Window U-value Research Articles

Simulation-based sensitivity analysis of energy performance applied to an old Beijing residential neighbourhood for retrofit strategy optimisation with climate change prediction

Due to the high energy consumption in the building sector and the ever-increasing urbanisation rate, the demand for retrofitting old buildings in urban areas is increasing in China, especially in metropolitan cities like Beijing. However, despite national and local policies calling for extensive building retrofitting, it is a challenge to determine a cost-effective retrofit strategy. Considering this, the study establishes a novel approach that analyses the sensitivity of building energy consumption on parameters defining the materials used on the building envelope, as well as the solar shading and airtightness of the building. This research builds EnergyPlus models using geometric data captured from the map, building fabric data from local design standards, and a set of varying activity schedules, and carries out simulations to calculate the building energy consumption of a residential neighbourhood in Beijing, China. The energy consumption data is then used for a sensitivity analysis using the Morris Method on 14 building envelope parameters in total. For different building shapes, the sensitivity analysis results highlight that the energy is most sensitive to infiltration, followed by window U-value and window SHGC. The solar absorptances and U-values of external walls and roofs are also found to have a moderate influence on total energy consumption. By using predicted weather files, this research further discusses the changing influences of these parameters considering climate change over the next few decades. The approach of this research is instructive for the analysis of buildings in other cities in cold climate regions due to the generalisability of the studied neighbourhood, and the result has the potential to inform the building management teams and policymakers to determine suitable retrofit strategies.

Evaluation of the Impact of Window Parameters on Energy Demand and CO2 Emission Reduction for a Single-Family House

This article deals with the determination of the impact of selected parameters on energy consumption for heating and cooling purposes and CO2 emissions. Mathematical modelling combined with planning a computational experiment was adopted as the research method. The database for creating the models was developed using building energy simulations performed with DesignBuilder software. A single-family house with an area of 101 m2 was the subject of this study. Four deterministic mathematical models for the estimation of annual energy demand for heating, cooling, total final energy demand, and CO2 emissions were developed. Four parameters affecting the energy balance of the house: the area of the glazing system (three levels), U-value of windows (two-, three- and four-pane), U-value of external walls (0.1, 0.15, 0.2 W/m2K) and location (Warsaw, Berlin, Paris) were considered. The article discusses in detail the influence of individual factors on the energy demand and their common interactions. It was found that the level of thermal insulation of the glazing system plays the most important role in saving energy. This factor was the only one to show a stable and significant reduction in house energy demand, and thus a reduction in CO2 emissions for all four objective functions.

Thermal performance characterization of supply-air double windows: A new guarded hot box protocol and numerical modelization

The window area constitutes a significant part of the façades of heritage buildings in French cities. An effective way to improve window performance as well as preserve the appearance of heritage buildings is to retrofit old single-glazed windows into supply-air double windows by putting a secondary window inside the existing window. For window renovation, the U-value is an important indicator. To accurately identify the U-values (i.e., the Ueq value, Uuse value and Uloss value) of supply-air double windows, a new guarded hot box (GHB) method was firstly proposed which improved the GHB setup and calibration/testing procedure. Based on GHB experiments, the thermal performance of different window configurations was identified and compared. The results showed that U-value reductions were in the range of 59.2%-80.8% by renovating a single-glazed window into a supply-air double window. Moreover, the window U-value was compared to the center-of-glass U-value, which is often used in building codes to simulate building energy consumptions. It was found that the deviations between the window U-value and the center-of-glass U-value were in the range of 21.6%-45.5% for different window configurations. The GHB experimental data were further utilized to validate a three-dimensional computational fluid dynamic (CFD) model. Based on the validated model, a parametric study was performed to investigate the impacts of different window configuration parameters on U-values. The results showed that the location of Low-E coating, coating emissivity and air flow rate had significant effects on U-values. In addition, it was found that improving the thermal conductivity performance of the internal window frame was more important than the external window frame.

