basics:building_physics_-_basics:heat_transfer:thermal_bridges
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basics:building_physics_-_basics:heat_transfer:thermal_bridges [2016/07/12 08:55] – [Introduction] sahmed | basics:building_physics_-_basics:heat_transfer:thermal_bridges [2016/07/18 14:23] (current) – removed sahmed | ||
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- | ===== Thermal bridges ===== | ||
- | ===== Definition and effects of thermal bridges ===== | ||
- | ===== Introduction ===== | ||
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- | Heat makes its way from the heated space towards the outside. | ||
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- | The effects of thermal bridges are: | ||
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- | * **Altered**, | ||
- | * **Altered**, | ||
- | \\ | ||
- | Both effects of thermal bridges can be avoided in Passive Houses: the interior surface temperatures are then so high everywhere that critical levels of moisture cannot occur any longer – and the additional heat losses become insignificant. | ||
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- | <WRAP center round box 60%> | ||
- | If the criteria for [[basics: | ||
- | [[basics: | ||
- | </ | ||
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- | ===== Normative definition of thermal bridges ===== | ||
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- | In [DIN10211] (Thermal bridges in building construction – Heat flows and surface temperatures - Detailed calculations) there are numerical procedures relating to the calculation of thermal bridges. Here, a thermal bridge is defined as follows (Section 3.1.1): | ||
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- | <WRAP center round box 60%> | ||
- | //Compared to thermal bridge free building components, there are two effects of thermal bridges which occur at each connection point between building components or at places where the composition of the building structure changes:// | ||
- | * //altered heat flow// | ||
- | * //a change in the interior surface temperature// | ||
- | </ | ||
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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, | ||
- | <WRAP center 60%> | ||
- | < | ||
- | $$H_{T} = \underbrace{\sum_{i}A_{i}U_{i}}_{1d}+\underbrace{\sum_{k}l_{k}\varPsi_{k}}_{2d}+\underbrace{\sum_{j}\chi_{j}}_{3d}$$ | ||
- | \begin{tabular}{ll} | ||
- | where& \\ | ||
- | $A_{i}$ & area of the building components, in m^2\\ | ||
- | $U_{i}$ & thermal transmittance of component $i$ of the building envelope, in W/(m^2\cdot K) \\ | ||
- | $ l_{k} $ & length of the linear thermal bridge $k$, in m \\ | ||
- | $ \varPsi_{k} $ & thermal transmittance of the linear thermal bridge $k$, in W/(m\cdot K) \\ | ||
- | $ \chi_{j} $ & thermal transmittance of the point thermal bridge $j$, in W/K \\ | ||
- | \end{tabular} | ||
- | </ | ||
- | </ | ||
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- | 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%> | ||
- | < | ||
- | $$U=\dfrac{1}{R}=\dfrac{1}{R_{si}+\frac{d_{0}}{\lambda_{0}}+\frac{d_{1}}{\lambda_{1}}+\dots+\frac{d_{n}}{\lambda_{n}}+R_{se}} $$ | ||
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- | \begin{tabular}{ll} | ||
- | where& \\ | ||
- | $R_{si}$ & inner heat transfer resistance , in m^2 \cdot K/W \\ | ||
- | $d_{n}$ & thickness of the $n$-th component layer, in m\\ | ||
- | $\lambda_{n}$ & rated value of the thermal conductivity of the $n$-th layer, in W/(m\cdot K) \\ | ||
- | $R_{se}$ & outer heat transfer resistance, in m^2 \cdot K/W \\ | ||
- | \end{tabular}\\ | ||
- | </ | ||
- | </ | ||
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- | The two-dimensional and three-dimensional heat flow proportion of the building envelope is expressed by thermal bridges. They are defined by geometric, constructive and/or material modification and usually exhibit a higher heat flow rate and lower surface temperatures than adjacent standard building components. They occur particularly at the component joints, edges, transitions and penetrations of the standard building components. They are depicted by the linear thermal transmittance $\varPsi$ with the unit W/(mK) and the point thermal transmittance $\chi$ in W/K. | ||
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- | [{{ : | ||
