Hot Box Method Research Articles

Hot box method to investigate U-values for straw bale walls with various structures

Straw bales are used increasingly as rural building material due to their relatively low thermal conductivity and availability as a local product. The U-value is a key parameter for evaluating thermal performance of building walls. The present study investigated the effects of different structural details concerning straw bale joints (SBJs), rebar arrangements, and various finishes on U-values for SBWs using the guarded hot box method. Firstly, modification coefficient was proposed to qualitatively evaluate SBJs, results showed that R-value for straw bales without SBJs was more than three times that of straw bales with SBJs, the optimal U-value was achieved for straw bale wall (SBW) when the inner pressure was 1386 N. Secondly, for rebar arrangements, the SBW with three bars yielded the minimum U-value of 0.53 W·m−2·K−1. Lastly, we found that plastering improved thermal performance of SBW by increasing the air tightness of the wall, and U-values for SBWs with plate tended to be larger than those with plastering attributed to that the uneven surface of the straw bales resulted in an incomplete contact between the plate and the straw bales, causing convective heat transfer, giving rise to U-values of SBWs. Thus, we recommend that clay or lime plaster should be attached to the straw bales first, before installing an exterior plate.

The impact of connector's thermal and geometrical characteristics on the energy performance of facade systems

For energy efficient buildings more accurate evaluation of heat loss through their external envelope is becoming more important. The thermal characteristics of facade systems with connectors are evaluated in different ways, and the results of the experimental measurements showed significant differences comparing to the calculated according to standardised procedure (EN ISO 6946:2017 [1]). To identify the reasons for these discrepancies, the impact of connector thermal conductivity and its dimensions on the thermal performance of facade systems was investigated by analysing the calculated (temperature field calculation software) and experimentally measured (hot box method) values. Two equations of correction factor a were obtained for wall systems with and without air cavities. The corrected heat transfer calculation provides the results of the heat transmittance of wall systems, which are similar as determined using the 3D numerical method with a difference up to 5%.This study identified that the higher is the thermal conductivity of the connector material, the lower part of heat flow through the connector must be included in the total heat transfer through the wall structure, because the heat transfer through the connector is limited by the heat inflow and outflow area of the connector. Additionally, increasing the thickness of the thermal insulation layers, the impact of the connectors on the overall heat transfer does not change significantly in the case of particularly conductive aluminium connectors and even decreases using less thermally conductive steel and stainless steel connectors. The results of this study will contribute to a more accurate assessment of the energy performance of buildings and will be useful in the development of new innovative facade insulation systems.

Thermal performance of hybrid vacuum glazing installed in the wood-aluminium frame

The current development in building energy efficiency towards nearly zero energy buildings (nZEB) represents a number of new challenges to design and construction. One of these major challenges is lowering the need for heating by means of highly insulated glazing units, unique glazing technologies, such as vacuum glazing (VG) or hybrid vacuum glazing (HVG). Hybrid vacuum glazing combines vacuum glazing with double glazing technology by adding one low-emissivity glass to the vacuum glazing using a thermally-improved spacer profile to eliminate the thermal bridge at the edge of the vacuum glazing. The gap between the vacuum glazing and the low-emission glass is filled with the inert gas Argon. The thermal performance of a hybrid vacuum glazing was tested using the guarded hot box method developed in accordance with the requirements of ISO 8990. Hybrid vacuum glazing has thermal transmittance about 0.43 W.m−-2.K−1.

Hot-box measurements to investigate the internal convection of highly insulated loose-fill insulation roof structures

The purpose of this study was to investigate how internal convection in loose-fill insulations affects the insulation properties of highly insulated roof structures. This study consists of laboratory measurements of roof structures insulated by two different blown-in insulations. The measurements are repeated with two temperature differences and air velocities for 300 mm and 600 mm thick insulation layers both with and without trusses, making a total of 24 case studies. The measurements were conducted with equipment using the calibrated hot-box method. The results of the tests show that internal convection can reduce insulation capacity significantly, especially with low-density loose-fill insulations, such as blown-in glass wool. A critical evaluation should be performed as to whether international standards and national building regulations take internal convection into account adequately. According to this study, 5 should be used as a critical modified Rayleigh number for horizontal roof structures with an open upper surface when used insulation material is loose-fill glass wool or wood fibre insulation as in this study.

