basics:building_physics_-_basics:thermal_bridges:thermal_bridge_definition
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basics:building_physics_-_basics:thermal_bridges:thermal_bridge_definition [2016/08/02 13:54] – [See also] mschueren | basics:building_physics_-_basics:thermal_bridges:thermal_bridge_definition [2022/07/30 14:50] (current) – [Effect on the building structure] wfeist | ||
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===== Definition and effects of thermal bridges ===== | ===== Definition and effects of thermal bridges ===== | ||
- | ===== Thermal bridges | + | ===== Thermal bridges |
- | ===== Introduction ===== | + | |
Heat makes its way from the heated space towards the outside. | Heat makes its way from the heated space towards the outside. | ||
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* **Altered**, | * **Altered**, | ||
+ | |||
* **Altered**, | * **Altered**, | ||
\\ | \\ | ||
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A general overview is possible if the procedure for determining the transmission heat losses $H_T$ of the building envelope is considered. The following equation in the norm DIN 14683 (Section 4.2) makes a distinction between one-dimensional, | A general overview is possible if the procedure for determining the transmission heat losses $H_T$ of the building envelope is considered. The following equation in the norm DIN 14683 (Section 4.2) makes a distinction between one-dimensional, | ||
<WRAP center 60%> | <WRAP center 60%> | ||
- | < | + | \begin{align} |
- | $$H_{T} = \underbrace{\sum_{i}A_{i}U_{i}}_{1d}+\underbrace{\sum_{k}l_{k}\varPsi_{k}}_{2d}+\underbrace{\sum_{j}\chi_{j}}_{3d}$$ | + | & |
- | \begin{tabular}{ll} | + | With\qquad&\\ |
- | where& \\ | + | A_{i}\qquad&\text{area of the building components, in $m^2$}\\\\ |
- | $A_{i}$ & area of the building components, in m^2\\ | + | U_{i}\qquad&\text{thermal transmittance of component $i$ of the building envelope, in $W/(m^2\cdot K)$}\\\\ |
- | $U_{i}$ & thermal transmittance of component $i$ of the building envelope, in W/(m^2\cdot K) \\ | + | l_{k}\qquad&\text{length of the linear thermal bridge $k$, in $m$}\\\\ |
- | $ l_{k} $ & length of the linear thermal bridge $k$, in m \\ | + | \varPsi_{k}\qquad&\text{thermal transmittance of the linear thermal bridge $k$, in $W/(m\cdot K)$}\\\\ |
- | $ \varPsi_{k} | + | \chi_{j}\qquad&\text{thermal transmittance of the point thermal bridge $j$, in $W/K$}\\ |
- | $ \chi_{j} | + | \end{align} |
- | \end{tabular} | + | |
- | </ | + | |
</ | </ | ||
Planar regular building components such as the roof areas and exterior walls have the largest share of the total heat flow. For these, heat transfer can be considered one-dimensional with good approximation. The reason for this is that no cross-flows occur in them on account of their homogeneous layered structure. The heat transfer coefficient is defined in the norm [DIN6946] and can be calculated with little effort using the familiar equation given below: | Planar regular building components such as the roof areas and exterior walls have the largest share of the total heat flow. For these, heat transfer can be considered one-dimensional with good approximation. The reason for this is that no cross-flows occur in them on account of their homogeneous layered structure. The heat transfer coefficient is defined in the norm [DIN6946] and can be calculated with little effort using the familiar equation given below: | ||
<WRAP center 60%> | <WRAP center 60%> | ||
- | < | + | \begin{align} |
- | $$U=\dfrac{1}{R}=\dfrac{1}{R_{si}+\frac{d_{0}}{\lambda_{0}}+\frac{d_{1}}{\lambda_{1}}+\dots+\frac{d_{n}}{\lambda_{n}}+R_{se}} | + | & |
- | + | With\qquad&\\ | |
- | \begin{tabular}{ll} | + | R_{si}\qquad&\text{inner heat transfer resistance , in $m^2 \cdot K/W$}\\\\ |
- | where& \\ | + | d_{n}\qquad&\text{thickness of the $n$-th component layer, in $m$}\\\\ |
- | $R_{si}$ & inner heat transfer resistance , in m^2 \cdot K/W \\ | + | \lambda_{n}\qquad&\text{rated value of the thermal conductivity of the $n$-th layer, in $W/(m\cdot K)$}\\\\ |
- | $d_{n}$ & thickness of the $n$-th component layer, in m\\ | + | R_{se}\qquad&\text{outer heat transfer resistance, in $m^2 \cdot K/W$}\\ |
- | $\lambda_{n}$ & rated value of the thermal conductivity of the $n$-th layer, in W/(m\cdot K) \\ | + | \end{align} |
- | $R_{se}$ & outer heat transfer resistance, in m^2 \cdot K/W \\ | + | |
- | \end{tabular}\\ | + | |
- | </ | + | |
</ | </ | ||