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- | ===== Efects ===== | ||
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- | ====Additional heat losses ==== | ||
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- | Die Auswirkungen von Wärmebrücken auf die Energiebilanz, | ||
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- | - pauschal über Wärmebrückenzuschlag $ \Delta U_{bw} = 0,10 \quad W/(m^2\cdot K)$ (EnEV) | ||
- | - reduzierter Wärmebrückenzuschlag $ \Delta U_{bw} = 0,05 \quad W/(m^2\cdot K)$ (DIN 4108 Beiblatt 2) | ||
- | - Ψ-Wert aus Wärmebrückenkatalogen z.B. (DIN EN ISO 14683) | ||
- | - Ψ -Werte aus Berechnung aus (DIN EN ISO 10211) | ||
- | - keine Berücksichtigung im Falle der [[grundlagen: | ||
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- | Der tatsächliche Anteil der Wärmebrücken an den Transmissionswärmeverlusten der Gebäudehülle kann im Grunde nur angegeben werden, wenn die Ψ -Werte für ein konkretes Gebäude berechnet werden. Es wird davon ausgegangen, | ||
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- | Eine allgemein gültige Angabe, wie groß die tatsächlichen Wärmeverluste durch Wärmebrücken sind, ist allerdings nicht möglich. Dazu sind sie in ihrer Art und Anzahl zu individuell, | ||
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- | ====Auswirkung auf die Baukonstruktion ==== | ||
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- | [{{ : | ||
- | Im Unterschied zu ebenen Bauteilen, kommt es an Wärmebrücken zu einer Änderung der Wärmestromdichte und damit meist zu einer lokalen Senkung der raumseitigen Oberflächentemperatur. Dieser Effekt wird begünstigt, | ||
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- | Das anfallende Tauwasser kann durch Kapillarwirkung der Baustoffe weiter in die Konstruktion eindringen, die Wärmeleitfähigkeit weiter erhöhen und damit durch weitere Auffeuchtung das Bauteil regelrecht durchnässen. Feuchteschäden an der Baukonstruktion und Schimmelpilzwachstum sind anschließend nicht mehr zu verhindern. Große Schäden gehen jedoch einher, mit generellen Fehlern bei der Planung, Ausführung und Nutzung von Gebäuden und sind kein reines Problem von Wärmebrücken. Sie sind jedoch Keimzellen, an denen es zuerst zu Problemen kommt. Das Risiko von Schimmelpilz im Innenbereich von Wärmbrücken und den damit möglichen toxischen Wirkungen auf den Menschen, muss nichtsdestotrotz gesondert betrachtet werden. Vor allem da Schimmelpilzwachstum bereits oberhalb der Taupunkttemperatur, | ||
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- | ====Anforderungen ==== | ||
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- | Anforderungen | ||
- | Die aktuellen Regeln der Technik (DIN 4108-2), schließen das Risiko von Schimmelpilz im Bereich von Wärmebrücken aus, wenn die minimalen Oberflächentemperaturen unter den vorgestellten stationären Randbedingungen nicht mehr als 12,6 °C betragen. Dies entspricht einem $f_{Rsi}$-Faktor von 0,7 : | ||
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- | <WRAP center 60%> | ||
- | < | ||
- | f_{Rsi, | ||
- | </ | ||
- | </ | ||
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- | Je höher der $f_{Rsi}$-Faktor ist, desto geringer ist die Wahrscheinlichkeit von Schimmelbefall. Bei [[zertifizierung: | ||
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- | <WRAP center 60%> | ||
- | < | ||
- | f_{Rsi, | ||
- | </ | ||
- | </ | ||
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- | ===== Siehe auch ===== | ||
- | * [[grundlagen: | ||
- | * [[grundlagen: | ||
- | * [[basics: | ||
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- | \\ | ||
- | |{{ : | ||
- | |//**This illustration shows a completely thermal bridge free building envelope, | ||
- | \\ as implemented in the " | ||
- | \\ Kronsberg by the architects Grenz and Rasch of Büro Faktor 10.\\ | ||
- | __Literature: | ||
- | \\ Development in Hannover Kronsberg", | ||
- | \\ charge from [[http:// | ||
- | (Literature -> Brief reports and technical literature about the Passive | ||
- | \\ House -> Final Reports: Climate-neutral Passive House Development | ||
- | \\ in Hannover-Kronsberg)**// | ||
- | \\ | ||
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basics/building_physics_-_basics/heat_transfer/thermal_bridges.1468306507.txt.gz · Last modified: 2016/07/12 08:55 by sahmed