Roxul Industrial Insulation Process - Manual (Page 112)

2.2.2 Thermal conductivity
The material property defining heat flow through
an insulation material is thermal conductivity, “λ”
(or “k“). It indicates the heat flow rate “Q” through
unit area of material “A” induced by unit
temperature gradient “∆T / L” in a direction
perpendicular to that unit area (Heat-Flux per unit
temperature difference “∆T” across a unit
thickness “L” of material).
The unit of thermal conductivity is
BTU.in/hr⋅ft
2 ⋅°F (W/m⋅K). The thermal conductivity
depends on the temperature, apparent density,
fiber material, fiber dimension and fiber structure
and orientation within the insulation and is made
up of the following parts:
Thermal conduction of the dormant air in
spaces between the fibers
Thermal radiation
Thermal conduction through the fibers
Convection
The dependency of these heat transport
mechanisms on apparent density and temperature
are shown in the graphs below. The individual
transport mechanisms cannot be measured
separately using existing measurement
techniques, but can be measured together to
allow thermal conductivity to be determined.
λ =
Expressed on an inch-basis this is:
=
Heat-Flux
Unit-Temperature-Gradient
Q / A
(ΔT ⁄ L)
= = BTU / hr ∙ ft ∙ °F
(BTU/hr) ⁄ ft 2
(F ⁄ ft)
= BTU ∙ in / hr ∙ ft 2∙ °F
(BTU/hr) ⁄ ft 2
(F ⁄ in)
109