Hidden Mold and How It Occurs - Wonder Makers



Hidden Mold and How It Occurs

By Ron Marullo

Introduction

 

Moisture problems in buildings are prevalent throughout the country. They are the single largest factor, excluding single catastrophic events like fire, hurricanes, etc., limiting the useful service life of a building. Elevated moisture in buildings also can lead to health problems for occupants.

Water is one of a building’s worst enemies, which is why we spend so much time and effort keeping it out. No matter how hard one tries, water will eventually find a way in. If it can’t drip through an obvious roof defect, it will come into a building sideways or uphill during a windy rainstorm. Water can flow in any direction by capillary action, or pass through walls invisibly as water vapor. Many people cannot comprehend the amount of water that can be deposited around a building during a rainstorm. Two inches of rain falling from a 1000 square foot roof can produce 1242 gallons of water.

Today’s buildings are designed to cope with moisture in all of its forms. But these efforts are rarely 100% successful. Existing buildings have a multiple of defects that create a path for water intrusion. Until recently, building design was not widely acknowledged as an important factor in preventing water problems. Sure, all of us understand a pitched roof will shed water easier than a flat roof, and grading away from a building is better than having the grade slope towards the building.

As recently as 2005, the American Institute of Architects (AIA) emphasized that mold problems were tied to the maintenance of a buildings plumbing and ventilation systems, not the initial building design. No more than a year later, an article in the AIA publication AIArchitects emphasized design details are critical in preventing mold problems.

In order to understand and prevent, or detect and start remediation of moisture problems, a building must be viewed as a complex system of interacting variables. A building consists of the building envelope and the subsystems contained within it. Building envelopes enclose the conditioned space and separate the interior and its occupants from the exterior environment. The building envelope is comprised of assemblies including the exterior walls, foundation, ceiling, roof, windows, and doors. Subsystems include the equipment that heats, cools and ventilates the conditional space, as well as the plumbing, electric and structural systems.

Airflow, heat flow, moisture flow, and biological and chemical reactions are all interacting mechanisms within a building. Climate, building envelope design, construction practice, conditions during construction, and building operation are all factors influencing moisture problems and solutions.

The materials we build with and the way we assemble them have changed dramatically over the past few decades. Well-insulated, nearly airtight, buildings hold heat better than older buildings but as a result, stay wet longer. New products, from composite lumber, to sheathing wraps, to flashing membranes all require new ways of building, and new ways of managing moisture. Added to the problem, is the renovation of existing buildings. Taking an old leaky house, covering the defects with a new, almost airtight, interior will result in hidden moisture and mold problems that might not become evident for years.

This paper is intended to provide an overview of various building assemblies and mechanicals that allow moisture to enter a building and allow mold growth to develop. Many basic and complex conditions will be outlined. Some we know about and understand. Some will result in visible mold growth. Many are interrelated with each other and must be understood to follow the moisture source to the areas of undetected, or hidden, mold growth. It is possible to have a basement problem that causes mold growth on the second floor.

Often, there are more variables involved and more complicated than the obvious. This paper will explain the historical changes in building construction, the various building and mechanical assemblies within a building and how they are subject to moisture intrusion. In addition I will cover how outdoor and indoor climate contribute to the prevention or introduction of mold growth. I do not cover metropolitan hi-rises, large big box retail stores, warehouses, schools, or hospitals. Although many of the same design principles apply to all buildings, there are certain other factors, not discussed here, that pertain to the larger buildings.

 

Back to Basics

 

One needs to have basic knowledge of building construction to understand the principles of moisture intrusion and the development of mold growth. Most of the time, there is more hidden (or non-visible) mold growth than visible. This paper is about building construction and how climate and moisture interact. It will outline some construction details so when investigating a mold problem, one might discover building defects. These are the same details that aid in the prevention of different types of moisture intrusion. Many buildings will not have many of these details. But finding the source will lead to the repair and complete remediation of the mold.

Understanding the path of the water will help identify the mold growth areas. A moisture problem suggests four questions.

• The source-Where did the moisture come from?

• The path- How did the moisture get to where it caused the problem?

• The moisture form-Was the moisture a vapor, liquid bulk, a condensate or a combination?

• The force-Did gravity, air pressure, capillarity, or diffusion carry the moisture from one place to another?

Most moisture problems fall into two types.

• Site specific moisture problems where the problem, source, and path are all close together: the location of the problem, the source of the moisture, and the path of the moisture are all close together and easily identified. For example, water leaking into the corner of the basement (form=bulk) is likely coming in through an opening in the basement wall (path). The water is coming from the downspout that is spilling water in this corner of the house (source) and gravity is carrying the water in (force). The water could also be surface water or a broken drainage pipe, but it is all associated in the same area and easily identified.