DEVELOPMENT OF INTERNAL THERMAL SYSTEM FOR RC WALL USING VACUUM INSULATION PANEL AND ITS EVALUATION OF THERMAL INSULATION PERFORMANCE

In order to upgrade internal thermal system for RC wall with less spatial loss, a new site-fabricable thermal system using quite thin vacuum insulation panel(VIP:5 mm and 8 mm) is developed here and compared with traditional XPS thermal system(45 mm) by Calibrated Hot box Method. In this new system, small pieces of plastic blocks in place of furring strips are arranged on lattice points of a square lattice on RC wall with VIPs in between them and with VIP joints covered by site spray foam. VIP system shown close score to Japanese Flat35 standards whereas XPS shown adequate score to it. For further upgrading of new system, improvement of joint heat bridge will be necessary.

Reduced-scale hot box method for thermal characterization of window insulation materials

Highly transparent and low thermal conductivity materials have attracted great interest for the applications in window insulation in recent years. Accurate characterization of the thermal properties including thermal transmittance (U-value) and thermal conductivity of window insulation materials is very important for developing next-generation materials. The conventional hot box method, which is commonly used to measure the U-values of building materials, requires sample sizes > 1.0 m2 to minimize the influence of parasitic heat loss on the measurement accuracy. This characterization challenge hinders the development of novel window materials which are not yet available for large-scale deployment. To address this issue, a reduced-scale hot box system (RHS) was designed to measure both the U-value and the thermal conductivity of specimens that can be more readily made, with sizes < 0.2 m × 0.2 m. The developed reduced-scale hot box system has a very simple testing system and can avoid the challenging thermal insulation requirement of the conventional hot box. The fast turnaround of the reduced-scale hot box system can help facilitate the development of novel insulating materials for energy-efficient windows.

Bio-Waste Thermal Insulation Panel for Sustainable Building Construction in Steady and Unsteady-State Conditions.

Apart from being used as an oil stock for bio-fuels production, an annual crop plant Brassica napus, thought to be an agro-waste, and used either as an animal feed, soil fertilizer or biomass for combustion and thermal energy production. Alternatively, as a bio-based and locally bio-sourced cellulosic material, it could be used as a thermal insulation in sustainable building fabrication, likewise woodchips, a bio-waste from the wood industry. In this study, the above-mentioned bio-waste materials’ thermal properties were identified using a sandwich panel from medium density fibreboard (MDF) and wood studs. Premanufactured panels have been filled in with randomly oriented short-cut rapeseed and with short-cut woodchips. A modified guarded hot box method was used to designate steady and un-steady state thermo-physical parameters of such insulation panels. The examined bio-waste materials absorbed thermal fluctuations of the exterior environment and kept the indoor building environment at constant temperature regardless of such fluctuations. The ability of bio-based sandwich panels to store heat energy was found to be similar to mineral wool. Additionally, VOC (volatile organic compound) emissions of tested materials were identified using gas chromatography-mass spectrometry (GC-MS) combined with headspace solid-phase microextraction (HS-SPME) to declare materials’ harmlessness to indoor environmental quality and human wellbeing. In conclusion, bio-based short-cut materials proved to be a viable environmentally friendly and energy efficient alternative to conventionally used thermal insulations.

Thermal Performance of Window with Vacuum Glazing. Case Study

This document deals with the determination of thermal transmission properties of wood-aluminium window with vacuum glazing. Test measurements are performed with guarded hot-box method at defined temperature difference. They describe how the support pillars influence temperature distribution on the surface and how the edge vacuum glazing influence the heat flow through window. The deformation of the temperature field due to support pillars is surprisingly small and its range is from 0.40 K to 0.51 K with temperature difference on both sides of approximately 33 K. Decrease of internal surface temperature from the middle of glass to the edge is about 18.92 °C – 12.37 °C = 6.55 K, and so it is considerable effect. The effect of the edge on the glazing is not explicitly quantified in the term of heat flow in this document, but is implicitly documented by means of surface temperature. Thermography was used to check if there are touching points between glasses where distance is from 0.15 to 0.2 mm. The vacuum glazing measured in this work was a sample, which was fabricated in Asian producer. The window was tested two years after delivery from the producer.

