Window U-value Research Articles

Establishment of Passive Energy Conservation Measure and Economic Evaluation of Fenestration System in Nonresidential Building of Korea

ECO2 (building energy efficiency rating program) and passive energy conservation measures (ECMs) were established as a basic study for targeted methodologies and decision support systems development in Korea to meet national regulations. The primary energy consumption and economic evaluation of nonresidential buildings was performed. Passive ECMs were classified as planning and performance elements. The planning elements are the window-to-wall ratio (WWR) and horizontal shading angle. The performance elements are the thermal transmittance (U-value) of the walls, roof, and floor and the U-value and solar heat gain coefficient (SHGC) of windows. This study focused on the window-to-wall ratio and the U-value and solar heat gain coefficient of windows. An economic efficiency database for the constructed alternatives was built; the target building was set and the Passive ECM List for the target building was derived. The energy consumption evaluation and economic evaluation were performed for each of the constructed alternatives, and a methodology for guiding energy efficiency decisions was proposed based on the performance evaluation results, and the optimal Passive ECM List for the target building was derived.

창호 전체 단열성능과 열교부위 열성능 상관관계 분석

Thermal bridge in windows can cause multiple problems, including heat loss and mold/condensation problems. The energy performance of window has been dramatically improved in recent years, however, mold/condensation are still challenging problems. This paper analyzed the relationship of thermal performance between the whole window and thermal bridging area. Also, it identified the factors which affect the local heat flow of window thermal bridging. The results show that the thermal performance of window thermal bridging area has no direct relationship with window U-value. The window type and the position of Low-e coating were identified as key factors affecting the thermal performance of window thermal bridging area.

Assessment of window performance in classrooms by long term spatial comfort metrics

Providing good environmental conditions for educational buildings has always been critical because of the negative influence of thermal and visual discomfort on students’ learning and performance. Unsatisfactory conditions mostly caused by overheating and glare due to the presence of direct solar radiation, could be compensated by appropriate window configurations. Since occupants are obligated to sit in defined positions in classrooms, spatial analysis of the thermal and visual field are necessary in addition to temporal evaluations. This paper applies both spatio-temporal thermal and visual comfort metrics, to assess the effect of window design on comfort by dynamic simulations in a south and north faced classroom model in Tehran’s Climatic condition. The SolarCal method, which considers the direct effect of solar radiation on occupants, validated by field measurements, is used to assess local thermal comfort. A solution space, presented in graphs has been defined for each orientation by the defined comfort targets. These graphs could be used to select the optimum glazing properties based on the desired window wall ratio. The defined solution areas are larger in the south orientation and could be extended by considering shadings. Results show that low U-values and low Solar Heat Gain Coefficient (SHGC) are needed in both north and south faced classrooms to decrease the overheating risk. Also the use of double glazing units is not always required or the most favorable option for all window sizes. Based on the results the most effective factors for window design in regard to spatio-temporal thermal comfort are Window Wall Ratio (WWR), glazing SHGC and window shadings, orientation, and window U-value respectively. However, WWR, orientation, and glazing Visual Transmittance (VT) and shadings have almost equal effect on spatio-temporal visual comfort.

Assessment of uncertainties for the GHP apparatus in determining U-value of double-glazed window with inter-pane blinds

This paper describes a procedure for determining the component uncertainties for the guarded heater plate apparatus – developed to compute from the measured thermal resistance – the ‘Unit heat loss rate’ of a double-glazed window with inter-pane venetian blinds. An example for computation of such uncertainties as per full-factorial design (162 trial runs) is also presented. The overall uncertainty for determining this U-value was determined to vary from 1.2% to 2.6% for the identified design. It was concluded that the fabricated apparatus has the advantage of getting an acceptable level of accuracy without using the complex and specifically designed equipment.

Effect of facade components on energy efficiency in office buildings

Properties of facade materials should be considered to determine which of them strongly affect building energy performance, regardless of the building shapes, scales, ideal locations, and building types, and thus may be able to promote energy efficiency in buildings. In this study, the effects of four fundamental facade properties related to the energy efficiency of office buildings in Tokyo, Japan, were investigated with the purpose of reducing the heating and cooling energy demands. Some fundamental design factors such as volume and shape were also considered. It was found that the reduction in both the solar heat gain coefficient and window U-value and increase in the solar reflectance of the opaque parts are promising measures for reducing the energy demand. Conversely, the reduction in the U-value of the opaque parts decreased the heating energy demand, and this was accompanied by an increase in the cooling energy demand in some cases because the total energy demands were predominantly for cooling. The above-mentioned promising measures for reducing building energy demands are thus recommended for use, and an appropriate U-value should be applied to the opaque parts based on careful considerations. This study provides some fundamental ideas to adjust the facade properties of buildings.

