Basics and Some Theory of AnTherm
Thermal Transmission and Thermal Bridges
Energy in the form of heat in building constructions is generally transmitted
by a combination of conduction, convection, and radiation. When considering heat
transfer within and through solid (homogeneous or non-homogeneous) building
materials, heat conduction is the primary transmission factor; the effects of
convection and radiation are typically negligible.
heat conduction |
Conduction through homogeneous planar building
components (i.e. composed of one or more layers of material with parallel
surface planes) occurs in a single direction: normal to the component
surface. This is referred to as one-dimensional heat flow, and is
characterised by a constant surface temperature over the entire surface
plane.
Such idealised conditions can, of course, only be assumed in limited
regions of an actual building structure. Geometries of non-planar components
(construction joints, floor-wall connections, balconies, etc.) give rise to
heat flow patterns of more than one direction, that is, to two- or
three-dimensional heat flow.
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thermal bridge
cold bridge
heat bridge |
Regions or elements of a building construction
characterised by multi-dimensional heat flow patterns are called thermal
bridges. In contrast to regions of one-dimensional heat flow, thermal
bridges are typically associated with local peaks of heat loss, which
correspond to characteristic drops in the interior surface temperature.
Thermal bridges, which in general occur at any junction between building
components or where the building structure changes composition, have two
consequences:
- a change in heat flow rate
and
- a change in internal surface temperature
compared with those of the unbridged structure.
Although similar calculation procedures are used, the procedures are not
identical for the calculation of heat flows and of surface temperatures.
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thermal bridge
(defined in ISO 10211) |
Part of the building envelope where the otherwise uniform
thermal resistance is significantly changed by:
- full or partial penetration of the building envelope by materials with a
different thermal conductivity
and/or
- a change in thickness of the fabric
and/or
- a difference between internal and external areas, such as occur at
wall/floor/ceiling junctions
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linear thermal bridge
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Thermal bridge with a uniform cross-section along one of
the three orthogonal axes
(as defined by ISO 10211)
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point thermal bridge
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Localized thermal bridge whose influence can be represented by a
point thermal transmittance (as defined by ISO 10211)
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See also: Thermal Heat Conductance,
Methods of thermal heat bridge survey in
international perspective
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