Integrated Measuring and Control System for Thermal Analysis of Buildings Components in Hot Box Experiments

In this paper, a novel integrated measuring and control system for hot box experiments is presented. The system, based on a general-purpose microcontroller and on a wireless sensors network, is able to fully control the thermal phenomena inside the chambers, as well as the heat flux that involves the specimen wall. Thanks to the continuous measurements of air and surfaces temperatures and energy input into the hot chamber, the thermal behavior of each hot box component is analyzed. A specific algorithm allows the post-process of the measured data for evaluating the specimen wall thermal quantities and for creating 2D and 3D thermal models of each component. The system reliability is tested on a real case represented by a double insulating X-lam wall. The results of the 72 h experiment show the system’s capability to maintain stable temperature set points inside the chambers and to log the temperatures measured by the 135 probes, allowing to know both the U-value of the sample (equal to 0.216 ± 0.01 W/m2K) and the thermal models of all the hot box components. The U-value obtained via hot box method has been compared with the values gathered through theoretical calculation and heat flow meter measurements, showing differences of less than 20%. Finally, thanks to the data postprocessing, the 2D and 3D thermal models of the specimen wall and of the chambers have been recreated.

Thermal Irregularities in Vacuum Glazing

This document deals with the determination of thermal irregularities of wood-aluminium window with vacuum glazing. Test measurements were performed with guarded hot-box method at defined different temperature difference. They describe how the support pillars influence internal surface temperature distribution and how the edge vacuum glazing influence surface temperature around edge of vacuum glazing in wood-aluminium window. The deformation of the temperature field due to support pillars is surprisingly small and its range is from 0.20 K to 0.46 K with temperature difference on both sides of approximately 20 K. Decrease of internal surface temperature from the middle of the vacuum glass to edge is about 20.04 – 16.15 = 3.89 K, and so it is considerable effect. The effect of the edge on the glazing is not explicitly quantified in the term of heat flow in this document, but is implicitly documented by means of surface temperature. Thermography was used to check if there are touching points between glasses where distance is from 0.15 to 0.2 mm. The vacuum glazing measured in this work was a sample, which was fabricated in the Beijing Synergy Vacuum Glazing Technology Co., Ltd. The window was tested one year after delivery from the producer.

Thermal Performance Testing for Window with Vacuum Glazing

This document deals with the determination of thermal transmission properties of wood-aluminium window with vacuum glazing. Test measurements are performed with guarded hot-box method at defined temperature difference. They describe how the support pillars influence temperature distribution on the surface and how the edge vacuum glazing influence the heat flow through window. The deformation of the temperature field due to support pillars is surprisingly small and its range is from 0.20 K to 0.46 K with temperature difference on both sides of approximately 20 K. Decrease of internal surface temperature from the middle of glass to the edge is about 20.04 – 16.15 = 3.89 K.

Dependencies of heat transmittance through the ventilated wall system on thermal conductivity of connectors crossing thermal insulation layer

The ventilated facade systems are widely used for improvement of energy efficiency and reducing of heat losses of newly built buildings and for existing buildings. To reduce the influence of point thermal bridges on heat transfer through the ventilated facades, previous often used aluminium alloy connectors as a change to stainless steel and reinforced plastic connectors. Different thermal characteristics of connectors using in ventilated facade systems, significantly influence the heat transfer coefficient of building’s walls. Previous empirical calculations of the heat transfer through ventilated facade walls with different connectors according to standard methodology and numerical modelling showed significant differences in results, therefore experimental research with the fragments of the ventilated facade systems were carried out using a guarded hotbox method. The aim of this experimental research was to analyse the heat flows through the ventilated wall system with different kind of heat-conductive connectors. Expanded polystyrene foam (λ – 0,031 W/(m∙K)) was used as thermal insulation material, thickness 300 mm, and three types of heat-conductive connections were installed: aluminum alloy (λ - 160 W/(m∙K)), stainless steel (λ - 17 W/(m∙K)) and glass fiber reinforced plastic (λ – 0,23 W/(m∙K)). The measurements in the guarded hotbox were useful for analysis of differences in results according to the standard and numerical calculations methods. The experimental studies showed that the results are very close to the numerical simulation results. The empirical calculation method gave similar results to the other two methods, except in the case of highly heat-conductive connectors.

