The Material Ecology of a House: Wood Framing

Material ecology of a house focuses on the interrelationships between the materials utilized in construction and the processes, labor, and environmental effects. This blog post addresses the exterior wall systems of a house for various types of construction with a specific focus on the wood frame construction type. Exterior wall systems are designed to shelter the interior spaces from heat, cold, wind, moisture, and noise while providing a sense of enclosure, privacy, and security for the occupants. The exterior walls also serve to carry the loads and spans of the roof as well as any additional floors. The most typical wall systems for houses in the United States are comprised of wood framing with insulation, exterior sheathing and siding, and interior gypsum board. But there are a handful of other wall systems that are also employed for houses in the US – including concrete masonry units (CMU), insulated concrete forms (ICF), rammed earth, straw bale, and other emerging forms such as 3D-printed concrete, which will be described in future blog posts.  

Wood frame construction is the most prevalent for housing in the United States. Wood framing is affordable, lightweight, transportable to various locations, and is a renewable resource. Sustainable forestry practices are important for providing wood to the construction industry while maintaining the health of forests over time. These practices involve considerations for the proper thinning of trees and brush that will clear space for older growth development and reduce potential rapid fire spread in the case of forest fire. The managed thinning of forests provides a large portion of lumber for home and building construction. Most common lumber framing products are sourced from hardwood species such as Cedar, Douglas Fir, Redwood, Oak, Pine, and Poplar. Softwood species are utilized for furniture and other finish wood products.

Wood frame lumber products have dimensional standards in the US, with both a nominal and actual dimension. For example, a common 2x4 member, which comes in varying lengths that can be cut to size, has the cross-section nominal dimensions of 2-inches depth by 4-inches in width – while the actual dimensions of a 2x4 are 1-1/2”x3-1/2”. This is a result of the processing of wood into lumber, which includes a time of shrinkage after the initial cutting of logs (since live trees retain a lot of moisture in their trunks) as well as the planing or finishing of the member, which smooths out the surfaces. While the original lumber cutting produces a 2x4, the actual dimension of the dried and planed member becomes about a half inch smaller in each its cross-direction due to the drying, shrinkage, and finishing that occurs. The length of lumber is fixed since the members are cut in length after drying and planing.

It is also important to acknowledge that wood is anisotropic, meaning it has different structural properties in its cross-section vs its length. The rings of the tree are a result of layers and years of growth developing from the center outwards. The height of the tree is also a result of the time of growth but follows the linear path of its fibril bundles’ longitudinal growth. Thus, the shrinkage of lumber when drying primarily occurs in the cross-section and is less significant along the length. When architects prepare design drawings for wood frame construction, the actual (not the nominal) dimensions are utilized for accuracy in plans, sections, and details.  

Homes that utilize wood framing will incorporate different types of siding and exterior finish materials. When you see a house that has a shingle siding as well as a house that has a masonry siding, or sometimes a combination of both, it is often clad over wood sheathing that is attached to the wood frame members of the exterior wall structure. There will also be a vapor barrier layer on the sheathing to prevent moisture transport through the wall system. Various forms of insulation materials are placed within the spaces between the wood framing members. Often times this will be batt insulation, a type of blanket with a radiant film (metallic paper) on its surface. Batt or blanket insulation comes in different thicknesses to provide the appropriate amount of thermal barrier for a given climate location. Super-cold and super-hot climate locations require thicker walls with wider wood framing members to incorporate a thicker insulation barrier. While this can increase the up-front costs of construction, it will reduce energy bills for heating or cooling throughout each year.

The interior finish layer of wood frame construction is most commonly a gypsum board that is screwed to the framing. Wood framing is also used for interior wall construction in combination with gypsum board and then paint, tile, or other finish material methods. Wood framing is favored by the construction industry because it is accommodating for the various trades that need to incorporate their components and systems – such as plumbing, HVAC, and electrical routing. Wood frame wall construction does not require any additional formwork or secondary material system during construction since it is self-supporting as it is installed (though sometimes temporary bracing members will be used). Wood frame construction is also flexible in the way it can be designed for heavier loads, when necessary, which can be accomplished by decreasing the spacing between studs (vertical members) or doubling up members to create heftier beams (horizontal members).

Wood framing has been favored as the primary construction method in regions that are earthquake prone, such as California. Wood framing is resilient and flexible and is designed with specific shear and torsion resistant brackets for the connections between wood members in seismic zones. While wood framing is preferred for its flexibility in responding to the dynamic forces of an earthquake, it is not as resistant to fires. Forest fire regions, which are also prevalent in California, are beginning to call for new construction standards of the exterior wall systems for single-family houses to prevent complete loss of structures during fires. Preferred wall systems of residential buildings in fire susceptive regions are made with masonry construction types, such as the insulated concrete form (ICF) blocks.

There can be a few other drawbacks with wood frame wall construction in different locations regionally. One drawback with wood frame construction can be the susceptibility to termite infestation. Termites thrive on the wood as a nutrient source and can degrade and break down the integrity of wood members over time. If you live in an area with known termites and begin to see sawdust piles or trails around the exterior walls of your house or at the windowsills, it is best to have professional termite inspectors assess the situation to prevent further damage.

In areas where rainfall and humidity conditions are high or variable, the wood members can subtly swell and shrink with respective wetting and drying cycles from humidity sorption. This can cause slight shifts in the parts of the house that connect to the walls – such as window and door frames, which may become tighter and more difficult to open and close during wet seasons but loose and with possible gaps in connection points during dry seasons. When architects and contractors design and build with wood frame construction, the swelling and shrinking of wood members is accounted for in the dimensional planning and layouts.

Wood members can also have some slight variations in plumbness, or how straight they are, causing slight variances in how flush or level certain aspects of walls and connections with floors and ceilings might be. Most wood frame members are planed smooth and plumb during the finishing process before the dimensional lumber is put on the market – but it is wise to have a contractor self-select the framing members from the point of sale if at all possible. When bulk framing members are supplied to contractors, many pieces may be deemed unusable, resulting both in wasted material and money.

Overall, however, the wood frame wall system is healthy both for the labor force working with the materials and the occupants of the finished home. Wood is also a renewable and biogenic resource, making it one of the more sustainable building materials available. The transport of wood framing holds one of the largest responsibilities for the carbon footprint related to this material. Transport factors include the mode of transport (i.e. diesel truck, train, cargo ship) as well as the distance from forestry sourcing to sawmill plants to construction sites. In some scenarios, wood framing may be higher impact on carbon footprint than a handful of other options. It is best to assess the impact factors on a case-by-case basis for project location, size, and best design options for the climate.

Future blogs about the material ecology of a house will discuss the other types of exterior wall systems. If you are planning for any new home construction or renovation and are interested in a healthy material ecology, reach out to AIDA, LLC today for a consultation. You can always find more information and healthy home resources at Aletheia Ida Design and Architecture, LLC (AIDA, LLC) at www.aletheiaida.com.