Roxul Industrial Insulation Process - Manual (Page 124)

across a unit thickness of material).
The unit of thermal conductivity is BTU.in/hr∙ft
2 ∙ °F
(W/m∙K).
λ =
(Q ⁄ t)
A·(∆T ⁄ L)
Apparent Thermal Conductivity
A thermal conductivity assigned to a material that
exhibits thermal transmission by several modes of
heat transfer resulting in property variation with
specimen thickness, or surface emittance. Thermal
conductivity and resistivity are normally considered
intrinsic or specific properties of materials and, as
such, should be independent of thickness. When
nonconductive modes of heat transfer are present
within the specimen (radiation, free convection) this
may not be the case. To indicate the possible
presence of this phenomena the modifier “apparent”
is used, as in apparent thermal conductivity.
Conductance, C
The conductive heat transfer coefficient, C, (thermal
conductance) is the thermal conductivity of a system
or a particular thickness under consideration; it is not
a material property. Thermal conductance is the heat
flow rate per unit area per unit temperature
difference across a system or piece of material - the
thickness to which the conductance is assigned must
be defined. The unit of thermal conductance is
BTU/hr∙ft
2 ∙ °F (W/m 2 K)
C = =
Thermal Conductivity
Applied Insulation Thickness
λ
L
Note: Within the ASTM standards and ASHRAE
handbooks the term “Conductance” is used both in
the strict sense (as defined above in relation to heat
transfer by conduction) and in a more general loose
sense to refer to the heat transfer coefficient by any,
or all, of conduction, convection and radiation (in
isolation or combination).
Resistivity
The quantity determined by the temperature
difference, at steady state, between two defined
parallel surfaces of a homogeneous material of unit
thickness that induces a unit heat flow rate through a
unit area. Thermal resistivity is the reciprocal of
thermal conductivity. Where a material exhibits
thermal transmission by several modes of heat
transfer the term “apparent thermal resistivity” is
applied. The units of thermal resistivity are
hr.ft.°F/BTU or hr.ft
2 .°F/BTU-in (mK/W).
Thermal Diffusivity
The ratio of thermal conductivity of a substance to the
product of its density and specific heat. The unit of
thermal diffusivity is:
= (m 2 /s) =
(BTU ⁄ (hr·ft·F) ft 2
(lb ⁄ ft 2 )·(BTU ⁄ lb·F) hr
Heat Transfer Coefficient, u
The proportionality coefficient defining general heat
flow in unit time through unit area induced by unit
temperature difference between the environments on
each side. Heat transfer coefficients can be defined
for heat transfer taking place by the mechanisms of
conduction, convection or radiation (individually or
combined).
Note: with the ASTM standards and ASHRAE
handbooks the term “conductance”, as defined in the
strict sense, applies to the conductive heat transfer
coefficient, C, but the term “conductance” is also
loosely applied to the heat transfer coefficient of any
layer with heat transfer by any, or all, of conduction,
convection and radiation. When used in this general
sense the symbol u
n
is used, where the subscript, n,
references the particular layer (or layers) under
consideration.
Overall Coefficient of heat transfer
(U-Factor or Thermal Transmittance), U
The proportionality coefficient defining heat
transmission in unit time through unit area of a
material or construction and the boundary films,
induced by unit temperature difference between the
environments on each side. Units are
BTU/hr.ft
2 .°F (W/m 2 K).
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