Abstract
An accurate evaluation of the thermal transmittance ( U -value) of building envelope elements is fundamental for a reliable assessment of their thermal behaviour and energy efficiency. Simplified analytical methods to estimate the U -value of building elements could be very useful to designers. However, the analytical methods applied to lightweight steel framed (LSF) elements have some specific features, being more challenging to use and to obtain a reliable accurate U -value with. In this work, the main analytical methods available in the literature were identified, the calculation procedures were reviewed and their accuracy was evaluated and compared. With this goal, six analytical methods were used to estimate the U -values of 80 different LSF wall models. The obtained analytical U -values were compared with those provided by numerical simulations, which were used as reference U -values. The numerical simulations were performed using a 2D steady-state finite element method (FEM)-based software, THERM. The reliability of these numerical models was ensured by comparison with benchmark values and by an experimental validation. All the evaluated analytical methods showed a quite good accuracy performance, the worst accuracy being found in cold frame walls. The best and worst precisions were found in the Modified Zone Method and in the Gorgolewski Method 2, respectively. Very surprisingly, the ISO 6946 Combined Method showed a better average precision than other two methods, which were specifically developed for LSF elements.
Highlights
The use of lightweight steel frame (LSF) systems has emerged as a viable alternative to traditional construction and its usage are increasing every year, mostly because of its great advantages, such as: cost efficiency, reduced weight, mechanical resistance, fast assemblage and others [1,2]
The accuracy of THERM models used for the LSF wall thermal performance evaluation was checked under two different verifications: (1) benchmark values for two test cases presented on ISO 10,211 [25], and (2) comparison with the analytic U-value provided for a wall assuming homogeneous layers
The U-values obtained by the six analytical methods for all the evaluated LSF walls are plotted on Figure 11
Summary
The use of lightweight steel frame (LSF) systems has emerged as a viable alternative to traditional construction and its usage are increasing every year, mostly because of its great advantages, such as: cost efficiency, reduced weight, mechanical resistance, fast assemblage and others [1,2]. Gorgolewski [8] adapted the ISO 6946 Combined Method to a more accurate analytical U-value calculation methodology for LSF building components, including cold and hybrid frames In this new suggested analytical methodology, the upper and lower R-values limits are still being used, but instead of an average between these limits Gorgolewski found an “algorithm” for estimating the adequate weighting between them [8]. ISO 6946 [20] states that the prescribed Combined Method is not suitable to estimate the U-value of cold and hybrid LSF elements, but it is not known how large is this methodology calculation error In this context, the main aim of this work—besides to perform a review of the analytical methods calculation procedures—is to evaluate and compare the accuracy of the above mentioned simplified analytical methods. The main concluding remarks of this work are described
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