This is an article from Architectural Record, and very interesting.
**Preventing Moisture-Related Problems in Residential Wood Framing **
**November 2007 **
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There is a popular phrase in the construction industry, “Don’t worry about the problems you can see, worry about the problems you can’t.” This is especially true when discussing moisture-related problems such as mold, fungi rot, and termites. The seeds of potential moisture problems are present in rainfall and humidity during the construction process and then released daily into the air through cooking, showers, and air conditioning. Unfortunately, these problems can begin to develop deep within the structure of the home behind layers of plaster and drywall, underneath tile and linoleum, or growing unnoticed in attics and basements. The scale of the problem and, subsequently, the cost of the repair are exponentially larger by the time the greenish-black fuzz creeps into plain view or a homeowner’s foot breaks through the floor in their kitchen nook.
Beyond the significant impact to the physical structure, moisture-related problems can dramatically affect the health of homeowners and their guests. Allergic reactions, flu-like symptoms, rashes, asthma, air illness, and even deaths have been caused by uncontrolled moisture that creates an environment ripe with molds and toxins that infect the indoor air quality day after day.
There is an estimated $9-10 billion spent to correct construction defects each year. 80 percent of those defects are moisture-related. 41 percent of the moisture-related problems developed in the envelope of the building. There are two reasons for the prevalence of moisture-related problems growing in the internal structure of homes and multi-family dwellings. First, moisture is everywhere. Second, there are weaknesses inherent in the wood materials most commonly used for the building frame that make them susceptible to these problems.
Strengths and Weaknesses of Wood Framing
Today, untreated or white wood, in its many forms, is used throughout 95 percent of the residential and multi-family homes in the United States. Wood is a very popular building material for a wide variety of reasons. It is a readily available, replaceable, natural resource. It is also easy to work with, cost-efficient and available in different forms that offer varying degrees of flexibility, strength, and size.
The framing package of a house often includes dimension lumber, plywood, oriented-strand-board, and engineered wood products. Dimension lumber is a solid piece of wood often referred to simply by its dimensions, a 2x4, 2x8, etc. Plywood is thin layers of wood (plies) glued together and ordered by the number of plies, for example, 4-ply, 5-ply, etc. Oriented-strand-board (OSB) is created by compacting many 1-inch x 1-inch wood chips together.
Engineered wood products (EWP) combine large wood chips and small wood chips to create the required board size. EWP supports more weight and structural load in floors and ceilings than the original dimension boards.
Moisture-related problems can begin deep in the envelope of a home, making them difficult to detect and expensive to correct.
All photos courtesy of WoodSmart Solutions, Inc.
Despite all of the reasons that wood has become such a popular framing material, there are significant weaknesses in untreated wood that make this material susceptible to moisture-related problems. Wood is an organic natural resource and, as such, occupies a spot in the food chain. Wood and the cellulose inside wood are a food source for decay fungi, mold, termites and other wood ingesting insects. It is also susceptible to dimensional instability, racks, splits and bows.
The Wood and Moisture Relationship
Wood is an organic and porous material that absorbs moisture. The combination of wood and moisture creates a feeding ground for microbial agents like mold, rot fungi, and termites. The effects of these agents to the wood structure they attack can be devastating. By better understanding these threats, building professionals can make preparations to prevent against the invasion of the living spaces they design. Surprisingly, the best protection against each wood menace is to better control the wood’s moisture absorption.
Mold is a fungus that eats wood. More specifically, mold secretes an enzyme that breaks down the cellulose in wood products into smaller pieces, so it can be absorbed. The process discolors and ultimately destroys the host material. In order for mold to grow and reproduce it requires an environment rich in oxygen, a food source, an acceptable temperature, and a sufficient amount of water. Typical indoor environments readily provide all of these factors—except one. Mold requires a higher quantity of moisture in the air than is comfortable to humans. The best defense against mold in buildings is moisture control.
Decay fungi are living organisms that invade damp wood and use the cellulose inside as a food source. Gradually, this rot fungi decomposes the wood and destroys its strength. Rot fungi, also called wood rot, can cause the decayed area to look brown and crumbly or create a white or yellow discoloration with a spongy texture. Some rot fungi can grow for long periods of time without producing any external evidence of the internal destruction, while others produce visible fruiting bodies on the surface of the decaying wood. When previously dry wood is exposed to moisture, either by being laid on moist soil or being located in a moisture-rich area of a house, it is likely that wood decay problems will occur. Most wood rotting fungi must have a direct supply of water at the site of the decay. The key to preventing rot fungi is to control the presence of moisture inside the wood.
Some rot fungi can grow for long periods of time without producing any external evidence of internal destruction, while others produce visible fruiting bodies on the surface of decaying wood.
Termites, subterranean and Formosan, also threaten the structural integrity of wood framing components. These insects eat the cellulose in untreated wood. Although termites are often considered a pest problem, they are also a moisture-related problem. Termites, often referred to as white ants, lack the typical insect exoskeleton, and, instead, appear milky and white and almost larval. This soft body makes them vulnerable to drying out, so termites must keep their homes moist and are attracted to moist food supplies like damp or rotting wood. Most termites are actually blind and navigate using scent and moisture trails. Formosan termites, normally existing in the Southeast United States, are especially voracious and are capable of infesting homes by air and building nests in the wall systems of homes. Termites can create their nests inside their food source and when they do so, they share their homes with fungi and bacteria that help them to destroy the wood.
Preventative measures that can be taken during home construction to help protect a home against termite infestation include: making sure that lumber and other wood components do not come into contact with the soil, placing a moisture barrier under basements to keep the area dry, treating wood components with a pesticide that make them unappealing to the termite palate, and ensuring that the wood in the home envelope is kept as dry as possible. To make an already constructed home less attractive to termites, start by eliminating moisture problems and remove excess food sources like firewood, lumber, paper, and tree stumps from the immediate surrounding area.
Some indications that a home may be infested with termites include: a temporary swarm of winged insects in the home or in the soil around the home, cracked or bubbling paint, frass (termite droppings), mud tubes on exterior walls, and wood that sounds hollow when tapped.
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