Window retrofit strategy for energy saving in existing residences with different thermal characteristics and window sizes

An adequate window system is one of the most important retrofit strategies for effective energy conservation of a building, because the U-value and solar heat gain coefficient of windows have enormous impact on the heating and cooling loads of buildings. Therefore, this paper presents methods for improving the energy efficiency of existing residences that have various window sizes and envelope insulations, through a window retrofit using optimal U-value and solar heat gain coefficient values. Furthermore, the window retrofit strategy has been standardized using analysis of the correlation between the properties of the retrofitted window and energy saving rates. The results show that the annual heating and cooling energy demand decreases by 7.9–16.7% when changing the U-value of the windows in a poorly insulated house, and that the relationship between the lower U-value and energy saving is strong for poorly insulated houses regardless of window size. However, for houses with better insulation and larger window sizes, the total energy usage decreases by 18.4–29.7% when the solar heat gain coefficient is lower, and the energy saving effect of the U-value decreases while that of the solar heat gain coefficient increases. Practical application: This study was focused on improving the energy efficiency of existing residences by applying retrofitting technology. By exploration of the contribution of the specific qualities of windows and the thermal envelope (insulation) system of buildings via simulation, it was determined that it is necessary to adjust the U-value and SHGC of retrofitted windows, in relation to the thermal performance and window-wall ratio of an existing residence, to achieve high energy efficiency.

바닥복사난방이 적용된 사무소 건물의 Downdraft 평가에 관한 연구

A radiant heating system keeps a space uniformly heated and comfortable. In addition, this system occupies only a small amount of space. Because of these advantages, many countries are now applying this system. However, when applying radiant floor heating, the thermal discomfort that can be caused by the external environment needs to be considered. Recently, the amount of glass area used in office buildings has been increasing. As a result, heat loss through the building envelope has increased, and increasing the risk of the occurrence of downdraft. Downdraft is defined as descending cold air induced by cold windows in colder weather, which may cause thermal discomfort to occupants. Therefore, in this study, the principle of how donwdraft occurs was analyzed. In order to estimate air movement in a room, numerical simulations are performed. A computational model of a room with radiant floor heating is constructed within the CFD program Star CCM+. Eight cases of radiant floor heating with varying surface temperatures, surface areas and window U-value are considered.

Comparative study of simulation and experimentation for the U-value of double-glazed windows with inter-pane blinds

This article applies the Taguchi design on a simulation as well as on an experimentation technique to determine the thermal transmittance (U-value) of a double-glazed window with inter-panes blinds so as to explore its performance potential for a particular Indian climate. Environmental parameters are selected per the composite climate in India. Simulated results show a maximum of 20% variation when compared with experimental ones. Analysis of results shows that Taguchi methods are effective in ranking the effect of various parameters on the U-value of identified glazing units. The low-emissivity glazing system ranks first, followed by slat angle, temperature difference, pane spacing, and temperature of hotplate (representing climatic conditions). The analysis of means is also carried out to identify the best combination of levels of various parameters.

Impact of convection on thermal performance of aerogel granulate glazing systems

Among currently available window systems, aerogel granulate glazing systems are considered promising for achieving good thermal and visual comfort in buildings. In aerogel granulate glazing systems, it is important to evaluate how convection in the granular-filled cavity affects their thermal performance because double glazing systems, which have a similar structure, show variable thermal properties related to the convection effect. In this study, it is shown that the granular-filled cavity hardly influences the air permeance, and therefore, convection can occur in such cavities. Nevertheless, a mock-up of commercially available aerogel granulate window shows practically identical window U-values in hot box measurement with various tilt angles. In other words, convection in the granular cavity does not affect the thermal performance of aerogel granulate glazing systems. The center-of-glass U-values are 0.28 and 0.69W/(m2K) for aerogel thickness of 60 and 30mm, respectively. The total window U-values (using a wood frame) are 0.69 and 0.84W/(m2K) for aerogel thickness of 60 and 30mm, respectively. Furthermore, the granular aerogel in the cavity subsided by ∼3% of its total volume after the hot box measurement. The risks posed by this subsidence under actual conditions should be assessed in later research.

