better sealants.

Under the above conditions when a failure in the weather-tight integrity occurred, water entered the building and into various components and did not dissipate and, therefore, mold developed.


  • NAHB Research Center
  • Northwest Wall and Ceiling Bureau (NWCB) of Seattle
  • Exterior Design Institute (EDI) of Virginia Beach, VA
  • The Sealant, Waterproofing and Restoration Institute (SWRI)
  • ASTM Manual Series MNL 16
  • ASTM Stp. 1187 and 1269
  • ASTM C1382, C1397, C1481
  • ASTM PS72-98
  • EIMA (EIFS Industry Members Association)
  • SPRI (Single-ply Roofing Institute Standards)
  • PIMA (The Polyisocyanurate Insulation Manufacturers Association)
  • “A”—From the July 2002 Engineering Times of NSPE

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