• Moisture problems where both source and path are not obvious at all and significant investigation is required to try and isolate them. For example, a cathedral ceiling roof, with recessed light fixtures (path) is rotting because of moisture (vapor), being sucked out from the apparently dry basement walls (source), and condensing on the underside of the roof sheathing during the cold winter months. The movement of water vapor by vapor pressure differentials is discussed later in the paper.

The term “moisture” can be interchanged with “mold growth”. Let us assume that every moisture intrusion into a building has the potential to become a mold problem. Understanding the source, path, form and force will lead to the uncovering of many hidden mold problems. Remediation of visible surface mold is only the first step of a process of solving the problem. Understanding the construction details of a building will aid in the discovery and remediation. It will also help identify missing components of this complex system.

 

Historical Building Construction Changes

 

Over time, there have been important changes in the construction of buildings and the way we operate them. These include the introduction of thermal insulation, the development of tighter building enclosures, the introduction of forced air heating and cooling systems, and in many parts of the country, the elimination of active chimneys.

Thermal insulation was developed in the 1950’s. The primary purpose was to reduce the heat flows into and out of buildings to improve comfort. Insulation levels were increased in the 1970’s when energy conservation became more important and building operating costs needed to be reduced. Thermal insulation achieved these goals. But an unintended consequence was that it also created a reduction of the drying potential of the building envelope. Since air flow and heat are reduced through the building assemblies (foundation, walls, roof), the building’s ability to dry is diminished should it get wet from interior or exterior sources. This impact of insulation is similar regardless of location or climate.

Tighter building enclosures have also developed since the 1950’s. This increase in tightness has occurred as a result of cost-saving new materials and building techniques (plywood sheets, drywall sheets, etc), the desire for interior comfort by eliminating drafts, and the need to reduce energy usage of heating and air conditioning. The by-product of this tighter building is a lower exchange of air between the interior, conditioned space and the exterior. The lower the air change, the less dilution of moisture and interior pollutants.

In commercial construction, these manifested itself in sick building syndrome (SBS) complaints and cases of building related illness (BRI). In residential construction, the tighter enclosures limited a chimney’s ability to exhaust combustion products (furnaces, water heaters, and fireplaces). Tighter buildings restricted exterior “make-up” air that assisted the chimney exhaust flow. However, the most noticeable symptom of this new tightness was the increase of moisture present inside of the building. This manifested itself with mold and mildew on interior surfaces in the heating (northern U.S.) and cooling (southern U.S.) climates, as well as condensation on the window interiors in the heating climates. Tighter buildings have also created an indoor air quality issue. Due to occupant complaints regarding health, the building community has been forced to resolve these problems by developing better systems and products.

Heating, ventilation, and air conditioning systems are also considerably new developments. Although the idea of circulating hot air around a building for heat has been around for a long time, the HVAC systems of today were developed in the 1950’s. Circulating large quantities of air around a tight building has lead to serious health, safety, and operating cost issues. A HVAC system must be “balanced” in order to achieve an acceptable level of comfort (and energy conservation) in the building. “Balancing” the system involves maintaining correct air flow between supply air (air coming out of registers that heat or cool the building and its occupants) and return air (air being sucked back into a different duct that is filtered and recirculated as supply air).

Supply ducts are typically in bedrooms or offices and returns are typically in hallways or common areas. A leaky duct or a room with a closed door can create more positive pressure in a bedroom or office and negative (or depressurizing) pressure in a hallway or common area. Negative pressure of a conditioned space results in the infiltration of hot, humid air from the exterior if the air conditioning is on inside. If the heat is on, a positive pressure from a bedroom or office can cause warm, moisture-laden air to exfiltrate into wall and roof cavities. Both conditions leave moisture in the building assemblies that take longer to dry due to high insulation levels. This is a classic hidden mold case. Moisture trapped in a wall assembly will condensate on the back of the exterior sheathing or interior sheetrock creating a mold problem that might not be recognized immediately.

Less discussed, but equally important, is the elimination of active chimneys. Although many residential buildings in northern heating climates still have chimneys, there is a growing trend in other parts of the country towards power vented, sealed combustion furnaces, heat pumps, and other heat sources that do not require an active chimney. Active chimneys are exhaust fans that expel large quantities of air from the conditioned space of a building. This results in frequent air changes and the dilution of indoor pollutants. The elimination of the chimney has led to an increase in levels of moisture.