Definition of an experimental procedure with the hot box method for the thermal performance evaluation of inhomogeneous walls

Research and development of high thermal insulation materials for the construction sector requires an accurate characterization of the wall's performance, since that is the main causes of thermal exchanges between the internal and external boundaries. This paper presents a test procedure developed within the EU Project EFFESUS for evaluating the steady-state thermal performance of a masonry wall. A large-scale mock-up of the inhomogeneous wall was tested in a guarded hot box (GHB) apparatus before and after the application of an aerogel-based material. The methodology proposed in this paper is structured in the following steps: (i) definition of the wall geometry and the percentage of stone and mortar, using walls’ photographic records and geometrical surveys; (ii) precise thermal characterization of the material used; (iii) hygrothermal assessment procedure based on infrared technology (IRT) survey, gravimetric test, and monitoring of the internal relative humidity (RH); (iv) steady-state and dynamic thermal simulation; and (v) detailed set-up of the test using the data retrieved from the thermal surveys and simulations. According to the results of IRT surveys and the dynamic simulations, the mock-up was divided into thermal homogeneous parts, verifying the uniformity of the surface temperature and the heat flux in an isothermal area. This approach was validated both for low and high energy performance walls. Results show that the thermal flux was reduced to one third after the application of the aerogel.

Static and Dynamic Thermal Characterization of Timber Frame/Wheat (Triticum Aestivum) Chaff Thermal Insulation Panel for Sustainable Building Construction

Common wheat is currently the most widely grown cereal and its grains are mainly used to produce white bread. Contrary to plant seeds, the plant itself may be considered an agricultural waste which is used as an animal feed, soil fertilizer, or pelletized solid fuel to produce heat energy. While wheat straw is used widely in sustainable building construction, mostly as straw bales, the use of the thin dry husk around the grain, the wheat chaff, remained unexplored. An insulation panel made of a medium-density fiberboard (MDF) envelope and wooden studs was manufactured. The 150-mm gap between the MDFs was filled with dry wheat chaff. The thermal transmittance of the panel was determined under static and dynamic thermal conditions using a modified guarded hot box method. The results were compared to an insulation panel of the same construction that was filled with mineral wool. The thermal transmittance of wheat chaff under steady-state and dynamic conditions was 0.307 W m−2 K−1 and 0.298 W m−2 K−1, respectively. While in steady-state conditions the mineral wool outperforms the wheat chaff insulation, after a period of dynamic thermal loading, the ability of both insulations to resist heat-energy dissipation becomes similar. In conclusion, wheat chaff agrowaste seems to be a promising environmentally friendly alternative to artificial thermal insulation. Moreover, the determination of the thermal transmittance of bio-based materials with relatively high specific heat capacities under dynamic thermal loading, provides more accurate results, compared to steady-state conditions.

Thermal Transmittance of Porous Hollow Clay Brick by Guarded Hot Box Method

The thermal property of a porous hollow clay brick was determined by measuring the thermal transmittance of the wall made of porous hollow clay bricks. Prior to the production of porous hollow clay bricks, nonporous and porous tiny clay bricks were prepared to determine the physico-mechanical properties by modifying the amount of wood flour and firing temperature. The bricks were produced by uniaxial pressing and then fired in an electric furnace. Their physico-mechanical properties were measured by water absorption, apparent porosity, bulk density, and compressive strength. The porous tiny clay bricks were produced with three types of wood flour: coarse wood flour (1–0.36 mm), medium-sized wood flour (0.36–0.15 mm), and fine wood flour (< 0.08 mm). The thermal transmittance of porous hollow clay bricks was determined through the guarded hot box method, which measures the wall made of porous hollow clay bricks and nonporous cement bricks. The two walls had a thermal transmittance of 1.42 and 2.72 \(\hbox {W}\cdot \hbox {m}^{-2}\cdot \hbox {K}^{-1}\), respectively. The difference in thermal transmittance was due to the pores created with fine wood flour (< 0.08 mm) as a pore-forming agent.