Review of the Experimental Methods for Evaluation of Windows’ Thermal Transmittance: From Standardized Tests to New Possibilities

One of the most important parameters that indicate the energy performance of a window system is the thermal transmittance (U-value). Many research studies that deal with numerical methods of determining a window’s U-value have been carried out. However, the possible assumptions and simplifications associated with numerical methods and simulation tools could increase the risk of under- or over-estimation of the U-value. For this reason, several experimental methods for investigating the U-value of windows have been developed to be used either alone or as a supplementary method for validation purposes. This review aims to analyze the main experimental methods for assessing the U-value of windows that have been published by national and international standards or as scientific papers. The analysis criteria include the type of the test in terms of boundary conditions (laboratory or in situ), the part of the window that was tested (only the center of glazing or the entire window), and the data analysis method (steady-state or dynamic). The experimental methods include the heat flow meter (HFM) method, guarded hot plate (GHP) method, hot box (HB) method, infrared thermography (IRT) method, and the so-called rapid U-value metering method. This review has been set out to give insights into the procedure, the necessary equipment units, the required length of time, the accuracy, the advantages and disadvantages, new possibilities, and the gaps associated with each method. In the end, it describes a set of challenges that are designed to provide more comprehensive, realistic, and reliable tests.

Simultaneous design and control optimization of smart glazed windows

The paper examines the benefits of simultaneous optimization of various design and control parameters for smart glazing windows of office buildings. The design variables include the window U-value, solar heat gain coefficient (SHGC), visible transmittance (Tvis), and window-to-wall ratio (WWR). The tint-switching control strategies reviewed include outdoor air temperature (OAT), indoor air temperature (IAT), incident total solar radiation (ITSR), and incident beam solar radiation (IBSR). These variables are optimized independently and simultaneously to determine the best specifications to minimize operating costs while maintained desired thermal and visual comfort levels. Specifically, electrochromic (EC) smart glazed windows are optimally designed and controlled when installed for a prototypical medium office building in Denver, Colorado. The optimization analysis results are considered to determine the breakeven installation costs required for smart glazing systems to be cost competitive compared to their static glazing counterparts. The analysis found that the annual energy cost savings from optimally designed and controlled smart glazed windows ranged from 11% − 18%. Of the design parameters, the U-value proved to be the most impactful on energy and cost reductions, followed by SHGC and WWR. Indoor and outdoor air temperature-based controls are determined the most effective operation strategies. Simultaneously optimizing for the design and control parameters provided the highest annual savings.

Do high energy-efficient buildings increase overheating risk in cold climates? Causes and mitigation measures required under recent and future climates

Contradictory findings are reported in the literature showing that high energy-efficient buildings have either higher or lower overheating risks compared to old buildings. A methodology is developed using the Global and Local Sensitivity Analysis to quantify the contribution and correlation of individual building envelope parameter to the change in indoor operative temperature. This methodology is applied to an archetype Canadian detached house as a case study to evaluate its overheating risk. The building envelope thermal characteristics studied represent houses built in different periods from 1950 to high energy-efficient buildings in Montreal under different weather generations: typical historical (1961–1990), recent observational (2016), and typical future years 2030 (2026–2045) and 2090 (2080–2099) generated based on RCP-4.5 and 8.5 scenarios.The results showed that the high energy-efficient buildings can be more resilient to climate change than old buildings if adequate ventilation is provided, where the decrease of window and wall U-value, and SHGC all contribute to the decrease in indoor temperature. While without adequate ventilation, the overheating risk in high-energy-efficient buildings can be higher than old buildings, where decreasing wall and window U-values and infiltration rate has a greater contribution to the increase of indoor temperature, while decreasing window SHGC has a lower contribution to the decrease in indoor temperature compared to the case with adequate ventilation. The results also showed that natural ventilation in the high energy-efficient buildings is sufficient to reduce the overheating risk under the current climate but will require additional interior and exterior shading under future climates.

Impacts of Courtyard Envelope Design on Energy Performance in the Hot Summer–Cold Winter Region of China

This paper aims to investigate the effects of courtyard envelope design on the energy performance of office buildings in the hot summer–cold winter region of China. Two types of courtyard buildings were simulated with 200 energy models by changing the following variables: window–wall ratio (south, north, east, and west walls), window U-value, wall U-value, solar heat gain coefficient, and orientation. The treed Gaussian process (TGP) sensitivity analysis method was employed to quantify the contributions of parameters related to courtyard design that result in the changes of annual cooling, heating, lighting, and total energy consumption. The results show that the courtyard envelope design has a significant effect on the energy consumption in this case study. The most influential factor affecting annual cooling energy consumption and the annual total energy consumption is the orientation, which is responsible for 37% and 81%, respectively, in two layouts to annual cooling energy use. The corresponding proportion increased to 45% and 86% in terms of the total energy consumption. The most influential factor influencing annual heating energy use is the window U-value, which explained nearly 60% of the changes to the heating energy use. The effect of the solar heat gain coefficient in two kinds of layouts is as high as 82% and 79% for lighting energy use in this case study. Due to the different courtyard forms, the magnitude of the effect of the parameters on energy consumption and the main trend of the effect is slightly different.

