Thermal Transmittance Values Research Articles

Influence of heating systems on thermal transmittance evaluations: Simulations, experimental measurements and data post-processing

Nowadays, understanding the actual performance of building components is one of the key factor to achieve energy savings. For this reason, on-site measurements are essential but the boundary conditions during surveys can affect the final results. This can occur during heat flow meter measurements, when the thermal transmittance value of a wall can be influenced by disturbing factors, such as the heating system power-on and off. Due to this, the aim of this study is to investigate the influence of these disturbing factors, moving away from steady-state conditions. This research is divided in two main steps: a first critical analysis of data obtained by in-situ measurements and an investigation of how the mentioned disturbing factors can affect the final results, employing a FEM code, where stationary conditions are not respected; a second analysis related to the data post-processing procedures, proposing a new supplementary approach able to exclude heat flow distortions and able to obtain measured U-values closer to the calculated ones, according to ISO 6946. Starting from simulations and on-site measurements, the proposed method was preliminary validated, analyzing actual case studies characterized by heating systems with radiators and obtaining preliminary satisfying results. The simulations allowed to assess a reduction in the difference between the measured and the calculated U-value that goes from +22.1% to +0.7%. Post processing of experimental data with the proposed methodology allowed to significantly reduce the difference between measured and calculated U-values (from +36.9% to −7.6% in the best case study). Starting from the preliminary results, the proposed approach seems to be promising with U-value corrections in accordance with the theoretical ones.

Heat transfer through window frames in EnergyPlus: model evaluation and improvement

ABSTRACTWindow frames can significantly affect the energy demand of buildings. Due to their complex non-planar geometry, two-dimensional, 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/m2K. 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/m2K in 52% of the occurrences, and to less than 0.47 W/m2K for aluminium windows.

Experimental investigation of thermal bridges in building at real conditions

In recent time the energy consumption of buildings may be reduced by the application of modern technologies in the construction industry. Modern building materials ensure a reduction of heat losses. However, studies show that thermal bridges may cause up to 30% of the additional heat losses through the building envelope. Therefore, a one key aspect in assessing the real thermal state of buildings is the identification of the heat losses through thermal bridges. The analytical, experimental and numerical methods are used for the assessment of thermal transmittance value of building. In the paper the authors present the experimental research on heat losses through the building under real winter conditions. Infrared thermovision technique has been used to the thermal bridges assessment in situ. IR thermography technique allowed the determination of the influence of thermal bridges on the additional heat losses. For the obtaining the surface emissivity the measurements have been also performed with the use of thermocouples system. Numerical validation of the experimental results has been performed.

Energy refurbishment in existing buildings: thermal bridge correction according to DM 26/06/2015 limit values

The recent Italian decree 26.06.2015, concerning minimum requirements of new and renewed buildings, allows a 30% derogation on mean thermal transmittance limit values in case of insufflation with insulating materials in cavity walls in buildings object of refurbishment. The purpose of the present paper is to study the thermal bridges optimal correction in order to verify the respect of the mean thermal transmittance mandatory values. A typical repetitive module of a building façade with windows, sub windows and rolling shutter boxes is analysed. Thermal bridges due to pillars, beams, lintels, thresholds, jambs and thickness reductions of walls situated under windows are considered. The thermal bridges correction is obtained by means of high performance insulating material layers on the outer side of thermal bridges. A 2-D numerical model is used to determine the linear thermal transmittance of junctions. Calculations carried out by the authors show that the limit values of thermal transmittance provided by the decree are too severe to be respected, even in case of derogation.

Calculation and compliance procedures of thermal bridges in energy calculations in various European countries

EPBD directive 2002 and its recast 2010 have led to significant efforts in Member States to improve the energy performance of buildings. Important aspect of this goal is the compliance of building energy performance assessment which needs developed procedures in order to be able to achieve stringent energy targets in practice. Transmission characteristics have a significant role in energy efficient buildings. QUALICHeCK project conducted a review of thermal bridges in energy calculation and compliance procedures in nine European countries (Austria, Belgium, Cyprus, Estonia, France, Greece, Romania, Spain, Sweden). Results showed that there are four main types of methods to take thermal bridges into account in transmission heat loss calculation: the detailed calculation based on linear thermal transmittance values, simple basic rules, default transmittance values, and mean U-values. Regarding the compliance, review showed that often there are no specific thermal bridge related compliance procedures. General conclusion of this study was that compliance frameworks needs to be extended in order to be able to assess as built energy performance. It is common approach in many countries that control mechanisms stop with building permit phase.