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In principle, the actual share of the thermal bridges in the transmission losses of the building envelope can only be stated if the Ψ-values are calculated for a specific building. It is assumed that heat flow simulations are associated with an uncertainty of ca. 5 %, other methods such as the use of thermal bridge catalogues are even associated with an uncertainty of up to 20 % (DIN EN ISO 14683, Section 5.1). **For Passive House buildings, the use of thermal bridge additions is not advised because they lead to overestimation of the heat losses.** | In principle, the actual share of the thermal bridges in the transmission losses of the building envelope can only be stated if the Ψ-values are calculated for a specific building. It is assumed that heat flow simulations are associated with an uncertainty of ca. 5 %, other methods such as the use of thermal bridge catalogues are even associated with an uncertainty of up to 20 % (DIN EN ISO 14683, Section 5.1). **For Passive House buildings, the use of thermal bridge additions is not advised because they lead to overestimation of the heat losses.** | ||
- | However, in general it is not possible to state how high the heat losses due to thermal bridges actually are. Their type and number are too individual and therefore depend on the respective building. For example, thermal bridges do not always have to have a negative effect on energy balancing; in the case of efficient new constructions, | + | However, in general it is not possible to state how high the heat losses due to thermal bridges actually are. Their type and number are too individual and therefore depend on the respective building. For example, thermal bridges do not always have to have a negative effect on energy balancing; in the case of efficient new constructions, |
==== Effect on the building structure ==== | ==== Effect on the building structure ==== | ||
- | [{{ : | + | [{{ : |
Unlike with regular building components, at thermal bridges the heat flow density changes and usually results in a reduction in the surface temperature on the inside in that area. This effect is more pronounced because air circulation in corners and edges is restricted. Cupboards and other furniture not only disrupt convection but also restrict radiant exchange with the surroundings. Because the water vapour content of the air depends on its temperature, | Unlike with regular building components, at thermal bridges the heat flow density changes and usually results in a reduction in the surface temperature on the inside in that area. This effect is more pronounced because air circulation in corners and edges is restricted. Cupboards and other furniture not only disrupt convection but also restrict radiant exchange with the surroundings. Because the water vapour content of the air depends on its temperature, | ||
- | The resulting condensation can penetrate further inside the construction due to the capillary action of the building materials, and the thermal conductivity may increase and thus the building component may almost be saturated. It will not be possible to avoid moisture damage to the building structure and mould growth may occur. However, large-scale damage is generally associated with errors in the planning, implementation and utilisation of buildings and is not a problem that is solely related to thermal bridges. These are only the points where the problems originate in the first place. Nonetheless, | + | The resulting condensation can penetrate further inside the construction due to the capillary action of the building materials, and the thermal conductivity may increase and thus the building component may almost be saturated. It will not be possible to avoid moisture damage to the building structure and mould growth may occur. However, large-scale damage is generally associated with errors in the planning, implementation and utilisation of buildings and is not a problem that is solely related to thermal bridges. These are only the points where the problems originate in the first place. Nonetheless, |
+ | |||
+ | In constructions suitable for passive house or EnerPHit-design, | ||
====Requirements==== | ====Requirements==== | ||
- | Requirements | + | Requirements |
- | The current rules for engineering practice (DIN 4108-2) rule out the risk of mould near thermal bridges if the minimum surface temperatures under the mentioned steady-state boundary conditions do not fall below 12.6 °C. This corresponds with a $f_{Rsi}$ factor of 0.7: | + | |
<WRAP center 60%> | <WRAP center 60%> | ||
- | < | + | $$ |
f_{Rsi, | f_{Rsi, | ||
- | </ | + | $$ |
</ | </ | ||
- | The higher the $f_{Rsi}$ factor is, the less the likelihood of mould growth will be. [[certification: | + | The higher the $f_{Rsi}$ factor is, the less the likelihood of mould growth will be. |
- | + | For [[certification: | |
- | <WRAP center 60%> | + | |
- | < | + | |
- | f_{Rsi, | + | |
- | </ | + | |
- | </ | + | |
+ | * [[http:// | ||
===== See also===== | ===== See also===== | ||
- | * [[basics: | + | * [[basics: |
+ | |||
+ | * [[basics: | ||
+ | |||
- | * [[basics: |
basics/building_physics_-_basics/thermal_bridges/thermal_bridge_definition.txt · Last modified: 2022/07/30 14:50 by wfeist