Hidden Mold and How It Occurs

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.