ALTERNATIVE METHOD FOR ON SITE EVALUATION OF THERMAL TRANSMITTANCE

Thermal transmittance or U-value is an indicator of the building envelope thermal properties and a key parameter for evaluation of heat losses through the building elements due to heat transmission. It can be determined by calculation based on thermal characteristics of the building element layers. However, this value does not take into account the effects of irregularities and degradation of certain elements of the envelope caused by aging, which may lead to errors in calculation of the heat losses. An effective and simple method for determination of thermal transmittance is in situ measurement, which is governed by the ISO 9869-1:2014 that defines heat flow meter method. This relatively expensive method leaves marks and damages surface of the building element. Furthermore, the final result is not always reliable, in particular when the building element is light or when the weather conditions are not suitable. In order to avoid the above mentioned problems and to estimate the real thermal transmittance value an alternative experimental method, here referred as the natural convection and radiation method, is proposed in this paper. For determination of thermal transmittance, this method requires only temperatures of inside and outside air, as well as the inner wall surface temperature. A detailed statistical analysis, performed by the software package SPSS ver. 20, shows several more advantages of this method comparing to the standard heat flow meter one, besides economic and non-destructive benefits.

Comparison of Various Analysis Methods Based on Heat Flowmeters and Infrared Thermography Measurements for the Evaluation of the In Situ Thermal Transmittance of Opaque Exterior Walls

There are several methods to obtain the in situ thermal transmittance value (U-value) of building envelopes from on-site data, including the three approaches of the progressive average method, average method considering the thermal storage effect, and dynamic method for deriving the U-value from heat flowmeter (HFM) measurements and the four methods with different formulas to analyze infrared thermography (IRT) measurement data. Since each of these methods considers different parameters and the non-steady characteristics of the heat transfer in building walls in their own way, discrepancies may occur among the obtained results. This study evaluates and compares the in situ U-values by using various methods of analyzing HFM and IRT measurement data. Further, by investigating buildings with similar materials and identical stratigraphies, but with different construction years, we analyze the discrepancy between the designed and measured values caused by material deterioration and evaluate the errors according to the analysis method. The percentage deviation between the U-values obtained by the three methods from the HFM data is found to be satisfactory, being within 10%. When compared with the results of the progressive average method, the deviations for the four different IRT-measurement-based methods vary greatly, being in the range of 6–43%.

Experimental measurement of hermetic edge seal's thermal conductivity for the thermal transmittance prediction of triple vacuum glazing

Thermal conductivity of hermetic edge-sealing materials plays an important part in the thermal transmittance (U-value) of the triple vacuum glazing. Thermal conductivity of Cerasolzer CS186 alloy and J-B Weld epoxy-steel resin were measured and validated with the mild-steel and indium using transient plane source method with a sensor element of double spiral and resistance thermometer in a hot disk thermal constants analyser TPS2500s are reported. The thermal conductivity data of Cerasolzer CS186 alloy and J-B Weld epoxy steel resin were measured to be 46.49Wm−1K−1 and 7.47Wm−1K−1, with the deviations (using analytical method) of ±4% and ±7% respectively. These values were utilised to predict the thermal transmittance value of triple vacuum glazing using 3D finite element model. The simulated results show the centre-of-glass and total U-value of 300mm × 300mm triple vacuum glazing to be 0.33Wm−2K−1 and 1.05Wm−2K−1, respectively. The influence of such a wide edge seal on the temperature loss spreading from the edge to the central glazing area is analysed, in which the predictions show wider edge seal has affected the centre-of-glass U-value to 0.043Wm−2K−1 due to the temperature gradient loss spread to 54mm and 84mm on the cold and warm side respectively.

