Designing for Durability

Strategies for achieving maximum durability with wood-framed construction
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Sources of Water and How to Control Them

Moisture loads placed on a building must be accounted for and balanced in the building envelope design. The character of these loads is a function of the climate, surroundings, and type of building. Managing moisture in structural wood products is essential in order to control swelling and shrinkage, and to prevent problems associated with pests and decay.

Potential exterior sources of moisture include rain (wind-driven rain in particular) and snow, as well as groundwater, adjacent irrigation systems, and outdoor air bringing in water vapor. Any design and construction features that may trap moisture and slow the wood’s drying should be avoided. Interior moisture sources include poor detailing of the building envelope resulting in air leaks and plumbing failures, poor ventilation and thermal design, and building occupants and their activities. A four-member family can generate up to 10 gallons of water vapor a day.5

The primary objectives when addressing moisture loads are to keep water from entering the building envelope in the first place, and to balance the relative humidity of the indoor air within the building itself. Moisture management can be achieved by best-practice design details that protect wood-frame buildings and envelope assemblies against decay, using four lines of defense. None of these measures are perfect in themselves, but, in combination, they can create conditions where the risk of moisture problems is negligible. They are summarized as the four Ds:6

Deflection: Rain deflection is critical in preventing bulk water from entering the building envelope, whether through wall or roof coverings, or openings such as doors and windows. Pitched roofs, overhangs, and flashing should be used to deflect water away from the structure. To minimize the effects of wind-driven rain, exterior wall coverings should be installed over a drainage plane. If water penetrates the exterior siding, the drainage plane is designed to direct water down the vertical surface to the outside via weep-holes, rather than allowing it to enter the wall cavity.

Drainage: For any water that penetrates the cladding, roof shingles, or other building envelope surfaces, a well-designed drainage path, such as the drainage cavity integrated in rainscreen walls and other drainable building envelope assemblies, will allow water in the cavity to flow down a water-resistant plane and then exit the building envelope.

Drying: Drying is the mechanism by which building envelope assemblies remove accumulated moisture by venting (air movement) and vapor diffusion. If, due to construction or maintenance errors, water penetrates the water-resistant membrane, the wood sheathing, studs, roof truss, and other wood elements in the building envelope can get wet. These elements must be allowed to dry. In properly designed building envelope assemblies, water will evaporate and the resulting vapor will go through the assembly’s outer layers, providing vapor permeability has been designed into the building envelope assemblies.

Exterior wall assemblies must be designed to allow sufficient drying both to the exterior and interior depending on the climate. The permeability of cladding, moisture barrier, vapor barrier, insulation (exterior insulation in particular), and interior finish materials will greatly affect the wall’s overall drying potential. Rainscreen cavities may also dry the wall and cladding if vented.

Experts caution that the drying ability of wall systems should not be relied on to compensate for serious flaws in other moisture management mechanisms, since only minimal amounts of water can be dissipated through drying. The bulk of the moisture protection in wall stems from deflection and drainage.

Durable materials: Designers should not discount the value of preservative-treated wood or naturally decay-resistant wood for applications such as cladding, shingles, sill plates, and exposed timbers or glulam beams, where moisture tolerance is necessary or where termite infestation is likely. Approved durable or preservative-treated wood is usually recommended when location of the member in question cannot be maintained at a safe moisture content, and when climatic or site conditions may not permit control of decay or termites by construction practices alone. The role, characteristics, and availability of naturally durable and preservative-treated wood species are discussed later in this course.

Designed by tvsdesign, the Branson Convention Center in Branson, Missouri, features a large overhanging roof. By sheltering the wood underneath, the roof deflects precipitation and helps to ensure long-term durability.

Photo courtesy of Brian Gassel, tvsdesign

Designed by tvsdesign, the Branson Convention Center in Branson, Missouri, features a large overhanging roof. By sheltering the wood underneath, the roof deflects precipitation and helps to ensure long-term durability.

Fungi Control

Decay of wood doesn’t happen mysteriously or without cause. Wood that is recognizably rotten is the product of a sequence of events involving a succession of microorganisms operating under certain conditions. Understanding the conditions under which wood in a building can break down is a first step in interrupting the process of decay and preventing wood deterioration.

Fungi can be a cause of wood deterioration. Not all fungi weaken wood: for example, mold will merely stain it. Staining fungi give wood a “blue stain” that goes deep into the interior of the tree and typically occurs before logs are sawn into lumber. According to the Forest Products Laboratory, unlike decay fungi, staining fungi and mold fungi feed off the wood’s free water and its sugars but don’t impair the strength of the wood. However, if the wood remains wet for too long, it can be eaten by decay fungi.

Molds are notorious for their contribution to poor air quality and potential impact on human health. Mold spores can grow and thrive where there’s moisture, including humid air—which means they can proliferate almost anywhere. They grow on many surfaces, wood included, and usually signal a deficiency in a building’s moisture management program.

The MC of wood is the deciding factor in the growth of fungi. Wood with an MC of 19 percent or less is dry enough to virtually eliminate the ability of mold to grow. The risk of mold increases with higher moisture content and relative humidity, particularly when these conditions are sustained for extended periods of time. Relative humidity greater than 80 percent is a cause for concern—even though mold growth can be slow initially, higher humidity levels will accelerate growth. Most fungi grow fastest in the 60-80 degree Fahrenheit range; at freezing temperatures, they either do not grow or grow slowly.7

 

[ Page 6 of 10 ]             
Originally published in Architectural Record

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