Cost-optimum analysis of building fabric renovation in a Swedish multi-story residential building

In this study, we analysed the cost-optimum level of building fabric elements renovation in a multi-story residential building. We calculated final energy use for space heating of the building considering a wide range of energy efficiency measures, for exterior walls, basement walls, attic floor and windows. Different extra insulation thicknesses for considered opaque elements and different U-values for new windows were used as energy efficiency measures. We calculated difference between the marginal saving of energy cost for space heating and the investment cost of implemented energy efficiency measures, in order to find the cost-optimum measure for each element. The implications of building lifespans, annual energy price increase and discount rate on the optimum measure were also analysed. The results of the analysis indicate that the contribution of energy efficiency measures to the final energy use reduces, significantly, by increasing the thickness of extra insulation and by reducing the U-value of new windows. We considered three scenarios of business as usual (BAU), intermediate and sustainability, considering different discount rates and energy price increase. The results of this analysis suggest that the sustainability scenario may offer, approximately, 100% increase in the optimum thickness of extra insulation compared to BAU scenario. However, the implication of different lifespans of 40, 50 or 60 years, on the optimum measure appears to be either negligible or very small, depending on the chosen scenario. We also calculated the corresponding U-value of the optimum measures in order to compare them with the current Swedish building code requirements and passive house criteria. The results indicate that all optimum measures meet the Swedish building code. None of the optimum measures, however, meet the passive house criteria in BAU scenario. This study suggests that the employed method of building renovation cost-optimum analyses can be also applied on new building construction to find the cost-optimum design from energy conservation point of view.

Solar shading control strategies in cold climates – Heating, cooling demand and daylight availability in office spaces

Modern office buildings often have large glazed areas. Incident solar radiation can lead to large cooling demands during hot periods although the solar radiation can help reduce heating demands during cool periods.Previous studies have shown that large parts of the net energy demand of an office building is related to window heat loss and cooling demands induced by solar irradiance. In this article, the authors found that, even in what traditionally has been considered to be a heating-dominated climate, cooling demands dominate the net energy demand of an office building. Solar shading systems are vital to reduce the cooling demand of an office building.Introducing shading systems might contribute to higher heating demands as well as higher demands for artificial lighting but at the same time it might be necessary in order to reduce glare issues.Simulations of a number of shading strategies have been performed for south- and north-facing office cubicles with varying floor areas, window sizes and window parameters. Energy demands for heating, cooling, lighting and ventilation fans have been assessed. The simulations show that the choice of shading strategy can have an impact on the energy demand of the offices. Depending on strategy, the energy demand can either increase or decrease compared to an unshaded one- or two-person office cubicle.In addition, the shading systems can contribute toward a lowered thermal transmittance value (U-value) of the window by functioning as an additional layer in the glazing unit when closed. Potential improvements of U-values have been studied in combination with the shading system’s effect on solar heat gains and daylight levels. Experimental investigations of in-between the panes solar shading system effects on window U-values are currently being carried out at the Research Centre on Zero Emission Buildings (www.ZEB.no).It was found that automatically controlled shading systems can reduce the energy demands of south-facing, small office cubicles, but that they should not be installed without a thorough case-by-case investigation as increased energy demands were found if an improper shading strategy was chosen. Upgrading to four-pane glazing will, however, always have a beneficial impact on the energy demand compared to two- and three-pane glazing.

Comparison of heating and cooling energy simulation using multi-years and typical weather data in South Korea

Typical weather data (TWD) consists of 8760 values of various selected meteorological parameters such as ambient temperature, solar radiation, relative humidity and wind velocity, and are originally derived from long-term data. In this paper, TWDs have been generated, using three different methodologies, from hourly meteorological data measured in Seoul, Daejeon, Daegu, Gwangju, and Busan. The aims of this study were to compare the simulation results from the three methodologies (ISO Test Reference Year, CIBSE Test Reference Year, and Typical Meteorological Year) with those from individual years and their long-term means to assess the important five components of the building envelope (wall-to-window ratio, window overhang, window transmissivity, window U-value, and wall U-value) for the influence on the heating and cooling load and to study the influence of the different weather parameters (temperature and solar radiation). Results of the comparison show that predicted monthly load and energy consumption profiles from the TWD tended to follow the long-term mean quite closely. The load calculations vary almost linearly as the building envelope components varied apart from the window U-value. The yearly average values of mean bias error and root mean squared error for the heating and cooling loads were evaluated for the three TWDs and long-term measured meteorological data. Practical application: This paper will enable engineers and building designers of buildings in South Korea to assess the heating and cooling loads with more confidence. The paper will enable designers to use the appropriate weather data when using computer simulation for the design. The designer will also be able to assess the energy efficiency better.