Overall, the advance in the building technology has greatly improved the comfort level and safety for building occupants and durability and energy conservation for the building itself. Care must be taken in how we use all of these new products. Engineered wood products, such as oriented strand board, I-joists, and other composites are here to stay. In fact, they are more desirable than other building materials and are significantly more environmentally responsible. But they must be better protected from moisture during the construction process.

The building assemblies, in which they are used, must constantly be analyzed and adjusted. The trend today is towards sustainability and energy security. Sustainability means more celluostic-based engineered materials, which increases mold risk. Energy security means higher levels of insulation, with resultant lower drying potentials, creating increased mold risk. Our better buildings have become a haven for hidden mold. Reaction times to water damage must be immediate in order to dry out buildings. Gone are the days when a building will dry out on it’s own.

 

Moisture Movement

 

In order to understand moisture movement and subsequent mold growth, the mechanisms governing such movement must be understood. The four moisture transport mechanisms are:

• Liquid flow

• Capillary suction

• Air movement

• Vapor diffusion

Each can bring moisture into a building assembly. Moisture movement can also be a combination of these mechanisms.

The most significant moisture transfer mechanism is liquid flow. Liquid flow is primarily responsible for moisture moving into the building from the exterior. This involves groundwater and rainwater moving under the influence of a driving force, typically gravity or air pressure. Leakage will occur if groundwater or rainwater is present, and there is an opening in the building envelope along with a driving force.

Architects or builders can seldom control whether groundwater or rainwater is present. They may be able to influence the source strength of the groundwater and rainwater by proper site selection (minimizing wind exposure and building on elevated dry ground). The number of window and door openings in a building can be designed with wind driven rain in mind and can also be influenced by flashings and siding details. The control of the effects of these driving forces is the most effective approach for controlling liquid flow.

In portions of the building envelope below grade, gravity (hydrostatic pressure) can be controlled by the use of a drain screen. Above grade momentum, surface tension, and air pressure differences can be controlled by the use of rain screens. Drain and rain screens function in the same way, that is, by retarding moisture against the building envelope and draining it downwards, by gravity, through an air space or, in some foundations, free draining backfill. These are part of the building assemblies and, for simplicity, they will be referred to as drainage planes.

Groundwater is also affected by the amount of rain and surface water entering the ground adjacent to the building. Proper installation of leaders and gutters, lawn sprinkler systems, and proper ground slope will reduce water near the building. The foundation drainage plane can be numerous things. Free-draining backfill, like sand or gravel, allows for the free flow of water downwards toward a subgrade drainage system, thus reducing hydrostatic pressure.

Foundation wall assemblies can include draining materials such as fiberglass panels, rigid plastic insulations with vertical channels, and drainage mats. The drainage plane, also with one of these items, usually includes waterproofing barriers or membranes. Many buildings nowadays are being built with these systems in place. But there are millions of buildings that have only a simple asphalt coating, known as dampproofing. Although dampproofing can help retard water vapor, it will not necessarily keep water out.

Moisture in basements and crawlspaces are the most common water intrusion area in the building. In an unfinished area, the moisture is usually identified very easily. Trace the source and correct the problem. In finished areas there are existing conditions behind walls and under floors that might never be discovered, unless there is small visible water stain or mold growth that can direct the investigation process.

However, the real problem in basements and crawlspaces is people accept the fact that it is normal to have a damp, smelly basement. What is not taken into account is the fact that the moisture-laden air (and possible mold spores with it) can migrate to the upper floors. The force behind this could be air movement or vapor pressure. Upper floor mold growth that is hidden behind furniture, inside wall cavities, or behind cabinetry can be caused by basement dampness. Sometimes the basement problem is corrected by simply opening foundation vents for proper ventilation.

Rainwater must be controlled by reducing and controlling the amount deposited on building surfaces and assemblies. Reducing rainwater is a function of siting the building and architectural design. Most buildings are sited, if cost allows, so they are sheltered from prevailing winds to reduce exposure to wind-driven rain. Architectural designs include roof overhangs to shelter exterior walls from rain, and details, such as angled window sills or trim, which shed water. The absence of these details could very well be the cause of mold growth inside the wall cavities in the areas where these details should exist.

Controlling rainwater involves the control of air pressure differentials across the exterior cladding by using a drainage plane, draining rainwater that enters the building assemblies by using a drainage plane and controlling openings around windows, doors, siding, or sheathing with a weather proofing barrier or membrane to resist rainwater entry, or by sealing openings.

