Cross Laminated Timber
Thermal performance and energy efficiency. Thermal conductivity is a measure of the rate of heat flow through one unit of thickness of a material subjected to a temperature gradient. The thermal conductivity of common structural wood is much less than the conductivity of metals with which wood is often mated in construction. It is about two to four times that of common insulating material. For example, the conductivity of structural softwood lumber at 12 percent moisture content is in the range of 0.7 to 1.0 Btu×in/(h×ft^2×oF compared with 1,500 for aluminum, 310 for steel, 6 for concrete, 7 for glass, 5 for plaster, and 0.25 for mineral wool.6
Since CLT is a solid wood panel, it also provides thermal mass. But the key measures of CLT's thermal performance are U-value (coefficient of heat transfer) and R-value (insulating ability). Both are related to panel thickness. Thicker panels have lower U-values; they are better insulators and therefore require less insulation.
The commonly used R-value for wood is 1.25 per inch of thickness. So a 7-inch-thick CLT panel would have an R-value of 8.75. Softwood in general has about one-third the thermal insulating ability of a comparable thickness of fiberglass batt insulation, but about 10 times that of concrete and masonry, and 400 times that of solid steel.7
In addition, since CLT panels can be manufactured using Computer Numeric Control (CNC) equipment to precise tolerances, panel joints fit more tightly, resulting in better energy efficiency for the structure. Because the panels are solid, there is little potential for airflow through the system. As a result, an extremely tight building envelope can be achieved.
| CASE STUDY Earth Sciences Building, University of British Columbia |
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Owner: University of British Columbia Under one roof, the University of British Columbia’s new Earth Sciences Building combines five stories, four science departments, and two distinct structural systems. The South Wing is a conventional reinforced concrete laboratory building. The North Wing, which consists of academic teaching and research spaces, lecture halls and offices, uses wood as the primary structural material, as does the connecting atrium. With its innovative combination of materials, the ESB exemplifies not only the design flexibility inherent to CLT, but its value as a complement to other materials. CLT is featured in the roof and generous exterior canopy structure surrounding the building. The structure is supported by glulam columns and beams, and floors are made from an innovative composite of laminated strand lumber and concrete. One of the largest applications of CLT to date, the North Wing also demonstrated CLT’s advantages in terms of speed of construction. According to the manufacturer, roof panels were placed on the building at a rate of one every 15 minutes. The panels span between glulam post and beam frames, which run across the building width on a 21-foot (6.4-meter) grid. The panels were simply secured to the glulam beams using self-tapping lag screws, which made for fast and reliable installation. The new generation of self-tapping screws commonly used in solid wood construction was originally developed in Europe, but examples are now readily available in North America from a number of suppliers and in a variety of types and sizes. In addition to being technically innovative, the use of mass timber products supported the environmental goals of the University and design team. “The project boasts a low embodied energy profile and sequesters carbon dioxide, thereby mitigating the impact of climate change,” says Jana Foit, ESB’s Project Architect and Higher Education Practice Leader of Perkins+Will’s Vancouver office. |
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