Thermal performance optimization of envelope in the energy-saving renovation of existing residential buildings

Based on the life cycle cost analysis, a thermal performance optimization model of building envelope was developed for the energy-saving renovation of existing residential buildings. The calculation formulas of the economic insulation layer thickness of the roof, external wall and floor over the basement were obtained simultaneously, and the optimal relationship of thermal performance among building envelope units was proposed; that is, the equivalent U-value of each opaque building envelope units should be equal. Moreover, the calculation methods of ultimate and optimum energy saving ratio were put forward. For a case study existing residential building in Beijing, compared to the energy-saving renovation scenarios determined by the limit value method (LVM), the saving ratio of insulation material of those determined by the optimization model ranges from 0.53% to 73.59%; and the saving ratio of insulation investment ranges from 0.21% to 64.93% when the building envelope energy saving ratio been taken the same value and changing from 0.65 to 0.85. The limit U-value of envelopes prescribed by Design Standard for Energy Efficiency of Residential Buildings in Beijing (DB11/891–2012) cannot meet the optimal relationship. Therefore, energy-saving renovation scenario determined by LVM cannot obtain the best economic benefit. The ultimate and optimum energy saving ratio achieved are inversely proportional to the U-value of windows. However, improving the performance of windows cannot necessarily achieve the best economic benefit. It is necessary to consider the technical advancement and economic rationality to choose the windows.

In Situ Evaluation of the U-Value of a Window Using the Infrared Method

The amount of heat lost through the envelope of a building is one of the most important variables that affects the energy performance evaluation of a house. In addition, it is especially important to estimate and accurately diagnose the amount of heat produced by windows. In Korea, windows’ U-values reflect a building’s initial design values and thermal characteristics that determine the thermal performance of an existing building, and is a factor that can overestimate the energy performance of a building. Therefore, there is a need for a field measurement method that can accurately measure the total U-value of windows in an existing house. This study provides a method of quantitatively measuring the total U-value of windows using the infrared (IR) method in ISO 9869-2. As a result of measuring the U-value using the infrared (IR) method, the Korean Standard (KS F 2278) for window performance test result values and the root mean square error (cvRMSE) for the U-value measurements using the IR method showed a high accuracy of about 3.29%. In addition, we confirmed that the IR method is an effective (cvRMSE about 7% improvement) method that can measure the comparison result faster than the heat flow meter (HFM) method, which is a conventional thermal performance measurement method.

Multiobjective optimization of building energy consumption based on BIM-DB and LSSVM-NSGA-II

The building envelope performance significantly influences the building energy consumption, and the major influencing factors of the building envelope include the external wall U-value, external wall solar absorptance, roof U-value, roof solar absorptance, external window U-value, and window-to-wall ratio. This paper proposed a framework that integrates Building Information Modeling (BIM) with LSSVM (least square support vector machine (LSSVM) and non-dominated sorting genetic algorithm-II (NSGA-II) to study the influence of building envelope parameters on building energy consumption and discover the best design. The DesignBuilder (DB) is used to perform energy consumption simulation and orthogonal experiment to obtain the sample data set. The LSSVM is utilized to learn the data set to establish a prediction model between the building envelope and energy consumption, which is used as the fitness function of the building energy consumption in NSGA-II. With the objective of minimizing building energy consumption and maximizing the indoor thermal comfort (measured by predicted mean vote or PMV), an NSGA-II multi-objective optimization model is established. The Pareto front is obtained through calculation, and the optimal combination of building envelope parameters is discovered using the ideal point method. A case of a proposed school building in China is used to demonstrate the feasibility and effectiveness of the developed framework, the results show that: (1) The LSSVM has great ability to predict building energy consumption, achieving an accurate prediction with a goodness-of-fit of 0.9549 and an RMSE of 0.0273; (2) The optimized envelope design parameters of the proposed building are presented, including the External wall U-value is 0.2, the External wall solar absorptance is 0.55, the Roof U-value is 0.2, the External window U-value is 0.4166, the External window U-value is 1, the Window-wall ratio is 0.215. (3) The developed LSSVM-NSGA II hybrid approach can effectively improve the building’s energy consumption and thermal comfort with the building energy consumption reduced by 10.6% and the thermal comfort increased by 32.2% than the initial values, respectively. The proposed LSSVM-NSGA II framework is able to optimize building envelope design parameters than its precedent, which can provide effective thinking for similar problems.