The drainage plane on the wall is basically an air space cavity between the exterior cladding, such as brick, stone, stucco, wood or vinyl siding and the building sheathing such as plywood or masonry block. When exterior air pressure is greater than the cavity and interior pressure, rain droplets are drawn through wall openings from the exterior to the interior. If the air space cavity (drainage plane) is large enough (3/8” to 1”) and unobstructed, the air pressure will be equalized, the rain droplets lose their force, therefore, flowing downward from gravity and exiting through bottom flashing and weep holes. Controlling openings is achieved by the use of flashings, caulking, and various membranes. If any of these details are missing or defective, look to that area for interior hidden mold.

During a rainstorm, the roof of a building is exposed to a greater amount of water than any part of the structure. Roof assemblies are designed and built to shed water. Even “flat” roofs have a slope to them that directs water towards roof drains. Roofing materials, on their own, are very good water repelling products. If a roof leaks due to the roofing materials, it is usually some installation fault, or just old age. Roofing materials take a beating over their lifetime. We must expect them to wear out.

But much more likely to be the cause of a leak than the roofing itself is poor flashing details at penetrations, roof edges, or where a roof changes planes. Roof leaks also occur due to insufficient or inadequate fasteners for sheathing, flashings, or roofing materials, and by the attempt to substitute caulk or roofing cement for flashing.

Roofing leaks are sometimes easy to detect, if you have attic access. Many times water stains are evident on wood framing members and even plumbing vent pipes and chimney penetrations. Sometimes you can see daylight through a hole or gap in the roof assembly. Still many roof leaks go unnoticed for long periods of time.

Most people do not notice a roof leak until they see water staining on a ceiling or wall below. Attic inspections are rarely performed. So one has the potential for hidden mold. Water gets in and puddles on the paper vapor barrier of the insulation, or the backside of the ceiling sheetrock. There lies the potential for mold growth. Cardboard storage boxes are another food source for mold.

Water intrusion through the actual roofing material is usually the result of exposed roofing nails, unsupported sheathing at the ends, slipping or missing shingles, or shingles (such as valley shingles) that should have been embedded with roof cement.

Exposed roofing nails are common on cap shingles on the ridge. These should have been covered with a dab of roofing cement. If smooth shank nails are used for roof sheathing, instead of ring shanks, they can work themselves loose over the years and rise upwards through the roofing. This is common among flat roofs. When inspecting for moisture or mold in an attic, look for water staining around the nails protruding through the roof sheathing. There are thousands of nails that hold down an average roof. It is highly unlikely that water will not get through one of them during the life of a roof.

Many common roof leaks occur due to improper flashing, especially where chimneys are present. A proper flashing detail should have just one bead of caulking on top of the counterflashing where it meets the chimney. If there are mounds of caulking at every possible joint and intersection, there is a very good chance of water problems and resultant mold growth on the wood framing around the chimney in the attic.

Another flashing detail that is often overlooked is the drip edge where the bottom of the roof sheathing meets the top of the fascia board. Most roofers just extend the edge of the underlayment paper and shingle over the top of the fascia to bridge the gap. The gap is the direct path to the inside of the roof assembly. Water (splashing or overflowing) from the gutter backs up into this gap, enters the roof assembly, and wets the insulation. This is the same path that a winter ice dam follows. A piece of drip edge flashing seals the gap and prevents water from backing up into the roof assembly. This flashing also protects the roof sheathing edge from capillary action of water wicking into the board. Not only is there potential for hidden mold growth at this area of the roof assembly, it is possible moisture can work its way down, through gravity, to the wall assembly below.

Skylights present flashing problems as well. This is most always an installation problem. Water intrusion though a skylight can move inside the ceiling and appear as a wet spot on the ceiling or wall some distance away. When investigating, remember to find the source and follow the path. What might appear as a drip edge problem, as mentioned before, could actually be a skylight problem.

A roof assembly is comprised of many dissimilar materials. Flashings must overlap and be allowed to expand and contract. If flashing is too rigid, it can crack and create an opening for water. Caulking can help prevent water intrusion, but it can dry out, crack and peel. Caulking should not be used as the primary barrier against water entry.

Roofs must be maintained. Missing shingles must be replaced. Damaged flashings must be repaired. Leaders, gutters, and roof drains must be kept clear of debris so they can drain. The roof assembly must be allowed to do its job and remove rainwater from the building envelope.

 

Liquid Flow

 

Liquid flow into wall assemblies can be just as troublesome as roof assemblies. Brick veneers, wood clapboards, vinyl siding, stucco, and even recently developed fiber-cement siding all leak. These siding materials can protect the walls from many things, but water is not one of them.

The common maintenance/repair strategy is to use caulk and sealants. Sealants cannot span cracks, cannot withstand movement, and will degrade from the sunlight, temperature, and oxidation. Caulks dry up, shrink, freeze and crack, and decompose. They fail over time.

