Windows are a key factor for designing energy-efficient buildings, particularly the frame area that can produce high thermal bridging. This paper deals with accurately estimating heat transfer through window frames under fluctuating film coefficients. The one-dimensional frame conductance model traditionally used by building simulation programs is analysed and an alternative model is proposed, which takes into account the non-planar morphology of studied frames. This model shows a positive agreement with the results obtained from a two-dimensional heat-transfer simulation program, demonstrating that the thermal performance of high-conductance non-planar frames strongly depends on the ratio between the boundary surface area and the projected frame area. According to the results, the traditional conductance model seems to be suitable for all frames with a thermal transmittance lower than 5 W/m 2 K; however, frames with a U-factor higher than 6.2 W/m 2 K need an alternative conductance model that better reflects the 2D nature of frame sections.

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Window frames can significantly affect the energy demand of buildings. Due to their complex non-planar geometry, twodimensional, time-consuming simulations are necessary to estimate with precision heat transfer through frames. However, in dynamic heat transfer calculation through computer-based building performance simulation tools, simplifications are made to reduce simulation time. In particular, EnergyPlus models window frames as rectangular profiles, and uses a one-dimensional heat transfer model. In order to evaluate whether this simplification is legitimate, thermal transmittance values were calculated for a selection of frames, on one hand through two-dimensional finite elements simulation, and on the other hand, through a spreadsheet that reproduces the steps of the algorithm currently used by EnergyPlus to calculate heat transfer through window frames. The comparison of both values showed relative errors higher than 20%, including for rectangular frames (without simplifying geometry). Aluminium frames without thermal break exhibited the highest absolute errors, up to 2.98 W/m 2 K. An alternative algorithm was proposed, which uses compactness factors for film coefficient calculation and reduced emissivities to take into account self-viewing surfaces. The proposed model improved the estimates, reducing the absolute error to less than 0.10 W/m 2 K in 52% of the occurrences, and to less than 0.47 W/ m 2 K for aluminium windows.

Within the framework of window labelling system elaboration in Argentina, this article questions the suitability of replicating calculation procedures from other countries with different climatic and technological contexts. This paper analyzes the methodological differences between existing standards for calculating window frame U-factor. It focuses on the impact of the radiation model applied on the frame outside boundary. Simulations were performed on different window frames under three sets of environmental conditions, first using the NFRC blackbody radiation model at the outside boundary, then using the ISO 15099 detailed radiation model. The comparison of the resulting U-factors shows significant differences (up to 12.7%) between both models. The impact of outside boundary radiation model choice increases with the non-planarity of outside frame boundary and decreases at high wind speeds. According to these results, the ISO 15099 detailed radiation model should be used on both sides of the frame, in order to maximise the accuracy of non-planar frames thermal transmittance calculation under any environmental conditions.

RESUMENLas ventanas influyen de manera significativa en las demandas energéticas para calefacción, refrigeración e iluminación de los edificios. En este trabajo, se caracterizan las propiedades energéticas de las tecnologías de ventanas disponibles en Argentina, mediante el cálculo de sus índices K, FS y TV (transmitancia térmica, factor solar y transmitancia visible). Se analiza, además, el impacto que tienen diversos parámetros, como las dimensiones de la abertura, la tipología, el material y la absortancia solar del marco, sobre los índices mencionados. Los casos de estudio se seleccionaron cuidadosamente para asegurar una buena representatividad de la muestra analizada, utilizando, en particular, el método de análisis de conglomerados. El cálculo de los índices se realizó con el apoyo de los programas WINDOW 6.3 y THERM 6.3. Los resultados indican que los valores de K están comprendidos entre 1.88 y 6.00 W/m2K, mientras que los valores de FS varían entre 0.08 y 0.80, y los de TV entre 0.03 y 0.80, según el material del marco, la tipología y el tipo de vidriado.Palabras clave ventanas, índices, energía, desempeño, simulación. ABSTRACTWindows significantly impact the energy demand for heating, cooling and lighting in buildings. This study characterizes the energy properties of window technologies available in Argentina by calculating their thermal transmittance (U), solar heat gain coefficient (SHGC) and visible transmittance (VT). Additionally, the impact of different parameters like window opening dimensions, typology, frame material and solar absorptance on the mentioned indices is analyzed. Case studies were selected carefully using cluster analysis methods in order to ensure that the sample examined was representative. The calculation of the U, SHGC and VT indices was carried out using WINDOW 6.3 and THERM 6.3 software. The results indicate that U-values are between 1.88 and 6.00 W/m2K, while SHGC values vary between 0.08 and 0.80, and VT values are between 0.03 and 0.80, depending on frame material, typology and glazing characteristics.