Analysis of the Calculation Method for the Thermal Transmittance of Double Windows Considering the Thermal Properties of the Air Cavity

The calculation method for the thermal transmittance (U-value) of double windows as specified by the Korean government (ISO 15099) is often inappropriate. To develop a more suitable calculation method, the thermal properties of the air cavity between the internal and external windows should be considered. Herein, seven cases of double windows were set up. The air cavities were designed in accordance with international standards and computational fluid dynamics (CFD) and used for the calculation of the U-values of the double windows according to ISO 15099 and 10077. All the calculated U-values were compared with experimentally obtained values. In accordance with the ISO 10077-1 method, the thermal resistance of the air cavity calculated using CFD could produce double window U-values that are similar to the experimentally obtained values. In most cases, the difference between the theoretical and experimental U-values was 5% and less than 0.14 W·m−2·K−1, implying that the U-values calculated using CFD and the ISO 10077-1 method are approximately equal to the experimentally obtained U-values. Korean regulations do not include ISO 10077-1 for double-window assessment. However, these criteria can provide a solution in improving the accuracy of the calculation of the overall thermal transmittance of double windows.

Overheating risk of a single-family detached house built at different ages under current and future climate in Canada

With the anticipated increase in temperature and solar radiation and frequency of extreme weather conditions due to climate change, buildings typically designed/built in Canadian cold climates would experience increased risks of summer overheating. This paper focuses on how these existing buildings perform under a current extreme year and projected future climates. Results show that the thermal conditions of a single-family detached house built in 1964 and 1990 are more comfortable than the house built to meet the current National Energy Code of Canada for Buildings (NECB) and high energy-efficient building (HEEB) without including natural ventilation by up to 50%. On the other hand, when natural ventilation is included, the house built to NECB and HEED are more comfortable. Sensitivity analysis is carried out to evaluate the influence of five design parameters, i.e. wall and roof insulation, airtightness, U-value and SHGC of windows. Sensitivity analysis shows that wall insulation, airtightness, and windows U-value are the three most significant parameters influencing the overheating risk without natural ventilation. With natural ventilation, the SHGC of windows is the most influencing parameter in reducing overheating risk. This paper confirms that the Canadian buildings have the overheating risk over the hot summer experienced over the past a few years and the risk will be increased in the future. Natural ventilation as a mitigation measure, which has been relied on by building designers in Canada will not be sufficient to remove excess heat or provide thermal comfort to residents. Other mitigation strategies such as shading to reduce the heat gain during the summer, are needed.

A Study on Changes of Window Thermal Performance by Analysis of Physical Test Results in Korea

The interest in zero energy buildings is increasing in South Korea. Zero energy buildings need to save energy by using passive technology. The window performance is important to the thermal insulation of the building. Also, the government regulates the window performance through regulation and standards. However, it is difficult to predict window performance because the components of the window have become complicated due to the various materials used in the glass and frame. Based on window performance standards and regulations, the quality of window performance was managed. In this research, to consider thermal performance in proper window design in South Korea, we confirmed the impact on the thermal performance of the window through various kinds of materials and shapes. The authors also propose a window shape classification and frame calculation method based on actual test results. The authors analyzed the thermal performance data of the windows provided by the Korea Energy Agency and confirmed the change in the thermal performance of the windows by year and by frame material. The average U-value of the window decreased from 2012 to 2015 and maintained similar values until 2017. In 2018, this value was decreased to comply. Also, the authors confirmed the U-value of the windows through actual physical experiments and confirmed the change in thermal performance by the construction of the windows based on the results. The results show, in the case of aluminum windows, the U-value corresponding to Grade 3 (1.4–2.1 W/m2·K) was as high as about 60%. Regarding the analyzed results of the U-values of PVC windows, Grade 3 (U-value of 1.4–2.1 W/m2·K) accounted for about 35%, and Grade 2 (U-value of 1.0–1.4 W/m2·K) for about 29%. This paper also confirmed that the frame U-value of the PVC windows is lower than the frame U-value of the aluminum windows. Therefore, the authors proposed the performance index of the glazing part in PVC and aluminum window design. The results of this research can be used as basic data to identify problems in the method of determining the performance of windows in Korea.

