Excerpts from the Canadian Wood Council “Building Performance Series No. 1”
Protection of buildings from moisture is an important design criterion, as important as protection from fire or structural collapse. Designers, builders and owners are gaining a deeper appreciation for the function of the building envelope (exterior walls and roof). This includes the performance of windows, doors, siding, sheathing membranes, air and vapor barriers, rainwater control layer and framing. The capabilities and characteristics of wood and other construction materials must be understood, and then articulated in the design of buildings, if proper and durable construction is to be assured.
Wood and water are typically very compatible. Wood is a hygroscopic material, which means it has the ability to release or absorb moisture to reach a moisture content that is at equilibrium with its surrounding environment. As part of this natural process, wood can safely absorb large quantities of water before reaching a moisture content level which is favorable to the growth of decay fungi. To ensure durable wood-frame buildings, the design of the structure and envelope should be based on an understanding of factors that influence the moisture content of wood and changes that occur due to variations in moisture content.
Understanding the moisture content of wood is crucial, as 1) varying moisture content leads to shrinking and swelling of wood members, and 2) high moisture content can lead to the growth of mould and decay fungi. Moisture content (MC) is a measure of how much water is in a piece of wood relative to the wood itself. MC is expressed as a percentage and calculated by dividing the weight of water in the wood by the weight of that wood if it were oven-dry.
Two important MC numbers to remember are:
1. 19 percent: We tend to call a piece of wood “dry” if it has a MC of 19 percent or less. This type of lumber is grade marked as KD (typically shown as KD-HT) for kiln-dried, and means dry at the time of manufacture. (Note: Some lumber is also marked S-DRY for surfaced dry, or dry at the time of manufacture).
2. 28 percent: This is the average fiber saturation point for wood where all the wood fibers are fully saturated. At moisture contents above the fiber saturation point, water begins to fill the cell cavity. Decay can generally only get started if the moisture content of the wood is above fiber saturation for a prolonged period of time. The fiber saturation point is also the limit for wood swelling.
Shrinkage and Swelling
Wood shrinks or swells as its moisture content changes, but only when water is taken up or given off from the cell walls. This only occurs when wood changes moisture content below the fiber saturation point. Wood used indoors will eventually stabilize at 8 to 14 percent moisture content; outdoors at 12 to 18 percent.
The amount of dimensional change is estimated at 1 percent of the width or thickness of lumber for every 5 percent change in moisture content. Shrinkage is to be expected in lumber across its width while longitudinal shrinkage is likely to be negligible, such as the vertical shrinkage of a wall stud. In a wood-frame structure, shrinkage occurs primarily in horizontal members such as wall plates and floor joists. In buildings designed to three to six storeys, the effects of cumulative shrinkage can affect the building envelope, such as the exterior cladding. Special consideration must be given to designs that allow for shrinkage. (Visit www.cwc.ca, select “design tools” and open the dimension calculator tool, or go to cecobois.com/en/calculators and open the lumber shrinkage calculator for determining the amount of shrinkage and swelling in wood.)
For example, when a wood-frame structure is combined with a brick veneer, a concrete block elevator shaft or stair tower, or a steel-frame building element, the cumulative effects of differential movement in a multi-storey building must be accounted for in the detailing and specifications.
Specification of dry lumber is an important step towards minimizing shrinkage. One advantage of using dry lumber is that most of the shrinkage has been achieved prior to purchase (wood does most of its shrinking as it drops from 28 to 19 percent). It will also lead to a more predictable in-service performance as the product will stay more or less at the same dimension it was upon installation.
Another way to avoid shrinkage and warp is to use composite wood products such as plywood, OSB, I-joists and structural composite lumber. These products are assembled from smaller pieces of wood glued together. Composite products have a mix of log orientations within a single piece, so one part constrains the movement of another. For example, plywood achieves this crossbanding form of self-constraint. In other products, movements are limited to very small areas and tend to average out in the whole piece, as with finger-jointed studs.
Control of moisture during construction is also important. Even when dry lumber is purchased and delivered to the jobsite, it can be wetted prior to or during construction. Procedures should be developed to:
• keep wood-based materials dry while in storage onsite,
• minimize wetting of installed materials, and promote drying of materials with venting, heating or dehumidification.
Wood materials that are exposed to wetting should be dried to 19 percent moisture content or less prior to enclosure within assemblies. On buildings that are exposed to significant wetting during construction, schedules should provide an allowance for proper drying to framing and sheathing materials. The weather barrier (i.e., the rainwater control layers), installed soon after assemblies are framed, can be used to minimize exposure to weather.
