Wood Rot in Homes
Wood Rot, What is it?
The microscopic organisms that discolor and decay wood belong to a huge group of primitive plants known as fungi. Unable to produce their own food, fungi feed instead on natural substances that make up organic materials like leather, cloth, rattan, paper, and wood.
Mushrooms that spring from lawns and tree trunks are fungal "fruits".
The most effective "method" of preventing fungal deterioration of wood is to keep it dry. Most fungi need a wood moisture content of at least 20% to carry on. With the moisture content of wood indoors over most of the United States cycling annually between 6% and 16%, it's too dry for most microorganisms to get started.
Occurring outside and inside homes, most mildews are black, but reds, greens, blues, and browns are possible. Even the familiar gray color of weathered wood is the work of mildew. Masses of dark spores and hyphae give mildews their characteristic splotchy look. Merely discoloring the surface they grow on, mildews have no appreciable effect on wood itself. Some mildews that feed on airborne organic matter can even grow on inorganic vinyl and aluminum sidings. Dew and rain supply needed moisture.
Mildews appear most often on eaves, decks, and porch ceilings. North-facing walls and those shaded by foundation plantings, trees and other obstructions that restrict sunlight and airflow are also candidates.
Beware, finishes applied over mildewed surfaces that are recoated without first killing the fungus will quickly discolor as the mildew grows though the new coating.
Mildew occurs indoors most frequently in baths, basements, and other areas prone to high relative humidity. It also shows up in places with poor air circulation such as behind furniture against exterior walls, and in closets and closed-off rooms. Mildew can form whenever the relative humidity of air near a surface exceeds 70%. This can happen when warm air near the ceiling cools as it flows down colder wall surfaces. The relative humidity of 70 (F air, for example, rises from 40% to 70% when it's cooled to about 52 (F. Spores and musty odors emitted by mildew growing in indoor microclimates can trigger allergic reactions.
Thermal bridges that lead to "hot spots" outside create "cold spots" inside. Exterior corners are notoriously mildew-prone because of poor air circulation inside and heat-robbing windwashing outside. In summer, water vapor from warm, humid air entering crawl spaces and basements below air conditioned rooms may condense on cooler joists and subflooring, creating conditions irresistible to mildew, as well as mold, and staining and decay fungi. Moisture condensed as ice from heated air leaking into attics in winter likewise wets rafters and sheathing when it melts.
Molds need a wood surface moisture content of about 20% to get started. To provide that, simply surround wood with air at 90% relative humidity at any temperature from 40 to 100 (F, and presto! That's why mold and mildew sometimes suddenly appear on furniture during the dog days of summer.
Guarding against decay fungi
While discoloration by mildew, mold, and staining fungi poses an appearance problem, attack of wood by decay fungi threatens its structural integrity. Aptly termed the "slow fire", wood decays or rots because these fungi eat the very cellulose and lignin of which wood cells are made.
In its early or incipient stages, decay can be difficult to detect, even with a microscope. Strength loss can be appreciable even at this stage. As the slow fire advances, wood's luster fades. Surfaces become lifeless, dull, and discolored. A musty odor is often evident. The rate at which decay progresses depends on moisture content, temperature, and the specific fungus.
Brown rots and white rots
Decay fungi fall into three major groups: brown rots, white rots, and soft rots. The latter are rarely found inside homes, though they occasionally degrade wood shakes and shingles on heavily shaded roofs in wet climates.
Brown rots are so-named because infected wood turns dark brown. Most commonly colonizing softwoods, brown rots consume cellulose, hardly touching the darker lignin. Mycelia appear as white sheet-like or fluffy growths on wood surfaces. Brown-rotted wood shrinks excessively and splits across the grain as it dries. Friable and crumbly, surfaces then show brown rots' hallmark cubical checking.
Water-conducting fungi are a special type of brown rot that show up infrequently in the southeast, northeast, and Pacific northwest. Sometimes called dry rot fungi, the name unfortunately suggests that dry wood can decay. Dry wood can't decay, period. What builders, inspectors, and homeowners alike routinely mislabel "dry rot" is almost always, in reality, wood that got wet, rotted, and dried out before discovery. Unique in their ability to pipe moisture from the soil over long distances through root-like rhizomorphs, water-conducting fungi wet otherwise dry wood in advance of their attack. Infecting softwoods and hardwoods, their light-colored mycelia look like large, papery, fan-shaped sheets. Dirt-filled porches, damp crawl spaces, and wood in ground contact are avenues for entry.
White rots impart a white, gray-white, yellow-white, or otherwise bleached appearance to wood. Most often infecting hardwoods, they feed on both cellulose and lignin. In advanced stages of decay, white-rotted wood is spongy, has a stringy texture, and lacks the cubical checking of brown-rotted wood. A thin black line often marks the advancing edge of incipient white rot in hardwoods. Ironically, this partially decayed or spalted wood is coveted by woodworkers for its unique figure.