Towards harmonizing the NFRC and CEN window performance simulation methods

Studies have found that the European Committee for Standardization (CEN) and National Fenestration Rating Council (NFRC) methods produce different U-values for the same window resulting in confusion when comparing products. A comparative evaluation of the NFRC and CEN U-value calculation methods was conducted for North American residential high-performance window products with focus on the most influential parameters in determining the whole window U-value for high-performance windows. Using two-dimensional conduction simulation software, four North American high-performance frame types with double, triple, and quad glazing combinations were simulated and calculated according to the NFRC and CEN standard methods. Overall, the trend showed that for the specific window combinations of this study, the higher the performance of the insulated glazing unit (IGU), the lesser the differences in the whole window U-value of both methods. The results showed an overall difference of 1 to 11% in whole window U-value when using the NFRC and CEN standards, lower than other studies. Generally, the NFRC standard resulted in the lower U-value for each case. Recommendations for harmonization of the two standards include aligning boundary conditions, frame cavity models, and material conductivities.

Phase Change Material Wallboard (PCMW) melting temperature optimisation for passive indoor temperature control

Incorporating PCMs into traditional building envelops is an effective way to minimise indoor temperature fluctuations and maximise thermal comfort in summertime. PCMW is of particular interest due to its ease of installation. This paper presents a theoretical analysis in which both outdoor air temperature and solar radiation are considered to predict the optimal melting temperature of an internal PCMW for passive indoor temperature control. Key factors influencing the selection of optimal melting temperature are studied including outdoor temperature, solar radiation, window size, air change rate (ACH), and external wall and window U-values. A case study was carried out on a lightweight building using real weather data for the summer months in the UK. The simulation was developed in EnergyPlus with the potential to reduce energy use while maintain the desired internal temperatures determined. The optimal melting temperature was found to be 23.4 °C during summer in the UK from a theoretical analysis, which showed good agreement with the numerical simulations performed. In summer, the percentage energy savings can be as high as 40% compared with a similar building without PCMWs and the period which temperatures are maintained within the thermal comfort range can be improved by up to 7.2%.

Energy optimization of high-rise commercial buildings integrated with photovoltaic facades in urban context

This research thoroughly explored the impact of archetypes and confounding factors on a proposed holistic design optimization approach for high-rise office buildings with integrated photovoltaic (PV) facades. The design optimization adopts the hybrid generalized pattern search particle swarm optimization (HGPSPSO) algorithm, which is incorporated with qualitative and quantitative sensitivity analyses for factor prioritizing and fixing. Different archetypes are modelled by changing floor plan sizes and shapes, while diverse urban contexts and internal load (heat gain) levels are investigated as major confounding factors beyond designers' control. Variation of these four simulation scenarios are then used to examine the uncertainty of sensitivity indices and optimization potential for passive architectural design parameters. The window geometry, thermal and optical properties are proved to be most important to the reference PV envelope design. The building plan shape is found to have little impact on the weighting of different design parameters, while the shape coefficient (SC) is determined to be almost linearly correlated with the HVAC (heating, ventilation and air-conditioning) demand. The office design with the highest shape coefficient can therefore achieve a net energy demand reduction up to 48.77%. The floor plan size also has minor impact on the sensitivity index for each design factor, but the energy-saving potential grows with decreasing floor sizes. On the contrary, confounding factors can greatly change the sensitivity analysis (SA) result. The window U-value becomes more important with an increasing internal load level and urban context density whereas the impact of the window light-to-solar gain ratio is reduced by peripheral shading. Furthermore, varying confounding factors can even change the dimension of optimization problems based on different factor fixing results. This research can provide early-stage design guidance for energy efficient buildings with a comprehensive analysis of pragmatic building archetypes, background contexts and operation scenarios.

Numerical simulation study of BIPV/T double-skin facade for various climate zones in Australia: Effects on indoor thermal comfort