The primary durability hazard with wood is biodeterioration. Wood in buildings is a potential food source for a variety of fungi, insects and marine borers. These wood-destroying organisms have the ability to break down the complex polymers that make up the wood structure. The wood-inhabiting fungi can be separated into moulds, stainers, soft-rot fungi and wood decay fungi. The moulds and stainers discolor wood; however, they do not damage the wood structurally. Soft-rot fungi and wood decay fungi can cause strength loss in wood, with the decay fungi responsible for deterioration problems in buildings.
Decay is the result of a series of events including a sequence of fungal colonization. The spores of these fungi are ubiquitous in the air for much of the year, but only lead to problems under certain conditions. Wood decay fungi require wood as their food source, an equable temperature, oxygen and water. Water is normally the only one of these factors we can easily manage. Wood decay fungi also have to compete with other organisms, such as moulds and stainers, to get a foothold in wood materials. It is easier to control decay fungi before decay has started, since these pre-conditions can inhibit growth rates at the start.
Decay and mould are terms that are often used interchangeably in the context of moisture-related wood damage. It is important to understand the distinction. Mould fungi can grow on wood (and many other materials), but they do not eat the structural components of the wood. Therefore, mould does not significantly damage the wood, and thus mould fungi are not wood-decay fungi. However, some types of moulds have been associated with human health problems, so the growth of mould in sufficient quantity and exposure to occupants is of potential concern regardless of physical damage to building products. Unfortunately, the relationship between mould and health is not yet fully understood. We live safely with some moulds in the air all the time, so clearly there are issues of thresholds, individual sensitivities and other variables that still need to be determined by health experts and building scientists.
Decay fungi, a higher order of fungi than moulds, break down basic structural materials of wood and cause strength loss, but are not associated with any human health problems.
Mould and decay do not necessarily occur together, nor are they indicators of each other. There tends to be a gradual transition from molds to decay fungi if moisture conditions continue to be wet.
Moisture Balance and Sources
Moisture flows within any building must be managed to prevent water accumulation or storage that may lead to premature deterioration of building products. Water will lead to deterioration by corrosion in steel products, by spalling and cracking in concrete products, and by fungi in wood products.
There are two general strategies to moisture control in the building envelope:
• limit the moisture load on the building
• design and construct the building to maximize its tolerance to moisture, to a level appropriate for the moisture load
The key design objective is to keep building envelopes dry, and to achieve moisture balance, where wetting and drying mechanisms are balanced to maintain moisture content levels at or below the tolerance level. The concept of “load” is well established in structural design, where dead loads, live loads, wind loads, seismic loads and thermal loads are fundamental to the design process. Similarly, moisture loads are placed on a building, and these loads must be accounted for and balanced in the building envelope design. The nature and magnitude of the loads will vary greatly depending upon the climatic situation, as well as occupancy of the building.
Moisture sources in and around buildings are abundant. Interior moisture sources include building occupants and their activities. Some studies have concluded that a family of four can generate 10 gallons of water vapor per day. Rainwater, especially wind driven, is the moisture source that impacts the performance of the envelope most.
The design of building envelope assemblies must be based on an evaluation of the probable exposure to moisture. For exterior walls, moisture load is primarily determined by:
• Macro-climate: regional climatic norms
• Micro-climate: site-specific factors such as siting, solar exposure, wind exposure and relationship to surrounding buildings, vegetation and terrain
• Building design: protective features such as overhangs and cornices
The levels of exposure can vary significantly on a single building, and the design of exterior wall assemblies can reflect these differences.
The Canada Mortgage and Housing Corporation published a nomograph (applicable to Vancouver, B.C.) to analyze exposures based on micro-climates and design factors. The principle criteria are overhang ratio and terrain (the primary influence on the microclimate of a given site). Analysis with a tool such as the nomograph allows the designer to further refine the criteria for wall type selection.
Overhang Ratio = Overhang Width
Overhang width equals the horizontal distance between the outer surface of the cladding and the outer surface of the overhang, while wall height equals the height above the lowest affected wood element (therefore, do not include concrete foundation walls).
A number of studies have concluded that the primary failure mechanism with respect to moisture is rainwater penetration through exterior walls. This has been particularly evident in several wet, humid coastal regions of North America, such as Wilmington, Seattle or Vancouver. Development of strategies for rain penetration control is the first priority in design for durability. Control of condensation caused by vapor penetration and groundwater are additional – though secondary – concerns. In both cases the strategy should meet the degree of the hazard or moisture load.
