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basics:building_physics_-_basics:thermal_bridges:thermal_bridge_definition [2016/08/16 12:59] – [Additional heat losses] mschuerenbasics: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 =====
-===== Introduction =====+
  
 Heat makes its way from the heated space towards the outside.  In doing so, it follows the path of least resistance. \\ A **thermal bridge** is a localised area of the building envelope where the heat flow is different (usually increased) in comparison with adjacent areas (if there is a difference in temperature between the inside and the outside). Heat makes its way from the heated space towards the outside.  In doing so, it follows the path of least resistance. \\ A **thermal bridge** is a localised area of the building envelope where the heat flow is different (usually increased) in comparison with adjacent areas (if there is a difference in temperature between the inside and the outside).
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     * **Altered**, usually decreased, interior **surface temperatures**; in the worst case this can lead to moisture penetration in building components and mould growth.     * **Altered**, usually decreased, interior **surface temperatures**; in the worst case this can lead to moisture penetration in building components and mould growth.
 +
     * **Altered**, usually increased, **heat losses**.\\     * **Altered**, usually increased, **heat losses**.\\
 \\ \\
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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, two-dimensional and three-dimensional heat flows. 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, two-dimensional and three-dimensional heat flows.
 <WRAP center 60%> <WRAP center 60%>
-<latex>  +\begin{align} 
-$$H_{T} = \underbrace{\sum_{i}A_{i}U_{i}}_{1d}+\underbrace{\sum_{k}l_{k}\varPsi_{k}}_{2d}+\underbrace{\sum_{j}\chi_{j}}_{3d}$$ +&\Large{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} & thermal transmittance of the linear thermal bridge $k$, in W/(m\cdot K) \\ +\chi_{j}\qquad&\text{thermal transmittance of the point thermal bridge $j$, in $W/K$}\\ 
-\chi_{j}  thermal transmittance of the point thermal bridge $j$, in W/K \\ +\end{align}
-\end{tabular} +
-</latex>+
 </WRAP> </WRAP>
  
 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%>
-<latex>  +\begin{align} 
-$$U=\dfrac{1}{R}=\dfrac{1}{R_{si}+\frac{d_{0}}{\lambda_{0}}+\frac{d_{1}}{\lambda_{1}}+\dots+\frac{d_{n}}{\lambda_{n}}+R_{se}} $$ +&\Large{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}\\ +
-</latex>+
 </WRAP> </WRAP>
  
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 ==== Effect on the building structure ==== ==== Effect on the building structure ====
  
-[{{ :picprivate:corner.png?nolink&200|Mould in a corner of the building}}]+[{{ :picprivate:corner.png?nolink&200|Mould in a corner; this is caused by the thermal bridge - we won't allow for such bad design in a passive house, so we won't have that}}]
  
 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, condensation may form on the affected areas.  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, condensation may form on the affected areas. 
  
-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 risk of mould fungus in the inner area of thermal bridges and the resultant toxic effect on occupants must be considered separately, especially since mould growth already occurs at a temperature higher than the dewpoint temperature without condensation being present. For a building physical analysis of the model, formation of mould can be assumed if relative surface humidity levels of 80 % prevail for a duration of 12 h/d   (Technical Report 4108-8).+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 risk of mould fungus to the inner surface of thermal bridges and the resultant toxic effect on occupants must be considered separately, especially since mould growth already occurs at a temperature higher than the dewpoint temperature without condensation being present. For a building physical analysis of the model, formation of mould can be assumed if relative surface humidity levels of 80 % prevail for a duration of 12 h/d   (Technical Report 4108-8)
 + 
 +In constructions suitable for passive house or EnerPHit-design, thermal bridges with such catastrophic consequences are generally avoided; we won't allow for such bad design in the recommended constructions, so we won't have that. While there can still be thermal bridges with some remaining heat losses((which have to be accounted for)), such massive temperature drops can always be avoided. A big part in avoiding catastrophic thermal bridges is the improved insulation level already for the regular components. This leads to a generally higher level of indoor surface temperatures to begin with, already reducing the risk. Internal insulation, however, is a special case in which there are even more stringent rules "how to avoid thermal bridges of the catastrophic type".
  
  
 ====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:
-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%>
-<latex> +$$
 f_{Rsi,min}=\dfrac{12.6^{\circ} C -(-5^{\circ} C)}{20^{\circ} C - (-5^{\circ} C)}=0.7 f_{Rsi,min}=\dfrac{12.6^{\circ} C -(-5^{\circ} C)}{20^{\circ} C - (-5^{\circ} C)}=0.7
-</latex>+$$
 </WRAP> </WRAP>
  
basics/building_physics_-_basics/thermal_bridges/thermal_bridge_definition.1471345197.txt.gz · Last modified: 2016/08/16 12:59 by mschueren