Environmental assessment of a nano-technological aerogel-based panel for building insulation

This study assesses the life-cycle environmental implications linked to the energy efficiency improvement by a nano-technological aerogel based panel insulation solution. A cradle to grave approach has been taken for the environmental evaluation of the product life-cycle, including its integration in an existing residential building for the evaluation of the building’s use phase. The model developed has been also assessed in the 5 European climate zones, evaluating the different performance due to the different weather conditions and the effect of increasing the thickness used. Also, an evaluation of the impacts achieved depending on the heating source used, together with the comparative analysis with other traditional insulation materials complete the paper.This innovative aerogel based panel takes advantage of nanotechnology to increase its lifetime and reduce its thickness, in-building installation time and cost in comparison to conventional insulating materials. As a result, due to its low thermal conductivity (0.015 W/mK), only 10 mm aerogel based insulation panel is needed to achieve the same level of insulation of 25 mm thickness of standard Expanded Polystyrene Panel. This difference increases when the passive house requirements of façade thermal insulation are considered with thermal transmittance values in the range between 0.1 and 0.15 W/(m2K). From the results, a reasonable thickness of insulation material is available only with Aeropan in comparison to Expanded Polystyrene, Extruded Polystyrene and Mineral Wool, demonstrating its suitability in the accomplishment of passive house requirements with a significant reduction of the space needed.Thus, net life-cycle environmental benefits were found in all scenarios making this product suitable for the retrofitting of existing buildings by both, external or internal thermal insulation, minimizing at the same time the space occupied by the insulation solution.

Numerical analysis of thermal transmittance of hollow concrete blocks

Since the 1970s, due to the worldwide energy crisis, some countries have adopted severe legislation to promote energy efficiency in buildings. The electrical power utilized in buildings is directly related to the equipment required to provide thermal comfort to occupants. In this context, one of the most important factors that determine the choice of these systems is the conduction load through the building envelope. In several countries, hollow concrete blocks have become common over the past few years. Therefore, for the assessment of building energy efficiency, the thermal transmittance evaluation of hollow blocks is required. This study obtains values of thermal transmittance for different cavity configurations of concrete blocks by performing computational fluid dynamics simulations. These transmittances are compared to those calculated using the methodology presented in international standards. In the “Results” section, it is shown that large cavities provide higher transmittance values due to convection and that thermal radiation may play an important role in the overall heat transfer through concrete hollow blocks. Some methods used to reduce the thermal transmittance are presented and discussed. Finally, thermal transmittance of cavities with the same wall height has been compared with the ones of a single block and different strategies are considered for reducing the thermal transmittance of concrete hollow blocks.

Experimental investigation of the influence of convective and radiative heat transfers on thermal transmittance measurements

Walls thermal transmittance value (U-value) is essential to identify the performance of a wall and it is used for the energy labeling procedure. If on one hand the heat-flow meter can measure heat fluxes across its plate giving back the wall's U-value, on the other hand, if air temperatures and wall's inner surface temperatures are measured, it is necessary to set the total heat transfer coefficient value in order to calculate the heat fluxes. Many correlations were developed in scientific literature to quantify the convective heat transfer coefficient and it is possible to distinguish similarity based and experimentally ones.In this paper, the actual total heat transfer coefficients in different case studies were obtained by measuring the physical parameters that are needed to define them. Convective and radiative contributions were separately evaluated and, finally, actual convective heat transfer coefficients were compared with the same coefficients obtained by applying the correlations available in literature and with the constant value suggested by the Standard.The aim of this study is to analyze the differences between the various coefficients values and their influence on the thermal transmittance evaluation, in order to better understand the existing correlations and the UNI EN ISO 6946 applicability. This is an initial part of a research which aims to provide an overall more accurate representation of the building behavior, forthcoming developments will deal with the analysis of heat transfer on the outer side of walls.

