information provided by the Western Red Cedar Lumber Association 

DENSITY

One of the lightest commercial softwoods, the density of Western Red Cedar at oven-dry conditions is approximately 21 pounds per cubic foot with a relative density (specific gravity) of 0.32. Comparative oven-dry densities of cedar and some other softwood species are given in the Table 1 below.

Cedar’s low density enhances its insulation value and makes it an easy wood to transport and handle.

DIMENSIONAL STABILITY

Like all woods, Western Red Cedar is hygroscopic and will absorb or discharge moisture to attain equilibrium with the surrounding atmosphere. The size differential between dry and unseasoned Western Red Cedar is given in Table 2 below.

Shrinkage in both the radial and tangential directions is given in Table 3 below.

Cedar has a very low shrinkage factor and is superior to all other coniferous woods in its resistance to warping, twisting, and checking.

THERMAL CONDUCTIVITY

Wood is an excellent thermal insulator. This is an important characteristic since good thermal insulators help keep buildings cool in the summer and reduce heating costs in the winter. The conduction of heat in wood is directly related to its density. Woods with low density have the highest thermal insulating value because such woods contain a high proportion of cell cavities. In dry wood, these cavities are filled with air, one of the best-known thermal insulators.

With its low density and high proportion of air spaces, Western Red Cedar is the best thermal insulator among the commonly available softwood species and is far superior to brick, concrete, or steel.

It has a coefficient of thermal conductivity (k value) at 12% mc of 0.74 BTU inch/ft 2h degrees F. The R-value (the reciprocal of k) for Western Red Cedar is 1.35″ (34mm) of thickness.

ACCOUSTICAL PROPERTIES

An important acoustical property of wood is its ability to damp vibrations. Wood has a cellular network of minute interlocking pores that convert sound energy into heat by frictional and viscoelastic resistance. Because of the high internal friction created by the cellular pore network, wood has more sound damping capacity than most structural materials. Floor, ceiling, and wall assemblies of wood can provide effective, economical sound insulation and absorption when properly utilized.

Western Red Cedar is particularly effective in sound damping and can help reduce noise or confine it to certain areas.

FLAME SPREADING

Flame spreading ratings describe the surface burning characteristics of interior finishes.  They are used to regulate the use of interior finish materials to reduce the probability of rapid fire spread. Materials are burned in a test furnace for a relative assessment of flammability. The lower the flame spread rating, the more the material resists the spread of fire.

Building codes in North America generally define as interior finish any exposed material that forms part of the building interior. This usually includes interior wall and ceiling finishes, flooring, windows, doors, and other wood products. US codes set the maximum flame spread rating for interior wall, and ceiling finishes in most buildings at 200.

The flame spread rating for Western Red Cedar is 69 (Class II rating), which is much lower than the average finishes.

Smoke-developed classifications reflect the amount of smoke released by burning material. They are used in conjunction with the flame spread ratings to regulate the use of interior finish materials where the potential to generate smoke or control smoke movement is of major fire safety importance.

The smoke-developed classification for Western Red Cedar is 98. US codes set the maximum smoke developed classification for interior wall, and ceiling finishes in most buildings at 450.

Western Red Cedar’s flame spread rating and smoke developed classification compare well with the ratings of many other softwood and hardwood species.

Western Red Cedar can be used for an interior finish in some building applications where other species would not be permitted because of its favorable performance.

DURABILITY

Historically, native peoples of the Pacific coast prized cedar for its long-lasting qualities and used wood and bark from cedar trees for most of their building needs.

Evidence of cedar’s durability are the many cedar artifacts still in good condition today.

Properly finished and maintained, cedar will deliver decades of trouble-free service. If exposed for prolonged periods to conditions where decay could be a factor, such as where the wood is in contact with the ground, cedar should be treated with suitable wood preservatives.

FASTENING

Western Red Cedar has good fastening properties, but its natural preservatives have a corrosive effect on some unprotected metals in close contact, causing a black stain on the wood. Fasteners should be corrosive resistant such as aluminum, brass, silicon bronze, hot-dipped galvanized, or stainless steel.  Read more about choosing the right fasteners for your job HERE.

Nails and screws used to fasten Western Red Cedar should be about one-third longer than those used to fasten hardwood species.  It also works well with a wide range of adhesives.

Because it is free of pitch and resin, Western Red Cedar has excellent gluing properties.

FINISHING

Although cedar is a naturally durable species, leaving it untreated is not recommended because a finish or protective coating will greatly increase its service life.

Cedar is free of pitch, and with its high degree of dimensional stability, it is the best of the softwoods for accepting paints, stains, oils, and other coatings.

WORKABILITY

Cedar takes a fine finish in all hand and machine operations, takes fasteners without splitting, and is easily sawn and nailed. When working with Western Red Cedar, sharp cutters are recommended.

With its straight grain and uniform texture, Western Red Cedar is among the easiest and most rewarding woods to work with.

PRODUCTS, GRADES, AND SIZES

There are few more versatile building materials than Western Red Cedar, which is ideal both for indoor and outdoor uses. Western Red Cedar lumber is available in visual stress grades for construction and finishing uses in a range of lengths, widths, and thicknesses. It is available in clear or knotty grades with smooth-surfaced, combed, or rough sawn finishes; kiln dried or unseasoned (green); flat grain and vertical grain.

Western Red Cedar is used to manufacture a range of specialty products such
as siding, paneling, roofing, structural decking, and outdoor decking.

Table 1.  Comparative Softwood Densities (PCF)

SPECIES                               OVEN-DRY DENSITY
Western Red Cedar                       21
Douglas Fir                                      31
Ponderosa Pine                              27
Southern Pine                                34

Table 2.   Size Differential Between Unseasoned and Dry Lumber

                                              SIZE DIFFERENTIAL
NOMINAL                               BETWEEN UNSEASONED
DIMENSION                           AND DRY LUMBER
(INCHES)                                AFTER SURFACING
                                                  (INCHES)
1½ or less                                   1/32
2 to 4                                             1/16
5 to 7                                             1/8
8 or more                                     1/4

Table 3.     Shrinkage of Western Red Cedar

FROM KILN DRIED
FROM GREEN (25% OR             (15% AVERAGE
GREATER MOISTURE                MOISTURE
DIRECTION                           CONTENT) TO:                            CONTENT) TO:

OF SHRINKAGE                  15%       12%        6%                       15%     12%       6%
0.96       1.2          1.8                        0       0.3         1.0
TANGENTIAL                       2.0        2.6         3.8                        0       0.7         2.1

Notes:

1. Radial shrinkage applies to the width of vertical grain lumber: tangential to the width of flat grain lumber.

2. Shrinkage does not begin until the fiber saturation point is reached.

3. 15% is the average equilibrium moisture content of wood during the summer in the Pacific Northwest.

4. 12% is the summer average equilibrium moisture content in the US’s dry areas.

5. 6% is the average equilibrium moisture content for interiors of heated buildings.

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