창호의 열관류율, 일사취득계수와 향의 배치가 건물의 냉난방 부하에 미치는 영향에 관한 지역별 비교연구

The primary goal of this research is to identify the impacts of window design on the energy use in buildings which takes up about 25% of the total energy consumption. Recently, efficient use of energy is gaining more importance in buildings. Window design, especially being dependent on glazing performance choices, is an important factor for reducing energy consumption in most of the buildings. It also is influenced by the latitude of the site and window orientation. This paper aims at identifying the influence of Window performance indicators(U-value, SHGC), orientation and latitude on the building energy consumption with systematically designed simulations. Comparative study has been performed for five different locations; Greenland, Korea, Singapore, Argentina and Chile along with the different window U-value and SHGC values. The results show that optimum window system with properly coordinated window performance indicators(U-value, SHGC), orientation achieves dramatic reduction of energy consumptions. Windows with low U-value could reduce heating loads and high SHGC could reduce cooling loads. The study also verifies that the windows installed at south facade is more energy efficient in the northern hemisphere while windows facing north is more energy efficient in the southern hemisphere.

Assessment of the technical potential for multifunction building zero-carbon renovation with EnergyPlus

Energy consumed in the building field accounts for 30-40% of social energy consumption in the entire world. The energy used in office-educational buildings, especially in developed countries, takes up a large proportion, because of the high occupant density, long running time and high brightness and comfort requirement. This research aims at predicting the renovation potential for office-educational building in the energy-saving aspect. One typical existing educational building in the UK has been chosen as the research object; EnergyPlus computer model, which accurately reflects the building style, material, structures and Heating, Ventilation and Air Conditioning system is established. After the simulation of energy performance and carbon emission of the original building, some insulation measures and renewable technologies (photovoltaics, solar thermal etc.) have been added as the renovation method. Then, theoretical calculation and computer simulation about the effect of the renovations above have been conducted to reflect the improvement. Carbon-saving effects of renovation methods were compared and analyzed. Only 2.2% of carbon emissions can be reduced by improving the air-tightness of the building and the U-value of windows, but great carbon saving achieved by adding renewable devices. About 40.8% of the carbon emissions reduced due to the application of all the renovation methods. The result of this study will provide some critical references for the choosing and prediction of renovation methods in the energy-saving field in the UK.

An investigation of design parameters that affect commercial high-rise office building energy consumption and demand

There are many factors that drive the energy consumption and demand in high-rise commercial office buildings. Understanding the effects of individual building parameters and two-factor interactions can be very useful for directing building audits and developing energy simulation models. A fractional factorial analysis was conducted to evaluate a large number of building parameters in an effort to quantify their effect on building energy consumption and demand. The analysis utilized building data collected from 22 building audits of high-rise commercial office buildings located in the downtown Chicago Loop area. Simulation results for three of the buildings show the effects of each factor and two-factor interactions on energy consumption and demand over a set of climate zones. The factors of primary importance include lighting and equipment power density, chiller efficiency, window U-value, the mass of interior furnishings and supply fan static pressure.

Application of artificial neural network to predict thermal transmittance of wooden windows

Thermal performance of windows depends on many parameters, such as dimensional characteristics and material properties of the components. The thermal transmittance U can be evaluated by a numerical method based on the CFD approach for the evaluation of the frame U-value (ISO 10077-1, ISO 10077-2) or by experimental campaigns on window prototypes, according to ISO 12657-1; in both cases significant effort and time are required.The paper aims at developing an artificial neural network (ANN) model to predict the U-value of wooden windows, with the same accuracy of the numerical calculation procedure cited above (therefore greater than the one of the simplified method), but in real time and on the basis of a limited number of parameters. In particular, after a preliminary analysis, only 10 main parameters were selected as network inputs: window typology (windows and French windows), wood kind (hardwood and softwood), frame and shutters thickness, glazing spacer, top frame junction characteristics (size and number of small and large non-ventilated air cavities), U-value of the glazing and glazing size; the U-value of the window is the ANN output. Data set for the training and test of the ANN model consist of respectively 256 and 26 wooden window samples (windows and French windows). In such hypothesis, the developed ANN model, based on a multilayer feed-forward architecture, provides in real time the evaluation of the window U-value with an error of about 1% with respect to the results provided by the CFD numerical procedure), obviously when the input parameters vary within appropriate ranges (corresponding to the variation range of the data used for the network training). The ANN model set-up, therefore, allows to easily determine with high accuracy the thermal performance of wooden windows, saving both money and time. A sensitivity analysis of the main design parameters was also carried out.

