The 20 Percent Solution
Q: I recently had an adjuster tell me that he would not pay for drying that reduced the moisture content of the building below 20 percent. What would be the consequence of stopping the drying at 20 percent?
A: This is actually a fairly complex issue. To answer this question we need to understand where the 20 percent moisture content (MC) figure comes from; what 20 percent MC does and does not apply to; and what are the consequences of leaving structural wood, at that MC, in contact with other materials in an assembly.
Why 20 Percent?The use of the 20 percent figure has its roots (no pun intended) in the moisture requirements for "dry rot" fungi to grow. According to the Canadian Wood Council, wood is not considered "dry" until it reaches an MC of 19 percent: "We tend to call a piece of wood "dry" if it has an MC of 19 percent or less. This type of lumber is grade marked as S-DRY for surfaced dry, or dry at time of manufacture."
The Forest Products Laboratory (FPL) Division of the United States Department of Agriculture refers to S-DRY as "shipping dry" and says that it is dried to that MC "to prevent decay in transit." Likewise, the Timber Harvesting and Forest Engineering Glossary defines "shipping dry" as wood dried to less than 20 percent MC and that it "Results in reduced shipping weight and less susceptibility to decay." Any lumber that has a moisture content that is greater than 19 percent is grade marked S-GRN for green lumber, according to the Forest Products Laboratory Wood Handbook.
In some of the literature dealing with "dry" and "wet" rot fungi, it states that a MC greater than 20 percent is required for "dry" rot to occur and that a MC of greater than 30 percent is required for "wet" rot to occur. Actually, "dry" rot is a misnomer. Moisture or water is necessary for rot to occur. "Dry" rot is a term that has loosely been applied to a type of decay fungi that has the ability to damage wood at a lower MC than the so-called "wet" rot fungi. More accurately, these fungi are referred to as "brown" rot.
Brown rot attacks the cellulose and associated carbohydrates rather than the lignin. The result is a light- to dark-brown friable residue. Examples include Serpula (Poria) incrassata and Serpula lacrymans, two Basidiomycetes that cause "dry" rot. These fungi form hyphae that are referred to as rhizomorphs. They have the capacity to transport water through the rhizomorph from a source that can be many feet away to an area where the wood is relatively dry.
One source states that the moisture content of the affected dry wood can be as low as 16 percent MC, hence the term "dry" rot. When the MC begins to exceed the fiber saturation point of 28-30 percent then "wet" rots begin to grow. Most correctly, these fungi are called white rot. They attack both the cellulose and the lignin, producing a whitish residue that may be spongy or stringy rot, or occur as pocket rot. All of these rot fungi fall into a category of macro fungi.
When 20 Percent MC AppliesIt is anticipated that this "shipped-dry" wood will achieve equilibrium with its surroundings. Depending upon the environment, the wood will either quickly or slowly dry to its equilibrium moisture content. EMC is actually a moving target, since the surrounding air is constantly changing and the wood will, in some circumstances, take on moisture (more humid air), and in other conditions (less humid air) it will give off moisture.
Up to this point, we have been talking about MC in wood used for structural framing. In the table published by FPL - MOISTURE CONTENT OF WOOD IN USE
(see Chart 1), FPL recommends that wood be "dried to a moisture content close to the midpoint between the high and low values the wood will attain in service." For framing, the recommendation is that the lumber be dried to a MC between 7 percent and 14 percent, depending upon where you live in the United States. The table above and the map below shows that the recommended moisture content values for wood used in woodwork, flooring, wood trim and laminates be dried to a MC somewhere between 6 percent and 13 percent, depending where you live. These recommended values are for wood items at the time of installation. Northern Hardwood Initiative has stated that "As a general rule, most hardwoods used for furniture and flooring in North America are dried to 6 to 8 percent MC. In coastal areas or the Southern U.S., this figure may increase to 8 to 10 percent as outside relative humidity is generally higher and buildings are not heated as extensively (see Chart 2). Some products, such as squares for turnings or dowels, will often be dried to 10 to 12 percent."
The Federal Emergency Management Agency recently published a table titled "Initial Restoration for Flooded Buildings - HURRICANE KATRINA RECOVERY ADVISORY" In this advisory, the MC of structural wood is considered "dry" when it is 15 percent or less (see Chart 3).
The California Department of General Services, Division of the State Architect - Product Acceptance Criteria document titled "WOOD STRUCTURAL PANELS - ORIENTED STRAND BOARD (OSB)" (AC 23-2) makes the following statements about the moisture content of OSB:
5.2 Panels shall be protected from moisture during transit and storage. Wetted OSB panels shall be dried to 16 percent moisture content or less prior to being installed.
5.3 If installed panels are wetted, they shall be dried to 16 percent moisture content or less prior to being covered. Alternatively, provide adequate ventilation on the backside to allow drying.
There are several references to surface mold growth occurring at or above 16 percent MC. Most of this information comes from determining the water activity at the substrate. There are hundreds of articles about water activity and mold growth. We know that there are molds that are xerophilic (relatively dry loving) that can grow in surfaces with a water activity of .61 which is the same as a 61 percent equilibrium moisture content. The American Conference of Industrial Hygienists, in their publication, "Bioaerosols: Assessment and Control," lists the MC of a number of building materials that will result in a 0.8 water activity. In the case of softwood (structural framing) the MC would be 17 percent. We know that there are many species of molds that will grow below that number. It is most likely that these molds will grow slowly and in some cases perhaps not at all. But the point is that it can happen and we do not have an accurate or economical way of making that determination. If there is a relative humidity of slightly over 70 percent, some molds can germinate in a 64-day period (see Chart 4).
Moisture Content and Other Building Materials in an Assembly You also have to take into consideration what happens if you were to leave structural wood with a MC of 20 percent in contact with other building materials. The building is a system and walls are part of an assembly. According to Oak Ridge National Laboratory, softwood has a moisture holding capacity that is 8.3 times greater than gypsum board. If you were to leave semi-wet wood in contact with paper-faced gypsum board, there moisture could then transfer from the wood to the paper. While the wood may not start to develop mold growth, the adjacent paper may very well become moldy. Also, if the wall assembly does not allow water or moisture to escape from the assembly, mold growth can occur on the wood over a period of time.