Hygroscopicity: Moisture and Wood              

Wood is a hygroscopic material. It gains and loses moisture depending on the climatic conditions to which it is exposed, causing variations in strength, stiffness, and shrinkage. The moisture content of freshly sawn lumber is higher than it will ever be under normal service conditions. Most wood products are either air-dried or kiln dried prior to use.

Wood shrinks as the moisture content (MC) is reduced below the fibre saturation point, which is the specific moisture content where the wood cell walls are fully saturated but there is no free water in the cell cavities. Drying below this fibre saturation point then removes water from the cell walls and cell wall shrinkage results in changes in member width and thickness. The fibre saturation point varies with species and is typically assumed to be around 24%.

Generally, wood shrinks very little along the grain (longitudinally), while the shrinkage across the grain can be significant. In addition, shrinkage along the growth rings (tangentially) is generally larger than the shrinkage across the growth rings (radially).

Natural climatic variation in temperature and relative humidity causes changes in moisture content of wood products. The moisture level of the wood will eventually reach equilibrium with that of the surrounding air, and will continue to adjust to changing conditions by gaining or losing moisture.

Wood in heated buildings can be subjected to a wide range of humidity levels over an annual cycle. In cold climates, winter humidity levels of 20 to 30% are common in houses, and may be even lower in other occupancies that generate little or no moisture such as offices. During the summer, outdoor humidity levels average 60 to 70% in most inland areas. These differences cause the equilibrium moisture content of wood to vary from 6% in winter to 12% in summer, assuming steady-state conditions are reached.

Lumber which has been air-dried or kiln dried to lumber grading standards will have a moisture content of 19% or less. Specifying and using dried lumber automatically reduces the amount of shrinkage because the wood뭩 moisture content at the time of installation is closer to the equilibrium moisture content that will be reached over time.

Panel products such as plywood or OSB (Oriented Strand Board) are at a lower MC at the time of manufacturing. Engineered wood products (EWPs), including products such as I-joists also tend to have MC lower than kiln dried lumber, as a result of the manufacturing process. As a result, panels and engineered wood products are very stable as long as they are kept dry.

Potential dimension changes in a building, due to wood shrinkage, should be accommodated with proper detailing. Wood shrinkage can be estimated using the Dimension calc.

Strength and stiffness properties of wood products generally increase with drying below the fibre saturation point.

Strength Variation Relative to MC  

Tabulated specified strengths for dimensional lumber are derived assuming that sawn lumber will be used in a dry service condition, where the average moisture content of a member over a year averages 15% or less and does not exceed 19%. When calculating a wood member strength and/or stiffness for use in a wet environment, it will need to be modified by a wet service condition factor. Wet service condition factors are built into wood design standards and take into account the property differences between green and dry service conditions.

Checking occurs when lumber is rapidly dried. The surface dries quickly, while the core remains at a higher moisture content for some time. As a result, the surface attempts to shrink but is restrained by the core. This restraint causes tensile stresses at the surface, which if large enough, can pull the fibres apart thereby creating a check. Splits are through checks that generally occur at the end of wood members. When a wood member dries, moisture is lost very rapidly from the end of the member. At mid-length, however, the wood is still at a higher moisture content. This difference in moisture content creates tensile stresses at the end of the member. When the stresses exceed the strength of the wood, a split is formed.

Checks and splits are naturally occurring characteristics of solid sawn wood products and structural design values for lumber and timbers have been developed taking into account the effects of checking and splitting. The size of checks and splits are limited by the grading rules for lumber and timbers.

Larger dimension sawn timbers are susceptible to checking and splitting since they are always dressed green (S-Grn). Furthermore, due to their large size, the core dries slowly and the tensile stresses at the surface and at the ends can be large. Minor checks confined to the surface areas of a wood member very rarely have any effect on the strength of the member. Deep checks could be significant if they occur at a point of high shear stress and the size of the check or splits exceeds the size specified in the applicable grading rule. Checking and splitting surrounding mechanical connections may cause some strength loss in the connection and further investigation may be required.
The possibility and severity of splitting and checking can be reduced by controlling the rate at which drying occurs. This may be done by keeping wood out of direct sunlight and away from any artificial heat sources. Furthermore, the ends may be coated with an end sealer to retard moisture loss.

It is important to understand and account for the effects of moisture on wood in order to build strong and safe buildings. To minimize dimensional change and the possibility of checking or splitting, it is equally important to specify wood products that are as close as possible in moisture content to the expected equilibrium moisture content of the end use, and to ensure that the investment made in purchasing dry wood products is protected by proper storage and handling, and by enclosure of the building as quickly as possible.

Please see details about the upcoming CWC publication on Moisture and Wood in the "What's New" section below.

Insulation and Ventilation of Wood-Frame Roof Assemblies
Part 4 - Low-slope and Unvented Roofs
Michael Steffen

Ventilation at Low-Slope Roofs
Airtight ceiling construction is essential at vented low-slope roofs if interstitial moisture levels are to be controlled and heat loss minimized. Vapor diffusion control measures for low-slope roofs are similar to those discussed for steep-slope cathedral roofs.       

Low-slope wood-frame roofs with insulation below the roof deck present special ventilation problems given their typically reduced amount of ventilation airflow.
At roof slopes less than 2:12, stack-induced airflow is very limited making it is difficult to develop a draft between in-take and exhaust vents. Ventilation that does occur relies primarily on wind-induced airflow that can be inconsistent at best. Such ventilation may also encourage the flow of moist indoor air into the roof if the ceiling construction is not airtight.