Maintaining indoor thermal comfort is crucial for the health and productivity of building occupants. Building envelope plays a major role in influencing the impact of outdoor climate and controlling the indoor thermal conditions. In this paper, comparative analysis of indoor comfortable temperature for four different types of building-integrated photovoltaic/thermal (BIPV/T) building models in a range of climate zones in Australia was conducted using TRNSYS simulation tool. In terms of system operational mode, the four types of BIPV/T building facade systems include a building-integrated photovoltaic single-skin facade (SSF), non-ventilated BIPV/T double-skin facade (BIPV/T-DSF), naturally ventilated BIPV/T-DSF and fan-assisted BIPV/T-DSF. In addition to the operational modes of the facade systems, two types of semi-transparent PV glazing with different visible light transmittance (VLT) were respectively applied to the models as external window glazing. The numerical results showed that the naturally ventilated BIPV/T-DSF with lower VLT (27%) PV glazing maintained a relatively better indoor temperature for the hot climatic conditions compared to the other operational modes, while the non-ventilated BIPV/T-DSF with higher VLT PV glazing (37.5%) offered more comfortable indoor temperature (i.e. 20 to 26 °C for office hours) for the cold climates in Australia. On the other hand, the naturally ventilated BIPV/T-DSF could basically maintain comfortable indoor temperatures from 22 to 27 °C during office hours without mechanical systems for the peak summer times for cool temperate climates in Australia. Moreover, it was found that the thermal insulation effect of semi-transparent PV glazing hardly affected indoor operative temperature in the ventilated modes of the BIPV/T-DSF. According to the sensitivity analysis, the change of U-value of internal window of the DSF would significantly lead to the change of indoor thermal comfort in both ventilated operational modes, but very few changes for the non-ventilated DSF. The variation of cavity depth had distinct impact on the indoor thermal comfort for fan-assisted DSF but slightly affected that of other modes. In addition, the changes of opening ratio for the ventilating louvers and fan airflow rate of the DSF also had a degree of influence on indoor thermal comfort for naturally ventilated DSF and fan-assisted DSF respectively.

Design strategies and measures to minimise operation energy use for passive houses under different climate scenarios

Here, the implications of different design strategies and measures in minimising the heating and cooling demands of a multi-storey residential building, designed to the passive house criteria in Southern Sweden are analysed under different climate change scenarios. The analyses are conducted for recent (1996–2005) and future climate periods of 2050–2059 and 2090–2099 based on the Representative Concentration Pathway scenarios, downscaled to conditions in Southern Sweden. The considered design strategies and measures encompass efficient household equipment and technical installations, bypass of ventilation heat recovery unit, solar shading of windows, window size and properties, building orientation and mechanical cooling. Results show that space heating demand reduces, while cooling demand as well as risk of overheating increases under future climate scenarios. The most important design strategies and measures are efficient household equipment and technical installations, solar shading, bypass of ventilation heat recovery unit and window U-values and g-values. Total annual final energy demand decreased by 40–51%, and overheating is avoided or significantly reduced under the considered climate scenarios when all the strategies are implemented. Overall, the total annual primary energy use for operation decreased by 42–54%. This study emphasises the importance of considering different design strategies and measures in minimising the operation energy use and potential risks of overheating in low-energy residential buildings under future climates.

Experimental Assessment of the Energy Performance of a Double-Skin Semi-Transparent PV Window in the Hot-Summer and Cold-Winter Zone of China

The energy performance of the semi-transparent PV (STPV) window was carried out in a hot-summer and cold-winter zone of China. Semi-transparent PV (STPV) windows generate electric, reduce the heating load and aim to utilize daylighting efficiently. In order to analyze the energy performance of semi-transparent windows, a comparison test rig was set up which includes two test rooms of the same size. One room was installed with the STPV window and the other with a conventional window. The lighting, thermal, and electrical performance of STPV window was tested and compared with those of conventional window in the same ambient environment. It was observed that the maximum power generation of the STPV (a-SiGe) window was 33.3 W/m2 on a typical sunny day. Compared with the conventional windows, the average solar heat gain (SHGC) and Uvalue of STPV windows were 0.15 and 1.6, respectively, which is better than those of conventional window. On a sunny day, the Useful Daylighting Illuminance (UDI) of the test room was up to 52.2% better than the UDI of the conventional room. The results could support the application of photovoltaic technology in buildings in Southwest China.

Condensation at the exterior surface of windows

New energy efficient windows have a higher risk for outdoor air vapour condensing to their exterior surface, when compared to older windows with lower thermal resistance. This external condensation can reduce visibility through the window, decrease owner satisfaction and affect the behaviour of window buyers and sellers. The purpose of this study was to analyse the impact of window U-value and other factors on the occurrence of external condensation. A combined heat and moisture transfer model was created and used for the calculations. According to the results, the duration and amount of external condensation are projected to increase in the future due to lower window U-values and climate change. Exterior surface emissivity, external shadings and building location had a big impact on the amount of yearly condensation hours, while window orientation and solar absorption coefficient had a smaller impact. There was also an interesting power-law-type correlation between yearly condensation hours and the median effective thickness of the condensation layer. The results help window manufacturers and building designers make more accurate decisions in their future work.