Rain Penetration Control
There are two general strategies for rain penetration control:
• minimize the amount of rainwater contacting the building surfaces and assemblies
• manage the rainwater deposited on or within assemblies
The dynamics of rainwater penetration are well established. Water penetration through a building assembly is possible only when three conditions occur simultaneously:
• an opening or hole is present in the assembly
• water is present near the opening
• a force occurs to move the water through the opening
This is true of all water penetration and has been expressed as a conceptual equation: water + opening + force = water penetration. The minimum size of opening which will allow water penetration varies in relation to the force driving the water. To control water penetration, it is necessary to understand the underlying driving forces that may be present. These can include gravity, surface tension, capillary suction, momentum (kinetic energy) and air pressure difference.
It follows that water penetration can be controlled by eliminating any of the three conditions necessary for penetration. Building design and detailing strategies can be developed that:
• reduce the number and size of openings in the assembly
• keep water away from any openings
• minimize or eliminate any forces that can move water through openings
These general water management strategies have been further articulated into a set of design principles called the 4Ds: deflection, drainage, drying and durable materials. With respect to rain penetration control, deflection refers to design elements and details that deflect rain from the building minimizing rainwater loads on the building envelope. Drainage, drying and durable materials are principles that deal with the management of water once it has reached or penetrated the envelope.
These principles can be applied to design at two distinct scales. At the macroscale, there are design patterns that involve the manipulation of building and roof form, massing, siting, material expression and even issues of style. At the microscale, there are detail patterns, which determine whether water management works or does not work. Detail patterns involve the relationships between materials, installation sequencing, constructability and economy of means. Many of these patterns, developed empirically by trial and error, have been used by builders for centuries, whereas others have been developed more recently as a result of scientific research and testing.
The principles are also applied to material selection. In most exposures, effective rainwater management is accommodated by multiple lines of defense. This is often referred to as redundancy. The concept of redundancy involves recognizing the inherent limitations of the design and construction processes. Perfection is not easily achieved and errors in design and construction do occur. Where the degree of moisture hazard is high, these errors may have significant impacts on the envelope performance. Redundant systems provide for back-up protection, in the likely event errors are made.
The 4Ds can be understood as four separate lines of defense against rain penetration and the problems that can result.
The deflection principle is evident in many building design patterns that have historically proven effective at reducing the amount of rainwater on exterior walls. These include: 1) placing the building so it is sheltered from prevailing winds, 2) providing sizable roof overhangs and water collection devices at the tops of exterior walls, and 3) providing architectural detailing that sheds rainwater. A pitched roof with sufficiently wide overhangs is the singular design element that can help ensure the long-term durability of wood-frame buildings. Deflection is applied at the smaller scale in detail patterns such as projecting sills, flashings and drip edges. Cladding and sealants are also considered to be part of the deflection line of defense. A water management strategy that relies only on deflection may be at risk in regions of North America where the hazard condition is high.
Drainage is the next principle of rain penetration control, second only to deflection in terms of its capacity to manage rainwater. Building design patterns that incorporate the drainage principle include pitched roofs and sloped surfaces at horizontal elements. At the detail level, drainage is accomplished by collecting incidental moisture accumulation in the wall assembly and returning it to, or beyond, the exterior face of the cladding by means of gravity flow. In its simplest form, this is achieved by adding a drainage plane within the assembly, between the cladding and the sheathing. In wood-frame construction, the drainage plane typically consists of a moisture barrier (building paper, felt or housewrap), and most importantly, how they work in combination with window and door flashings. Drainage is generally the primary means of providing redundancy in a wall assembly.
A drainage cavity is a more elaborate feature that introduces an airspace between the cladding and the drainage plane/sheathing. The airspace serves as a capillary break to prevent water from excessively wetting the drainage plane. The airspace, particularly when it provides a pressure equalization function, also can be seen as another means of deflection, in that pressure-equalization neutralizes the primary driving force behind rain penetration (air pressure differential), and thereby reduces the amount of moisture being driven through the cladding into the drainage cavity.
Drying is the mechanism by which wall assemblies remove moisture accumulations by venting (air movement) and vapor diffusion. The drying potential of both the cladding and the wall sheathing/framing must be considered. Cavities introduced for drainage purposes also offer a means to dry the cladding material by back venting. Drying of sheathing and framing is often a separate matter and is greatly affected by the selection of moisture barrier and vapor barrier materials. Exterior wall assemblies must be designed to allow sufficient drying to either the exterior or the interior. The permeability of cladding, moisture barrier, vapor barrier and interior finish materials will greatly affect the overall drying potential of the wall.