Low-slope roofs with insulation below the roof deck are subject to condensation problems, particularly for buildings in heating climates. To avoid the risk of condensation, it is recommended that a conventional roof assembly, with insulation above the deck, be used. This type of assembly requires rigid insulation, and is generally a higher cost assembly to build, however the reduction in condensation risk may justify the additional costs.

If low-slope roofs are insulated below the deck, ventilation should be provided at minimum 1/150 ratio, meaning that the minimum net free ventilating area for vents be a 1/150 ratio of the area of the space being ventilated.
Where roof framing is not open, cross-strapping of minimum 1-?in. height can be installed over the framing members to interconnect the framing cavities and provide a degree of cross-ventilation that is helpful for avoiding dead spots.

Unvented Roofs
Current research indicates that ventilation is not necessary from a technical standpoint in all insulated roof assemblies. Ventilation is recommended at cathedral ceilings and low-slope roofs in heating climates, however, ventilation should be considered a design option in mixed and cooling climates.
For many roof designs, it is difficult to achieve proper ventilation in accordance with basic principles. For example, the conditions found on existing buildings can present problems when insulation is to be added during retrofit work. Roof forms on new buildings can be complex, prohibiting a simple and functional ventilation scheme.

Where venting is problematic, unvented roofs can perform well in all climates ?including heating climates ?when proper measures are taken to control interior humidity levels and minimize air leakage from the building interior into the roof assembly.

Insulation selection is an important consideration in the design of unvented roof assemblies. Foam insulation is inherently more airtight and vapor resistant than low-density insulations such as fiberglass batt. Where foam insulation has been used in walls and low-slope roofs, it has generally provided excellent moisture and thermal performance. The same performance can be expected at cathedral ceiling assemblies. In unvented roofs, foam should be applied directly to the underside of the roof sheathing and carefully air sealed at framing members and penetrations.

An unvented roof can also be achieved by placing the foam entirely above the roof deck. This is a common practice in low-slope roofs, and not unlike the exterior insulation detail of cathedral ceilings.

Michael Steffen is a registered architect and Quality Director at Walsh Construction Company in Portland, Oregon.

A longer version of this article is found in the Spring 2004 issue, Number 27 of Wood Design & Building. For more information visit www.woodmags.com, click on the Wood Design & Building logo and then select Back Issues.

Wood Trusses - Strength, Economy, Versatility

Wood trusses are engineered frames of lumber joined together in triangular shapes by galvanized steel connector plates, which are commonly referred to as truss plates. Wood trusses are widely used in single- and multi-family residential, institutional, agricultural and commercial construction.

Their high strength-to-weight ratios permit long spans, offering greater flexibility in floor plan layouts. They can be designed in almost any shape or size, restricted only by manufacturing capabilities, shipping limitations and handling considerations. In North America the wood truss industry has grown to the point where more than 60% of residential roofs are now built with wood trusses.

Advantages
Strength: Trusses provide a strong and efficient wood system specifically engineered for each application.

Economy: Through efficient use of wood and by providing a system that is quickly installed in the field, wood trusses provide an economical framing solution.

Versatility: Complex shapes and unusual designs are easily accommodated using wood trusses. The versatility of wood trusses also makes it an excellent roof framing system in hybrid construction where wood trusses are used together with steel, concrete or masonry wall systems. Wood floor trusses are also commonly used in residential and commercial applications.    

Environmental: Wood, the only renewable building material, has numerous environmental advantages. Wood trusses enhance wood뭩 environmental advantages by optimizing wood use for each specific application. Improvements in materials, design and manufacturing technologies have increased wood truss competitiveness. Wood trusses can be constructed and spaced to optimize lumber strength and conserve timber resources. For example smaller dimension lumber is used in the truss webs and the typical roof truss spacing of 600 mm on centre optimizes roof framing.

Fire & Sound: Fire-resistance ratings, based on standardized tests, are a measure of the fire resistance of roof and floor assemblies. Depending on sheathing, ceiling construction, and insulation, truss assemblies have achieved fire resistance ratings up to 2 hours. Not all truss assemblies require a fire resistance rating. The building occupancy, the building size, number of exits and the use of sprinklers will determine what fire resistance rating is required. Floor truss assemblies can be optimized to reduce sound transmission. In apartments, this limits noises from upper or lower units.

Truss Design
The building designer who must specify the shape and span of the truss, where the truss will be supported, and what the loads on the truss will be, initiates the truss design. Typically, the building designer or builder will contact the truss fabricator who will supply a fully engineered truss. The truss plate manufacturer usually designs the truss on behalf of the truss fabricator.

In North America, designs are based on the structural requirements of the Building Codes using design standards referenced in the Building Codes and approved material properties for lumber and the proprietary truss connector plates.

Please look for next month뭩 issue of the Wood (IN)Site newsletter where we will discuss specific issues relating to installation of wood trusses including bracing do뭩 and don뭪s, as well as safer and better erection techniques. We will also examine trusses as part of the Prefabricated Wood Components Market outlining the future and advantages for prefabricated wall, floor and roof components.

New CWC Publication on Moisture Management and Wood

CWC is currently completing a new publication on moisture management and wood products. The publication is part of CWC뭩 Building Performance Series and is intended primarily for builders and material suppliers, but also contains useful information for architects and engineers.

The bulletin examines the relationship between wood and moisture. It provides solutions to prevent wood from absorbing significant amounts of moisture so that optimal performance and durability of both lumber and engineered wood products are ensured. The publication also presents guidelines to follow should wood inadvertently become wet before installation, and examines the question of mould clean-up.

The publication will be available both in hardcopy and in PDF format in approximately two weeks. Professionals that are interested in obtaining a copy should visit the CWC website or the Wood Durability website. Hardcopies will be available directly from CWC by calling 1-800-463-5091.