When the exterior cladding leaks, neither caulks or sealants will keep water out of the building envelope. To keep the building dry, there must be a water management system beneath the cladding. There must be a continuous drainage plane (also known as the rain screen) with integrated flashings and weep holes with an air space between the cladding and the drainage plane where water can flow. All materials must be overlapped to direct water down and out, and have gravity do the work.

This rainwater management system for walls has the same principle as the systems for roofs and foundations. The principle is to carry the water away from the building envelope. The more we understand how these systems should work, the easier it will be to discover deficiencies, which in turn, will lead to the prevention of moisture/mold problems.

The most common drainage planes on houses are the various building papers, i.e., asphalt saturated felt, plastic house wraps, and coated papers. Foam sheathing also works well. As in any water management system, the drainage space is critical.

Foam sheathing drainage planes work well with vinyl siding because the air space is created by the design of the material. Vinyl siding also has built-in weep holes. Brick also works well over foam sheathing as long as there is an unobstructed air space and weep holes. Wood siding does not work well over foam sheathing or even building papers unless it is spaced away from the wall with furring strips. This is seldom done.

Wood siding is usually fastened right to the wood sheathing with a paper or house wrap in between. There is nowhere for trapped moisture to go and will be difficult to dry out. More than likely, this trapped moisture will migrate into the wall assembly, thus creating another hidden mold potential. More of this will be discussed later in the section on capillary suction.

Stucco has had its problems over the years. Decades ago, stucco hard coat was developed as an inexpensive, cost-efficient alternative to brick. It was much less expensive to stucco a building instead of bricking it. But hard coat stucco became a water problem when asphalt-impregnated felt papers rated at 15 pound and 30 pound felt paper were downgraded in quality and relabeled #15 felt and #30 felt. What used to be 15 pound felt (that weighed 15 pounds per 100 square feet), now weighs less than 7 pounds per 100 square feet under the new #15 felt label. The older, heavier felts absorbed water and swelled up when the scratch coat of stucco was applied over them. They were intended to swell, and as they dried, they wrinkled, shrank, and debonded from the back of the stucco. This created a thin, channeled drainage plane. The newer (and lighter) papers allow the stucco to stick and bond to it. Water easily penetrates the stucco cladding and the felt paper stays wet because there is no drainage. Eventually the felt paper starts to rot and there is an unprotected wall. Some applicators added a second layer of felt allowing it to bond to the stucco coat. This left the first layer intact and the space between the two papers created the drainage plane.

A next generation synthetic stucco was developed that was applied to a rigid foam board. This appeared to be a great looking, inexpensive product. Architects promoted it because additional foam board could be layered over the first piece to create many dimensional details. The problem was there was no drainage. Many wall assemblies on buildings failed due to trapped moisture. This moisture migrated into the wall assemblies of buildings and created a lot of mold remediation work.

The widespread failure of this product helped draw attention to mold problems. This problem was solved by the introduction of channels into the back of the foam panels for the purpose of drainage. This was a turning point for many architects, engineers, and designers. Water intrusion into a building and the potential for mold was now looked at much more closely.

In general, exterior cladding works well. Under normal rainfall conditions, these sidings shed water the way they are designed to. Most of the water intrusions into the wall assemblies are due to a flashing defect around doors, windows, or miscellaneous house penetrations.

Flashings serve important functions. They direct water out of construction joints and building assemblies, and they prevent water from flowing along wall surfaces by providing a drip edge. Flashings are very different than caulking or sealing. Caulks and sealants are used to close off an opening, where flashings are used to shed water and promote drainage. Flashings must direct water away from the wall assemblies. If they do not extend beyond the exterior of the cladding, water can be drawn back into the wall assembly. It is important that flashings extend to the back of the air space, attaching to the wood sheathing or masonry wall. This allows building papers and house wraps to overlap them and create a positive drainage plane system.

Even if flashings are installed properly, you can have leakage from the windows and doors themselves. Windows are manufactured like a picture frame with mitred corners. Water gets in, gravity draws the water down to the bottom, then creates another potential water problem. Most windows have weep holes built in for this reason. Windows and doors are integrated into the drainage plan by top (or head) flashings that are overlapped by the building paper or house wrap.

When investigating moisture leaks or mold growth in wall assemblers, the obvious areas are around doors and windows. The space between the door and window frame and the rough framed opening in the wall assembly is usually filled in with leftover fiberglass cellulose insulation or expandable spray foam, which also has cellulose in it. A small leak due to improper flashing or an improper drainage plane can lead to a breeding ground for mold growth.

Mold growth in back of baseboard molding does not always mean there was water on the floor and it wicked up behind the baseboard. If there is a window overhead or a door adjacent, it is very possible water is intruding from above or from the door threshold.

