EWPAA Structural Plywood and LVL Design Manual - Engineered ...

15.4 Thermal TransmissionThermal transmission, or more specifically for this section, heat flow through building materials is of primeimportance in this day and age where efficient energy usage is so important. Therefore, it is imperativearchitects, engineers and building designers are at least conversant with the topic.Terms and DefinitionsUnit thermal conductivity (k), a fundamental heat transmission property, is a measure of the rate of heatflow through unit area of a material of unit thickness subjected to a unit temperature gradient.Thermal conductivity of wood is affected by:• density – increases with increasing density;• moisture content – increases with increasing moisture content;(density and moisture content have the greatest influence)• extractive content – increases with increasing extractive content;• grain direction – about the same in the radial and tangential directions but can be about twice thisalong the grain;• natural characteristics – increases with the amount of knots, checks, etc;• temperature – increases marginally with temperatureThe unit of thermal conductivity (k) is the Watt/m°C where:1 watt = 1 Joule/second or 1 Newton metre/secondUnit thermal conductivity (k) of softwood timbers at 12% moisture content is in the range 0.11 to 0.18W/m°C compared with 216 for aluminium, 45 for steel and 0.9 for concrete.Thermal resistivity (R) is the reciprocal of unit thermal conductivity, i.e.1 R = m C/W(15.2)kThermal conductivity and thermal resistivity refer to thermal properties of homogeneousmaterials of uniform composition and specifically relate to a thickness of 1m of thematerial.Thermal resistance (r) refers to the individual resistances of the barriers encounteredduring the transmission from one side to the other of the system of barriers.The thermal resistance of an individual barrier, i.e. plywood, etc. is obtained thus:where:rikT iimaterial thickness (metres)=unit thermal conductivityTi=ki= thickness of the barrier (m)= unit (1m thick) thermal conductivity (W/m°C)TABLE 15.1 gives a range of thermal resistances for various thicknesses of softwoodplywood having an average density of 550kg/m 3 .233