Thermal resistance and conductivity of recycled construction and demolition waste (RCDW) concrete blocks

In Brazil, studies to reuse construction and demolition waste are a special issue because a large amount of this material has been delivered to the public landfills and in illegal places. Some researchers have suggested reusing this material in building elements, such as bricks or blocks. It is possible to find a lot of researches in physical/mechanical characterization, while little effort has been made to characterize recycled construction and demolition waste blocks (RCDW) for their thermal properties. The aim of this work was to characterize the RCDW thermal resistance and conductivity in order to provide subsidies for a building's thermal performance analysis. The hot-box method was adapted, together with measuring techniques with a heat-flow meter to determine the RCDW thermal properties. The results indicated that the RCDW block overall thermal resistance and thermal conductivity in the solid region was within the intervals of 0.33≤RT≤0.41m2KW-1 and 0.60≤l≤0.78Wm-1K-1, respectively. The lower resistance and conductivity values are justified by the presence of aggregate with a lower density and lower thermal conductivity than the natural aggregate.

A new simple method to measure wall thermal transmittance in situ and its adaptability analysis

The Simple Hot Box - Heat Flow Meter Method is proposed to measure the wall thermal transmittance conveniently in situ with the high accuracy on the basis of the previous study, while it avoids the heavy equipment of the Hot Box Method and overcomes outdoor and indoor thermal environment limitation of the Heat Flow Meter Method. According to these, this study mainly analyzed the determination method of the hot box dimension and the effect of the heating temperature by the numerical simulation. Results shows due to the local heating from the hot box, the wall can be divided to four zones with the different thermal characteristics, where a one-dimensional heat transfer zone is the optimum one to past the thermocouples and heat flow meters. With increasing the hot box dimension, the one-dimensional heat transfer zone increases linearly and the heat dissipation zone is constant, defining the minimum hot box dimension. The minimum dimension increases linearly with increasing the wall thickness or equivalent thermal conductivity. When temperature difference is more than 20°C, its effect can be ignored on the minimum dimension. And the maximum system error decreases by up to 4.4–7.5% with the temperature difference increasement from 10°C to 30°C. Taking wall thickness, wall equivalent thermal conductivity, temperature difference as variables, a regression formula is obtained to determine the minimum box dimension and the recommended values of the hot box dimension are also offered.