Liquid flow can be compared to running a garden hose around a roof, or against a wall, or around the foundation. It creates the most amount of water that affects a house at any given time. Other moisture transfer mechanisms may not cause moisture intrusion into buildings at the same volume as liquid flow, but they can result in just as severe moisture/ mold problems.

 

Capillary Suction

 

Another moisture transfer mechanism is capillary suction. Capillary suction primarily moves moisture into porous materials. It can move moisture into the building envelope from the outside. Capillarity is the function of pore size and available moisture. If there is large pore size, like gravel, moisture will travel downward with gravity and there will be no capillary action. If there is small pore size, such as concrete, capillarity can occur. Water can be drawn through the pores in the concrete. Capillary suction will not exist in materials that do not have pores. Steel, glass, and most plastics are examples of these types of materials.

But if these types of materials are closely placed next to one another, the space between them can become a capillary pore. An example of this is a 1960’s style bathroom design of plastic laminate wall panels, such as Marlite, and vinyl cove base molding. Two non-porous items, but a haven for moisture from water splashed on the floor. Water gets sucked under cove base and capillary suction moves it up the backside of the base and wall. In addition, there is a potential for hidden mold growth on the glue on the back of the cove base. Capillarity takes place both below and above grade. Below grade deals with foundations, and above grade deals with wall and roof assemblies.

At some depth below most building sites, a natural source of water can be found. Because water is often found on top of a basement slab floor, many people use the term “hydrostatic pressure” to describe the condition. However, for a true hydrostatic condition to develop, the water table must be at or above the floor elevation. This hydrostatic pressure occurs in some basements, rarely in crawlspaces, and almost never in a slab-on grade concrete floor.

Most of the time what is occurring is water is being drawn upward through the soil by capillary suction. Examples of building controls for this are foundation waterproof membranes (drainage planes), capillary breaks such as polyethylene plastic sheeting between the foundation footing and wall, or gravel placed under the concrete slab floor. These breaks interrupt the capillary flow by creating a barrier or a “large pore” area where the water will disperse.

Capillary suction in masonry block foundation walls can be controlled by the application of a cement parge coat to seal the mortar joints, or a waterproofing membrane. Many times it is impractical to repair a wet basement without tremendous expense. But a constantly wet carpet or water that wicks up from the floor to a finished sheetrock wall creates mold problems that must be addressed. Maybe it is as simple as removing wet, moldy cardboard storage boxes, repacking the contents in new boxes (preferable plastic) and relocating them off the floor onto a shelf.

Moisture migration into wall assemblies is common because all exterior claddings, with the exception of vinyl siding, are “reservoir” products. They absorb moisture even when they are sealed with a coating on all sides. For many reasons, moisture will find its way into siding and exterior trim through minute gaps no matter how tightly it is sealed.

Improper installation of siding, such as siding too close to grade or being installed directly on a substrate that retains moisture, or a poor water shedding detail such as an unpitched ledge, are all reasons moisture penetrates the wall assembly. The drainage plane in wall assemblies is necessary to create the capillary break. Wind and wind-driven rain cause positive air pressure against the exterior cladding. This difference in pressure creates a vacuum-like effect. Moisture is both driven and sucked through the siding or trim through capillary suction.

Even with proper water-shedding details, capillary suction can draw water vertically through siding boards, especially at butt joints and even nail holes. Water can even be drawn vertically into a brick wall assembly through the mortar joints.

Capillary suction in porous cladding materials can be minimized by sealing or filling the pores. The most common application is paint. The paint film seals the capillary pores on the surface of the cladding. Wood siding and trim should be sealed on all exposed sides, including the ends.

Where capillary suction occurs between the overlaps on horizontal siding, the seams can be sealed with caulking, thereby eliminating the capillary pores. Although this is the most common approach to every potential water problem, caulking usually fails, due to expansion and contraction, if not properly maintained. Horizontal wood siding can have spacers to separate the pieces, creating a capillary break, or installed on furring strips, as discussed before, to create a drainage plane. Brick veneer and EIFS stucco claddings must have a drainage plane to drain and dry the wall assembly.

When investigating for moisture/ mold, the exterior defects can be traced to the corresponding interior areas. A missing drip cap flashing above a window with a wood siding butt joint nearby will certainly allow water into the wall assembly above the window. And, remember, there is usually that left over cellulose fiberglass insulation stuffed in the gap right above the window. At some point, a mold growth problem will develop.