Thermal behaviour assessment of a novel vertical greenery module system: first results of a long-term monitoring campaign in an outdoor test cell

Vertical greenery modular systems (VGMSs) are an increasingly widespread building envelope solution aimed at improving the aesthetical quality of both new and existing facades, contemporarily achieving high energy efficiency performance. Within a research project, a new prototype of VGMS was developed, designed and tested. An experimental monitoring campaign was carried out on a test cell located in Turin (northern Italy), aimed at assessing both biometric parameters and energy-related issues. Two different types of growing media and two plant species, Lonicera nitida L. and Bergenia cordifolia L., have been tested on a south-facing lightweight wall. Results have been compared to the same wall without VGMS and plaster finished, in order to characterise the thermal insulation effectiveness in the winter period and the heat gain reduction in the summer period. Measured equivalent thermal transmittance values of the green modular system showed a 40 % reduction, when compared to the plastered wall, thus noticeably impacting on the energy crossing the facade during the heating season. Benefits of the VGMS are measured also during the summer season, when the presence of vegetation lowers the outdoor surface temperatures of the wall up to 23 °C compared to the plastered finishing, with a positive effect on outdoor comfort and urban heat island mitigation. Nevertheless, as far as the entering energies are concerned, not significant reduction was observed for VGMS, compared to the reference plastered wall, since the green coverage acts as a thermal buffer and solar radiation is stored and slowly released to the indoor environment.

Methodical Aspects of Determination of Parameters Which Characterize Thermal Balance of a Plastic Tunnel

Abstract The paper presents methodology of measuring the values which characterize the thermal balance in a horticultural facility. Thermal transmittance through a plastic tunnel cover and the ratio of solar radiation energy conversion into heat were analysed in detail. The research was carried out in real time with closed ventilators, where no plants were cultivated during the tests. The utility surface area was 144 m2, the cover was 289 m2 and its volume was 508 m3. Basing on the standard relations, first of all, thermal transmittance through a cover was measured. The ratio was measured with the use of standard criteria equations. Then, a correlation equation, which makes its course dependent on the climate parameters (the wind speed and temperature difference between the interior of the facility and its surroundings), was found. Knowing the course of the ratio in the function of the climate parameters, the value of the ratio of solar radiation conversion into heat stored in the facility was measured. Both parameters (thermal transmittance and solar radiation conversion ratios) were calculated from the thermal balance equaition where the change of the energy stored inside the facility was compared to the heat losses through a cover (for thenual transmittance) and the profit (in case of solar radiation conversion). For the obtained values, using the non-linear estimation procedure (with quasi-Newtom method with maintenance of the correlation coefficient of 0.001) the change of both ratios in the function of easily measured parameters of climate inside and outside the facility was found out. In the investigated conditions, the average value of thermal transmittance through a cover was 5.32 W·m−2·K−1, and the solar radiation conversion into heat was 0.36. The values and their course from the climate parameters inside and outside the facility may be used for controlling the amount of supplied heat and location of ventilators.

Thermal bridges in a prefabricated wooden house: comparison between evaluation methods

ABSTRACTThe major aim of this study was to create a numerical model of thermal bridges in a wooden structure and to discover differences between factual values of linear thermal transmittance utilized in the finite-element (FE) modelling and values given by the European normative method (EN ISO 14683). Thermal bridges of wooden structures were detected using thermocamera FLIR S65 for infrared detection. These measurements allow displaying the temperature distribution of the thermal scan and proving the positive correlation between heat flux and change in the evaluated temperatures of a thermal bridge. A three-dimensional FE model of the structure was prepared for the thermal analysis using the engineering software ANSYS Workbench 13. The transmission heat loss coefficients of a prefabricated wooden house were established based on the Czech normative method (ČSN 730540-4). Linear thermal transmittance values calculated from the thermal analysis and the normative method given by EN ISO 10211 were compared. The results show that even if the transmission heat loss coefficients meet the requirements designated by standards, the real heat losses are increased by thermal bridges involved in the construction. Their influence is determined by the linear transmittance value which is usually not exactly established.