Durable materials must be selected for use at all locations where moisture tolerance is required. Where deflection, drainage and drying cannot effectively maintain the moisture content of wood components below 28 percent, the decay resistance of the wood must be enhanced. For wood framing components, this is achieved by pressure treatment with wood preservatives. The use of treated wood where sill plates are in contact with concrete foundations is a common detail pattern that follows this principle.
Building design patterns involving architectural expression should be reconciled with long-term durability considerations. Weathering properties and maintenance requirements should be considered. For example, face brick applied to wood-frame walls must be rated for exposure, and masonry wall ties must be sufficiently corrosion-resistant. Wood siding and trim with direct exposure to weather should be either naturally decay-resistant or treated wood materials.
There are three basic exterior wall type options for wood-frame buildings, each based on a distinct conceptual strategy for rainwater management: face seal, concealed barrier and rainscreen. When designing exterior walls for a given building, there is a need to select an appropriate system and be consistent through the design and detailing phase, and to clearly communicate the details of the system to the construction team.
Face seal walls are designed to achieve water tightness and air tightness at the face of the cladding. Joints in the cladding and interfaces with other wall components are sealed to provide continuity. The exterior face of the cladding is the primary – and only – drainage path. There is no redundancy. The “face seal” must be constructed – and must be maintained – in perfect condition to effectively provide rain penetration control. However, such reliance on perfection is questionable at walls exposed to rainwater. As a rule, face seal walls should only be used where very limited amounts of water will reach the cladding surface, such as wall areas under deep overhangs or soffits or in regions where the degree of moisture hazard is not high.
Concealed barrier walls are designed with an acceptance that some water may pass beyond the face of the cladding. These walls incorporate a drainage plane within the wall assembly, as a second line of defense against rain penetration. The face of the cladding remains the primary drainage path, but secondary drainage is accomplished within the wall. An example of a concealed barrier wall is wood siding installed directly over an asphalt-saturated felt moisture barrier and plywood sheathing. The water-resistant felt constitutes the drainage plane.Vinyl siding and drainage EIFS (exterior insulated finish system) installed over a moisture barrier also should be considered concealed barrier walls, although drainage in these cladding systems is enhanced by provision of some airspace – however discontinuous – behind the cladding. A concealed barrier strategy is appropriate for use on many exterior walls and can be expected to perform well in areas of low to moderate exposure to rain and wind. Performance in high to severe exposure conditions, however, is not assured. In all cases, the integrity of the second line of defense is highly dependent on correct detailing by the designer and proper installation by the builder. To maximize performance and service life of the assembly in high exposure conditions, consideration should be given to the use of a rainscreen assembly.
Rainscreen walls take water management one step further by incorporating a drainage cavity (3/8-in. minimum width) into the assembly, between the back of the cladding and the building paper. The drainage cavity offers enhanced protection from water intrusion by acting as a capillary break, thereby keeping most water from making contact with the moisture barrier. The airspace also serves to ventilate the backside of the cladding, which facilitates drying of the cladding, and mitigates against potential moisture accumulation in the wall framing caused by reverse vapor drive. Examples of rainscreen walls include brick veneer (usually installed with a one or two-inch airspace) and stucco cladding installed over vertical strapping (typically pressure-treated 1x3s at 16-in. o.c. on center). Rainscreen walls are appropriate for use in all locations where high exposure to rain and wind is likely.
Pressure-equalized rainscreens represent an advancement of the basic rainscreen strategy. These walls incorporate compartmentalization and increased venting of the drainage cavity to improve performance. As wind blows on a wall face, air passes through vents into the cavity behind the cladding. If this air is contained appropriately by subdividing the drainage cavity with compartment seals, an equalization of pressure occurs across the cladding, thereby eliminating one of the key driving forces behind water penetration. This strategy is most commonly applied to brick veneer walls, though conceptually it is possible to enhance any rainscreen assembly with this technology. Pressure-equalized rainscreens are appropriate for use on all exposures and offer the highest performance potential with respect to water management.
Wood-frame buildings have an established record of long-term durability. With the correct application of building envelope design principles, all materials can perform well with regards to durability. The imperative for durable construction goes beyond creating healthy buildings, as we must build durably to minimize the environmental impacts of our society. In fact, wood buildings perform well against other materials when considered from a life-cycle cost perspective that factors things like greenhouse gas emissions, water pollution index, energy use, solid waste and ecological resource use. However, the environmental advantages of wood can only be achieved if the building is designed and constructed for long-term durability. With passion and eloquence, the architect James Cutler has spoken of “honoring the wood” through the building design and detailing process. This would include the concept of protecting wood from moisture, which is the essence of designing for durability.