End user impacts of automated electrochromic windows in a pilot retrofit application

Automated electrochromic (EC) windows, advanced thermally improved window frames, and a dimmable lighting system were installed in a single, west-facing conference room in Washington DC. The EC windows were commercially available, tungsten-oxide switchable devices, modulated automatically between either fully clear or fully tinted transparent states to control solar gains, daylight, and discomfort glare. Occupants were permitted to manually override the automated EC controls. The system was monitored over a 15-month period under normal occupied conditions. The last six months were used in the analysis. Manual override data were analyzed to assess the EC control system design and user satisfaction with EC operations. Energy and comfort were evaluated using both monitored data and simulations. Of the 328 meetings that occurred over the six-month period, the automatic system was manually overridden on 14 or 4% of the meetings for reasons other than demonstration purposes. When overridden, occupants appeared to have switched the individual zones with deliberation, using a combination of clear and tinted zones and the interior Venetian blinds to produce the desired interior environment. Monitored weekday lighting energy savings were 91% compared to the existing lighting system, which was less efficient, had a higher illuminance setpoint, and no controls. Annual performance was estimated using EnergyPlus, where the existing condition met the ASHRAE 90.1-2007 prescriptive requirements except for a higher window U-value. Annual energy savings were 48% while peak demand savings were 35%.

Applying a multi-objective optimization approach for Design of low-emission cost-effective dwellings

Modern buildings and their HVAC systems are required to be not only energy-efficient but also produce fewer economical and environmental impacts while adhering to an ever-increasing demand for better environment. Research shows that building regulations which depend mainly on building envelope requirements do not guarantee the best environmental and economical solutions. In the current study, a modified multi-objective optimization approach based on Genetic Algorithm is proposed and combined with IDA ICE (building performance simulation program). The combination is used to minimize the carbon dioxide equivalent (CO 2-eq) emissions and the investment cost for a two-storey house and its HVAC system. Heating/cooling energy source, heat recovery type, and six building envelope parameters are considered as design variables. The modified optimization approach performed efficiently with the three studied cases, which address different summer overheating levels, and a set of optimal combinations (Pareto front) was achieved for each case. It is concluded that: (1) compared with initial design, 32% less CO 2-eq emissions and 26% lower investment cost solution could be achieved, (2) the type of heating energy source has a marked influence on the optimal solutions, (3) the influence of the external wall, roof, and floor insulation thickness as well as the window U-value on the energy consumption and thermal comfort level can be reduced into an overall building U-value, (4) to avoid much of summer overheating, dwellings which have insufficient natural ventilation measures could require less insulation than the standard (inconsistent with energy saving requirements) and/or additional cost for shading option.

Sensitivity analysis of building energy use in different climates

Key design parameters (in terms of their influences on building energy consumption) through sensitivity analysis for fully air-conditioned office buildings in five cities (Harbin, Beijing, Shanghai, Kunming and Hong Kong) representing the five major climate zones (severe cold, cold, hot summer and cold winter, mild, and hot summer and warm winter) across China have been identified. These include: i) building envelope - window shading coefficient, window U-value and window-to-wall ratio; ii) internal load – equipment, lighting and occupancy; iii) HVAC system - outdoor fresh air, summer and winter thermostat set points, fan efficiency and fan static pressure; and iv) HVAC plant – chiller COP and boiler efficiency. The influence coefficient (IC, a ratio of the percentage change in computed output (building energy use) to the percentage change in input design parameter was used to indicate likely energy savings. For instance, Hong Kong has a lighting load IC of 0.359, suggesting that a 20% reduction (e.g. from 18 to 14.4 W/m 2 ) in lighting load could result in more than 7% reduction in the total building energy use. Furthermore, Hong Kong also has a summer thermostat set point IC of -1.131, suggesting that a 5% increase in the indoor temperature during the summer (e.g. from 26 to 27.3 o C) could result in about 6% energy savings. Given the growing awareness of adaptive thermal comfort, this could be a no-cost energy-efficient measure with great energy saving potential.

Efficiency of electrically heated windows

This study defined, derived and calculated the efficiency of electrical energy use in electrically heated windows which may be used for improving thermal comfort near glazing. In a cold climate, a warm glass surface is a unique possibility for thermal conditioning. The effects of surface and outdoor temperatures and the U-value of the window on the efficiency of a heated window were analyzed. The calculated results were compared to previously measured ones. The efficiency of a common heated window with a U-value of 1.1 W/m 2 K was about 78% at an outdoor temperature of −10 °C. The highest efficiency of 89% was calculated for a highly insulated window. Efficiency was proportional to the outdoor temperature and practically independent of the inner surface temperature of the window, the effect of which was less than 1%. The correlation of the calculated efficiencies shows that efficiency is primarily dependent on the U-value of the unheated window and can be expressed with very good accuracy for engineering purposes by a simple linear equation of the U-value. The results show that heated glazing is an efficient method for thermal conditioning when properly used.