Roof assemblies will also allow capillary suction to occur. Wood shingles and shakes absorb deposited rainwater. If the wood shingles or shakes are painted on all sides before installation, or installed on wood furring strips to create a drainage plane, the roof assembly will function properly with correct maintenance. The problem occurs when the wood shingles or shakes are installed, without sealing, onto plywood roof sheathings. Rainwater gets through the wood shingles or shakes, and gets trapped in between them and the roof felt paper. They dry to the exterior, but the inside moisture buildup leads to rotting, mildew, and mold.

Asphalt shingles are not as susceptible to water intrusion as wood, but capillary suction can certainly occur by the edge from gutter splash back or ice dams. This area at the end of the roof, or the eaves, is always an area to investigate for hidden mold growth. It is a tight and complicated building section that is prone to moisture intrusion and usually has poor ventilation.

 

Air Movement

 

The third moisture transport mechanism is air movement. Moisture can move into the building envelope from the exterior and from the interior depending on exterior and interior conditions. Warm air naturally migrates towards cool air. A cold climate building that is being heated will see hot, interior conditioned air move towards the colder building envelope. The opposite is true in a warm climate. The hot, humid exterior air will move towards the air-conditioned, cool interior environment.

Air moves as a result of the difference in air pressure. Moisture, in the vapor state, will move with the air. There needs to be an opening or a hole in the building envelope for this transfer to take place. In order to have a moisture problem, the airflow must be slow enough to allow the hot, moisture-laden air to condense within the building assembly before it exits the air leakage path. Warm air must have significant time to come in contact with a colder surface within the building assembly, such as a cold water pipe. With proper time, air will deposit some of its moisture on the colder surface in the form of condensation. This is a very basic description of what can happen inside the building assembly.

Condensation occurs on building components when they are below the dew point of the air that is in contact with them. This is a complex calculation that involves the R-values of the wall or roof components and the temperature and relative humidity of the interior and exterior air. A dripping water pipe is not a potential mold problem if left by itself. It becomes a problem when the dripping water is deposited on a material that can become a food source for mold growth.

Controlling moisture movement that is caused by air movement can be done by reducing the air moisture, controlling air tightness of a building, and controlling the air pressure. Although exterior moisture levels cannot be controlled, interior moisture levels can be controlled by reduction of moisture at the source, dilution, and dehumidification. Source controls involve identifying and correcting the problem.

Leaky basements, evaporation of moisture from exposed ground in crawlspaces, non-vented clothes dryers and bathroom exhaust fans all put moisture into the air. Proper waterproofing of building assemblies and proper ventilation prevent moisture from remaining in the air. Proper drainage of air conditioning condensate water will prevent moisture from reentering the air.

In cooler climates, a common moisture problem is the indoor storage of firewood, which releases moisture as it dries out. Dilution involves air change, or the exchange of interior, moisture-laden air with exterior, dry air. Dilution is achieved by mechanical ventilation, an active chimney, or opening the windows when the outside air is drier than the inside. This dilution removes common, everyday moisture that is retained by furniture, carpeting, and building materials. This moisture, along with people breathing and perspiring, cannot be controlled at the source.

If dilution cannot fully remove it, dehumidification can. Dehumidification cools moisture-laden interior air, removes moisture by condensation, and recirculates drier air. Dehumidification, by the use of air-conditioning, is the most effective moisture control in cooling climates.

Air tightness, or the resistance to air flow, is controlled by exterior and interior building air retarders, such as polyethylene sheeting, insulation within the building assemblies, and caulking, which seal openings throughout the building, created by plumbing pipes, HVAV ductwork, and electric wires.

Air movement is driven by air pressure. Air moves from high-pressure areas to lower pressure areas. If moisture is present in the air, it will be carried along. Moisture problems can be caused by air leakage. Warm, moist indoor air that leaks into a wall or ceiling cavity can condense when it reaches a colder surface, typically the backside of the exterior sheathing. Wet sheathing can sustain mold growth. Cool, outside air leaking into a house can cool interior surfaces such as drywall, causing moist, inside air to condense on the interior side. Wet drywall can lead to mold growth.

Air pressure differences across building assemblies are created by the stack effect, chimneys, wind and mechanical systems. The stack effect in a building is caused by the tendency of warm, heated air to rise upwards and leak out of the upper portion of a building. As warm air rises, inside air pressure builds up in the upper portion and can become greater than the exterior air pressure. If a hole or opening is found, this hot air will exfiltrate (or leak) to the exterior.

In the lower portion of a building, inside air pressure is lower than the exterior air pressure, causing infiltration of the air. This stack effect usually causes moisture problems on the higher floors of a building. Warm, moist air leaking into a cold attic will cause condensation. This can lead to mold growth on building materials or stored contents. Attic ventilation is necessary to move this moisture out. Active chimneys act as an exhaust-only ventilation system. This helps to draw out moisture-laden air and expel it to the outside.

