Engineering and Construction

A wood light frame building can be designed to mini- mize waste in several ways. It can be dimensioned to utilize full sheets and lengths of wood products. Most small  buildings can be framed with studs 24 inches (610 mm) o.c.  rather than 16 inches (406 mm). A stud can be eliminated  at each corner by using small, inexpensive metal clips to support the interior wall finish materials. If joists and rafters are aligned directly over studs, the top plate can be a  single member rather than a double one. Floor joists can  be spliced at points of inflection rather than over girders;  this reduces bending moments and allows use of smaller  joists. Roof trusses often use less wood than conventional  rafters and ceiling joists.
Laminated strand lumber and rim joists, wood I-joists,  laminated veneer lumber beams and headers, glue-laminated girders, parallel strand lumber girders, and OSB sheathing are all materials that utilize trees more efficiently  than solid lumber. Finger-jointed studs made up of short  lengths of scrap lumber glued together may replace solid  full-length wood studs.

Framing carpenters can waste less lumber by saving  cutoffs and reusing them rather than throwing them  automatically on the scrap heap. In some localities,  scrap lumber can be recycled by shredding it for use  in OSB production. The burning of construction scrap should be discouraged because of the air pollution it  generates.

Although the thermal efficiency of wood light frame  construction is inherently high, it can be improved  substantially by various means, as shown in Figures 7.17-7.21. Wood framing is much less conductive of heat than  light-gauge steel framing. Steel framing of exterior walls is  not a satisfactory substitute for wood framing unless the heat  flow path through the steel framing members can be broken  with a substantial thickness of insulating foam.

Figure 7.17 Insulation levels in walls of light frame buildings can be increased from the R13 to R-15 (RSI-90 to RSI-104) of a 2 X 4 stud wall to R19 (RSI-132) or more by using either 2 X 6 framing and thicker batt insulation (left) or 2 X 4 framing with plastic foam sheathing in combination with batt insulation (right). The foam sheathing insulates the wood framing members as well as the cavities between them but can complicate the process of installing some types of siding.

Figure 7.18 Window and door headers in 2 X 6 framing require special detailing. Two alternative header details are shown here in section view: (a) The header members are installed fl  ush with the interior and exterior surfaces of the studs, with an insulated space between. This detail is thermally effi cient but may not provide suffi cient nailing for interior fi  nish materials around the window. (b) A 3 X 2 spacer provides full nailing around the opening.

Figure 7.19 Two alternative corner post details for 2 6 stud walls, shown in plan view: (a) Each wall frame ends with a full 2 X 6 stud, and a 2 X 4 nailer is added to accept fasteners from the interior wall fi nish. (b) For maximum thermal effi ciency, one wall frame ends with a 2 X 4 stud fl ush with the interior surface, which eliminates any thermal bridging through studs.

Figure 7.20 With the two framing methods shown here, walls can be insulated to any desired level of thermal resistance.

Figure 7.21 A raised-heel roof truss provides plenty of space for attic insulation at the eave.