Potential of Vegetation in Improving Indoor Thermal Comfort and Natural Ventilation

According to the EPBD (2010) whilst improving the thermal performance of buildings good or at least tolerable thermal comfort conditions must be provided. But better thermal insulation and more airtight buildings increase the risk of summer overheating which makes mechanical cooling inevitable. This idea has been verified by the tendency of the last decade, when people were willing to install and use more frequently air conditioning devices during the summer heatwaves – increasing their energy consumption and electricity bills at the same time. We cannot neglect the importance of studies triggering an efficient way to minimise the cooling load of residential buildings by obstructing solar radiation. The usage of plants in front of transparent surfaces of the façade can avoid indoor overheating. Deciduous plants obstruct buildings’ solar access so that the microclimate around the building is improved too. The use of Green Infrastructure in different levels of planning processes, which would provide sustainable solutions for urban management, is also prescribed in the EU Biodiversity Strategy 2020. Of course in order to investigate the actual effect of trees on indoor thermal comfort we should take into consideration a list of other factors: such as orientation the type and thermal properties of the windows / transparent structures used, and the thermal transmittance values and heat storage capacity of the building. If we have taken into consideration the mentioned factors during simulation we can prove the effectiveness of vegetation for each case. Simulations are made on the base of transparency measurements carried out during the summer of 2014. The shading efficiency of trees is a species-specific attribute because of the varying crown structure and leaf density. Our analyses aimed at the quantification of the transmissivity of characteristic individuals of three frequently planted species (Celtis occidentalis, Sophora japonica, Tilia cordata). The measured data were the amount of transmitted shortwave radiation, compared with a measurement point under unobstructed sunlight. In preliminary studies we have shown that depending on species – a tree in front of the façade can decrease the solar gain on internal horizontal surface up to ~18-30 per cents. As the tree obstructs the solar access of the wall and that of transparent surfaces, a difference in indoor comfort is to be observed too.

Impact of Growing Conditions of Spruce Wood on Thermal Performance of Windows

In order to eliminate this wood handicap comparing to other material bases, chiefly PVC, which are used for window production, it is inevitable to know the thermal transmittance values of wood grown in Slovakia region, eventually in Czech Republic, to be able to modify the calculated or measured values of window thermal transmittance. Solution for the research problem is documented by the spruce or fir wood example, which are used the most in window production in SR.

Masonry wall panels retrofitted with thermal-insulating GFRP-reinforced jacketing

Today there is a need to provide thermally efficient walls, while at the same time to increase the mechanical properties of old unreinforced masonry walls that will not require large amounts of energy in the retrofitting or deconstruction processes. To address this problem, this paper gives the results of shear tests carried out on masonry panels made of solid bricks retrofitted with a new technique based on the use of glass fiber-reinforced polymers (GFRP) grids inserted into a thermal insulating jacketing. This was made of different low-strength lime-based mortars. Tests were carried out in laboratory and results were used for the determination of the shear modulus and strength of the wall panels before and after the application of the GFRP reinforcement. Retrofitted panels exhibited a significant enhancement in the lateral capacity when compared to the control panels. The thermal performance of the proposed mortars was also investigated both with and without GFRP. Low values of thermal conductivity were found, especially for the samples with GFRP; a reduction of the thermal transmittance value in the 34–45 % range was also obtained by applying 45 mm layer of coating in conventional masonry walls.