When wind blows over a building it creates a positive air pressure on the windward side and a negative pressure on the leeward side. Infiltration occurs on the windward side and exfiltration occurs on the leeward side. When combined with the stack effect, moisture problems are most likely up high on the leeward side, than down low on the windward side. Warm, moist air (high pressure) leaking into the cooler air is being drawn to the leeward side of the building (low pressure) and, if there is insufficient ventilation, it will exfiltrate through the wall and roof assemblies.

It has been said that mechanical systems do not alter air pressures. However, systems are rarely in a perfectly “balanced” state of supply and return. Leaky ductwork and return systems that utilize leaky wall, floor or ceiling cavities can create all types of over and under pressurizations that can cause air pressure to infiltrate or exfiltrate building assemblies which create condensation which can lead to moisture/mold problems.

As discussed earlier, closed doors create HVAC imbalance by putting too much supply air (positive pressure) in a room. With no way to circulate back into the return system, this can become a problem. One might argue that doors usually have a 1” air space at the bottom. But, that is not enough to achieve the correct “balance”. When investigating moisture/ mold problems, look for air leakage in the systems and the problem is usually found.

 

Vapor Diffusion

 

The last moisture transport mechanism is vapor diffusion. Moisture vapor travels in much the same way as air movement, i.e., the pressure differential across the building assembly. It is a scientific fact that moisture always moves from areas of higher temperature and relative humidity (high vapor pressure) to areas of lower temperature and humidity. The greater the pressure difference, the greater the push.

Air movement carries moisture and penetrates building assemblies through holes and openings, while vapor diffusion is the transfer of moisture through tiny pores in building materials. The amount of moisture that passes through a material depends on the material’s permeability and the vapor pressure that pushes the moisture through. A material is rated in perms, or the capacity for water vapor to pass through by diffusion. For comparison purposes a 6-mil sheet of polyethylene is rated at 0.06 perms while a sheet of ½” drywall is rated at 37.5 perms. The higher the perms, the greater amount of water vapor is able to diffuse through. By painting the drywall, the permeability is reduced down to 6.6.

A vapor barrier is a material or coating that is part of the building assembly that reduces the passage of water vapor by diffusion into the building assembly. Its purpose is to keep moisture from getting into an exterior wall or ceiling cavity where it can condense on building materials and create a potential moisture/ mold problem. Vapor diffusion problems, while the least occurring of all the moisture movement mechanisms, can be the most difficult to uncover. This is the most invisible of all the mechanisms. Yet the understanding of vapor diffusion is important because when there is no visible evidence of a moisture/mold problem, this could be the answer.

  

Conclusion

 

One may suspect hidden mold if a building smells moldy, there was previous water damage, or the residents are complaining about health problems. One cannot see the source so one must do the investigation. One must have a basic knowledge of building construction to try to locate the moisture source and trace its path.

Mold may be hidden in places such as the backside of dry wall, wallpaper, or paneling, the topside of ceiling tiles, or the underside of carpets and pads. Other locations could be inside wall cavities with leaking or condensing pipes, inside ductwork, up in attics or down in basements, or even behind furniture where condensation can occur. Other likely locations are ceilings, walls, and floors that have multiple layers of building materials from renovations over the years. There can be moisture sources in the basement that is affecting upper floors by diffusion. Or mold spores can be carried by air movement and cross contaminate other areas of the building. One must have an understanding of how moisture and the building interact with each other to solve mold problems.

I touched on many general construction details and practices. (You could write a paper just about flashing details for skylights). They can point a moisture investigation in the right direction. Or they can rule out a specific source by discovering it was installed properly. Whatever the case, with hidden mold, one must start somewhere.

The key is to understand and follow the water path. Contrary to popular belief, water does not always flow downward, by gravity, in a straight line. Its path can be diverted sideways, and even upwards, by accesses in the building’s structure. The forces behind moisture flow will take the path of least resistance. If one accepts this fact, one will understand the path and be able to determine where the moisture/mold problems lie.

Solving mold problems are like solving crimes. First one must gather all the evidence, examine it in a logical manner and then come up with the correct conclusion. Hopefully, I have aided my reader in knowing where to look to solve the problem. When one looks in the right place one usually gets the correct result. Happy mold hunting!

References

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Problems ASHRAE Journal, November.

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(SBS), March.

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Rousseau, J. 1983 Rain Penetration and Moisture Damage in Residential Construction

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United States Department of Agriculture, 1998 The Ins and Outs of Caulking.

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United States Environmental Protection Agency, 2007. Hidden Mold, April.

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