Abstract
A well-insulated, airtight and thermal bridge free building envelope is a key factor for nearly zero energy buildings (nZEB). However, increased insulation thickness and minimized air leakages increase the effect of thermal bridges on overall energy efficiency of the nZEBs. Although several more prominent linear thermal bridges are accounted for in the practice the three-dimensional heat flow through vast array of fixation elements, mounting brackets and other point thermal bridges are usually neglected due to time-consuming model preparation routine, lack of input data as well as high number of different thermal bridges that have to be assessed for a single project. In this study a new method was proposed for predicting three-dimensional heat flow and the point thermal transmittance of thermal bridges caused by full or partial penetration of the building envelope with metal elements with uniform geometry in third dimension based on multiple two-dimensional numerical heat flow calculations. A new parameter (equivalent length of thermal bridge) was defined which incorporates the effect of additional thermal transmittance in third dimension when multiplied by the difference of two thermal coupling coefficients derived for two-dimensional cross section. Multiple linear regression model was fitted on database with 102 cases and verified with separate case of window to wall connection incorporating metal penetration at fixation points. The proposed methodology can be useful in general practice where the design team lacks the skills or software tools for conducting detailed numerical analysis in three dimensions.
Highlights
A well-insulated, airtight and thermal bridge free building envelope is a key factor for nearly zero energy buildings that becomes mandatory from year 2021
Where L3D is the thermal coupling coefficient obtained from a three-dimensional numerical calculation, Ui is the thermal transmittance of the building envelope adjoining the thermal bridge, Ai is the respective area of the adjoining part of the building envelope in the calculation model, ψk is the linear thermal transmittance of additional thermal bridge if present and lk is the respective length of additional linear thermal bridge
In practice a very rough estimation of point thermal transmittance is sometimes calculated as the actual length of the thermal bridge multiplied by the difference (L2D - L2Dref) in thermal coupling coefficients derived from two-dimensional numerical calculations based on respective cross sections
Summary
A well-insulated, airtight and thermal bridge free building envelope is a key factor for nearly zero energy buildings that becomes mandatory from year 2021. Increased insulation thickness and minimized air leakages increase the effect of thermal bridges on overall heat loss nZEB building envelope. The contribution of thermal bridges to overall heat loss is highly dependent on architectural and structural solution but can be as high as 30 – 40% of total heat loss of the building envelope [1,2,3,4,5] and influence of moisture safety [6]. Based on experience with less insulated structures it is usually expected that various point thermal bridges have small contribution to overall heat loss, but several recent studies have shown that within highly insulated building envelope the highly conductive metal fixing elements and cladding systems have considerable effect on effectiveness of thermal insulation layers [7]. The selection of software tools for three-dimensional numerical heat flow modelling is limited due to high price and steep learning curve (ANSYS [9], COMSOL Multiphysics [10], PHYSIBEL Trisco and Solido [11]) compared to numerical heat flow calculation tools in two dimensions where several open source or free to use tools (LBNL Therm [12] etc) are generally available
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