단열 기준에 따른 기둥-보 목조주택의 단열 상세 및 에너지 성능

국립산림과학원에서는 한옥의 전통건축기술을 바탕으로 한국형 기둥-보 목조주택인 ?그린 프로젝트를 2006년부터 진행해오고 있다. 최근에는 에너지 절약 기술요소를 적용하여 건물의 에너지성능을 개선하고자 하였다. 본 연구에서는 강화된 단열기준에 따른 기둥-보 목조벽체의 단열상세를 제시하고 그에 따른 건물 에너지 절감 효과를 평가하고자 하였다. 건물외피의 열관류율은 현행법 수준과 저에너지주택인 패시브하우스 수준인 두 단계로 나누었으며 건물에너지 성능을 평가하기 위하여 건물의 기밀성능은 국내 신축 목조주택 수준인 <TEX>$3.0h^{-1}$</TEX>과 패시브하우스 수준이 <TEX>$0.6h^{-1}$</TEX> 두 단계로 나누어 열관류율과 기밀성능의 조합에 따라 4가지 경우에 대한 건물에너지 성능 평가가 이루어졌다. 대상건물은 서울에 위치하였으며 에너지 성능은 상용 건물에너지성능 평가 프로그램인 CE3가 사용되었다. 강화된 열관류율에 따라 시뮬레이션을 진행한 결과 연간 난방에너지 요구량은 <TEX>$14kWh/(m^2{\cdot}a)$</TEX> 절감효과를 나타냈다. Han-green project, which pursues Korean style post and beam timber house with traditional construction technique of Han-ok, has been carried out in KFRI (Korea Forest Research Institute) since 2006. Recently, the improvement of its building energy performance was studied with energy-saving elements. This study was conducted to provide the insulation details of building envelopes in a post-beam timber house for recent enhanced insulation standards and following effect on building energy performance. The level of thermal transmittance (U-value) values of building envelopes was composed of two stages: present Korean insulation standards and passive house. To evaluate building energy performance, the building airtightness values of two stages was ACH50 = <TEX>$3.0h^{-1}$</TEX> for common domestic timber house constructed recently, and ACH50 = <TEX>$0.6h^{-1}$</TEX> for passive house. Consequently, four cases of the building energy performance according to the combination of U-value with airtightness were evaluated. The test house for evaluation was located in Seoul and its energy performance was evaluated with CE3 commercial building energy simulation program. The result showed that enhanced insulation from level I to II reduced <TEX>$14kWh/(m^2{\cdot}a)$</TEX> of annual heating energy demand regardless of airtightness.

Impact of Opaque Building Envelope Configuration on the Heating and Cooling Energy Need of a Single Family House in Cold Climates

The building envelope plays a crucial role in reducing operational energy demand. The two main properties of the building envelope to look at in this perspective are thermal transmittance (U, W/m2K1) and thermal inertia, which is often expressed by a metric called periodic thermal transmittance (Yie, W/m2K1). These two properties are also traditionally connected to two different energy demands: while thermal transmittance is crucial to reduce heating energy demand, thermal inertia has an impact on energy demand for cooling. However, a question may rise about the impact of each property on the other demand – i.e. the impact of thermal insulation on the cooling energy demand and the impact of thermal inertia on the heating demand.A parametric analysis on the influence of the thermal inertia on the energy performance of a single family house in a Nordic climate has been carried out to answer to this question. “Ideal envelopes” have been modelled and simulated, meaning that used thermophysical properties do not represent any specific configuration, but the entire spectrum of technological configurations.The results show that the influence of the thermal inertia on the heating energy need is very limited. Solutions characterized by very high thermal inertia do not allow heating energy demand to be sensibly decreased. Periodic thermal transmittance has instead an impact on the heating load. The impact of the thermal inertia is also assessed in the warmer season, and the results show that this parameter does not significantly contribute to a better behavior (especially when the upper limit of the indoor air temperature is controlled). Limitations to value of thermal transmittance are also pointed out to avoid non-energy effective conditions when the total (heating plus cooling) annual performance is considered.

Aramid/glass fiber-reinforced thermal break – thermal and structural performance

Energetically weak points in thermally insulated building envelopes are formed by thermal breaks that are implemented to structurally connect external balconies to internal slabs. Current thermal breaks comprise stainless steel bars that penetrate the insulation layer and thus cause significant thermal losses. A new thermal break composed of highly insulating aramid and glass fiber-reinforced polymer (AFRP and GFRP) components and aerogel granulate insulation materials was developed and the first prototypes of the load-bearing components were experimentally investigated. The use of AFRP leads to an excellent thermal performance with linear thermal transmittance values of below 0.15W/mK. The experimental prototype investigations confirmed the targeted ductile failure mode through concrete crushing in the component–concrete interfaces. The serviceability limit state conditions are met for a targeted balcony cantilever span of 4.0m. The material-tailored components can be manufactured by fully automated processes such as filament and tape winding and pultrusion to economically produce large quantities within a short time.