The goods on wood
How best to use this renewable resources
Joseph Lstiburek is a god in the building science world. Google his name and you will find a wealth of technical information on building sustainable homes, as well as straight talk on what works and what doesn’t.
As a former custom homebuilder with a Ph.D. in mechanical engineering and owner of Building Science Corp. (www.buildingscience.com), he knows his stuff, writes prolifically, and is an internationally recognized expert on moisture-related building problems and indoor air quality.
I’ve learned a lot from Lstiburek, who refers to green building as smart building. His Builder’s Guide series, published by the Energy & Environmental Building Association (EEBA) (www.eeba.org), is composed of what are probably the most useful, detailed, and yet easy-to-read guides on designing and building healthy, durable, energy-efficient, and environmentally responsible homes.
Wonders of wood
Lstiburek loves wood products because they’re renewable, inexpensive, and oftentimes locally grown and manufactured. I recently read one of his articles on advanced wood framing, also known as Optimum Value Engineering (OVE).
He calls the United States “the Saudi Arabia of cellulose” (the stuff wood’s made of). Saudi Arabia may have oil, but we have cellulose and dirt! According to Lstiburek, “the future lies in better wood products and better use of them.” This is the essence of OVE—wise and judicious use of wood in homes.
Specifically, OVE includes building techniques aimed at optimizing the use of structural-grade wood in all framing. It provides the added benefit of increasing the whole-wall R-value well beyond what conventional framing permits. But it doesn’t stop there; OVE is a holistic building process that gives you better performance at less cost while minimizing environmental impacts. Here are the essential points:
• Basic house and room dimensions are chosen to use standard-size materials efficiently (e.g. 2-foot increments).
• Engineered wood and certified, sustainably-harvested wood products are used wherever possible.
• All framing members in the floor, walls, and roof are aligned and located on 24-inch centers. Floor stiffness is enhanced with manufactured I-joists or thicker subflooring (glued and screwed).
• Doors and windows are located to coincide with 24-inch stud spacing.
• Headers above doors and windows are “right-sized” according to loads and code span tables. Insulated headers are often used to boost whole-wall R-value.
• Entire house is sheathed with at least 1-inch polystyrene foamboard to mitigate thermal bridging and decrease the potential for vapor condensation in the wall. Shear resistance can be augmented by diagonal metal strapping.
• Air infiltration is minimized by caulking/taping joints and sealing around all penetrations between the conditioned and unconditioned spaces.
Some OVE methods are not as popular because they can add labor to construction processes:
• Two-stud wall corners, which save more wood and can be better insulated but limit nailing surfaces for exterior siding and interior drywall. Drywall clips must be used, but this provides the added benefit of reduced drywall cracking since the corners are floating.
• Single top plates, which also save wood but reduce rough ceiling heights, so that drywall must be trimmed, or nonstandard wall studs must be used. Also, plate joints should be reinforced with metal stitch plates.
OVE can reduce total wood volume by 25 percent or more over conventional framing. The whole-wall R-value of a 2-by-6 OVE wall with 1-inch extruded polystyrene foam is about an R-20, compared to R-14 with conventional 2-by-6 framing. Through this method, fewer trees are cut, homes use less energy, and less waste is sent to landfills. Another benefit is that contractors can shift to these methods with minimal training. Yes, OVE does require better planning and more careful construction, but isn’t that what we all want?