伝統技能で施工された土壁（木舞・貫入り）の熱物性測定

Tsuchikabe (wall made of a mixture of soil, water, sand, and straw) made of natural materials does not induce sick building syndrome. While being built, tsuchikabe requires no VOC-containing materials. A building with tsuchikabe produces fewer CO2 emissions. It therefore holds promise for reducing LCC (CO2). Buildings with tsuchikabe, however, are on the decrease because of the long work period and high labor costs. The authors intend to verify whether the indoor environmental control performance of the tsuchikabe, which uses natural materials, is worth its building price. To begin with, the thermal insulation performance of the tsuchikabe (composite material) built by a plasterer using traditional skills was measured as reported in this paper. Two types of specimens were used: one measuring 900 mm on each side that allows measurement of equivalent thermal conductivity with the calibrated hot box method (JIS A1420 Annex B) (hereinafter Specimen A) and the other measuring 300 mm on each side that allows measurement of direct thermal conductivity with the heat flow meter method (JIS A1412-2) (hereinafter Specimen B). Specimen A has the same shape and size as the actual material. Specimen A was prepared over a foundation of two nogs, six bamboo laths, and 28 wattles, with plasterers applying a covering of ara tsuchi (a mixture of paddy field earth, rice straw and water) (55 mm in thickness) and a browning coat of nakanuri tsuchi (mixture of river sand, mountain earth, water and rice straw) (both sides: 10 mm in thickness). Specimen B was prepared over a foundation of 12 wattles and a browning coat of nakanuri tsuchi (60 mm in thickness). As it turned out in our case, the interval of wattles used in the specimens ranged from 22 to 24 mm. This spacing is smaller than that of the standard building procedure. The authors confirmed the occurrence of dry shrinkage of tsuchikabe as reported in this paper. The volumetric shrinkage (average) under the steady state (air dry condition) is 4.7% for Specimen A and 5.5% for Specimen B. Plasterers generally estimate dry shrinkage of tsuchikabe prior to building one. The equivalent thermal conductivity of Specimen A as a composite material was 0.392 W/m·K. The thermal conductivity of Specimen B was 0.344 W/m·K. The equivalent thermal conductivity of Specimen A was estimated using the thermal conductivity of a single material (wattle, earth, and nog (cedar)). The thermal conductivity of the wattle part (wattles and earth) is 0.394 W/m·K, and the thermal conductivity of the nog part (wattles, earth and nogs) is 0.336 W/m·K. The authors consider the difference between both measurement values is attributable to nogs. Assuming the wattle part and the nog part conduct heat in parallel, the apparent thermal conductivity of the composite material can explain the measurement value at a sufficient accuracy. The thermophysical property of tsuchikabe built by a plasterer was measured as reported in this paper. The thermal insulation performance of the tsuchikabe is no larger than that of other building materials when compared in terms of equivalent thermal conductivity. However, analysis confirmed that sufficient thermal insulation performance is duly expected to occur because of tsuchikabe's thermal capacity. When evaluation of the humidity adjusting performance of the wall progresses, it will be able to evaluate the indoor environmental control performance and the validity of the construction price.

Calibration Method of Cool Box Used for Measuring of Thermal Conductivity Coefficient

One of the methods used for determining of the thermal conductivity coefficient value of building materials is the Calibrated and Guarded Hot Box method. This method is based on the principle of measuring the heat flux density that passes through the test specimen at a temperature difference between warm and cool parts of the measuring box. For measuring of thermal conductivity coefficient is at the Faculty of Civil Engineering of VŠB-TUO used the Cool Box. Measuring in Cool Box uses the same principle of measuring as Calibrated and Guarded Hot Box. For creating temperature difference during the measurements at the Cool Box is not used heat, but cold. Measurement of the thermal conductivity coefficient by a Cool Box is very complex in terms of measurement accuracy. That the resulting values as accurate as possible, it is necessary to calibrate itself refrigerated cabinets and all sensors that are used for measurements. This paper describes the principles of calibration of Cool Box and other components, which are used for measuring the thermal conductivity coefficient of selected natural building materials. Determination of the thermal insulation properties of selected materials is part of a long-term research focused on conventional and unconventional natural building materials.

Mechanical characterization and thermal conductivity measurements using of a new 'small hot-box' apparatus: innovative insulating reinforced coatings analysis

The insulation of the building envelope contributes to the reduction of annual energy consumptions. The development of new materials, such as fibre reinforced insulating coatings, could be useful in order to obtain an effective solution for the improvement of energy performance and for reinforcement of the walls.The evaluation of the thermal and mechanical characteristics of building coatings with good thermal insulation properties and mechanical resistance is the aim of the present paper. A new experimental apparatus, Small Hot-Box, built at the University of Perugia, was used for the evaluation of the thermal conductivity of four different coatings (with and without a reinforced structure). No European standards are available for this innovative facility, but it takes into account some prescriptions of EN ISO 8990. The apparatus was calibrated with materials of known thermal conductivity. The thermal conductivity can be calculated with both the thermal flux meter and the Hot Box method. Good values of the thermal conductivity, in the range of 0.09–0.11W/mK were found for all the samples, except for one (0.21–0.24W/mK).Mechanical tests were also carried out in laboratory on all the samples and results were used to evaluate the shear modulus and strength of the wall panels.