U-value

Thermal transmittance also known as U-value, is the heat flow rate divided by the area and temperature difference in the surroundings of both sides of a system at a steady state (BSI, 2014); and is a concept utilised to define the insulation properties for building materials. Construction elements with lower U-values are more effective to reduce the energy consumption in buildings due to its capacity to insulate from external weather conditions (usually utilised for colder climates).

U-value rates in windows are comparatively high when compared to other building elements such as floors (0.25 W/m²·K), roofs (0.16 W/m²·K) and walls (0.30 W/m²·K) (Jelle et al., 2012). Likewise, traditional window technologies such as single glazing windows have a U-value comparatively high (5.8 W/m²·K), when analysed against double glazing windows (1.2 W/m²·K), and their impact on the energy consumption of the latter is relatively low (Aguilar-Santana et al., 2020). This contrast makes traditional static glazing technologies a conspicuous element of improvement aiming to reduce housing energy consumption, and therefore a challenge for modern building codes worldwide.

Purpose of experimentation

This paper evaluates a series of static windows that utilise passive technologies to improve heat and optical performance in buildings. This would be achieved by calculating their thermal transmittance coefficients via experimental process, using the heat flux meter method described in the international standard ISO 9869-1:2014. The paper objective includes analysing the significance and implications for testing windows under standardised conditions, as well as providing experimental results on the application of this code. The analysis aims to report the heat flow and temperatures on the glazing surface, as well as the assessment of thermal imagery and the formulae applied to the experimental setup.

Calculation of thermal transmittance

For the calculation of U-values in windows and particularly for commercial products, ISO 10077-1:2017 and ISO 10077-2:2017 are a recommended method to utilise since it combines the geometry of the window and their thermal interactions among components (BSI Standards, 2017b, 2017a). Using this method guarantees the consideration of most elements in a traditional window assembly, these including: glazing(s), frames, shutters and blinds. These methods are particularly helpful when comparing rating reports in commercial window products defined as “declared values” by manufacturers, also denoted as “U_w”.

For the purpose of this research, the U-value calculations will focus on the thermal transmittance of glazing “U_g” or “glazing U-value”; this is to promote an equilibrated comparison of insulating capacities among prototypes. Moreover, the selection of this methodology presents a baseline for comparison given the novel characteristics of the glazing technologies involved, where the heat transfer focuses mainly in the central area of glazing.

For the focus of this experimental process, the selected code was the heat flow meter method (average process) due to its scalability, accuracy and adaptability. Since it is a non-invasive method, it could be adjusted to different glass sizes and save the experimentation process from the use of duplicate specimens. Contrastingly, the size restrictions and measurement periods linked to this standard make this method challenging for achieving a calculation balance. However, the use of infrared thermography helps in the validation of results, comparing the collected data throughout the experimental process (Figure 5).

Heat flow meter method (ISO 9869–1:2014)

Considering that a steady state plausible for on-site experimentations, the ISO standards recommends the average method to estimate the thermal transmittance. Their calculation are based on recording the heat flow rate, ambient and interior temperatures for relatively long periods (at least 72 hours). This method is valid only if the heat transfer coefficients are constant during the entire testing period (BSI, 2014).

Using the average method allows a more robust validation of data, being that it assumes the glass transmittance as a quotient of the mean density of heat flow rate, by the mean temperature difference between the interior and exterior sides of the specimen (defined in equation 1). This assumption is only valid however, when the experimentation occurs isolated from direct solar radiation and is carried out at intervals of 0.5 hours for longer periods (BSI, 2014). Calculations of the thermal resistance and conductance of materials are included in equation 2 and 3, accordingly.

where,

U = thermal transmittance (W/m²·K).

R = thermal resistance (m²·K/W).

Λ = thermal conductance (W/m·K).

Φ = heat flow rate (W).

q = density of heat flow rate (W/m²).

T_sij = interior surface temperature or φ_i (°C).

T_sej = exterior surface temperature or ϕ_e (°C).

By using this method, it is endorsed an accuracy rate in the limits of 14 to 28 %; this as a result of the data acquisition system, operational errors and its variation over the sampling period. A thermal network model is presented in Figure 2 detailing the resistances involved for the analysis of windows. In this particular case, it is described for the utilisation of vacuum glazing with aerogel pillars and the resistance model is included for both the window centre of pane (U_g) as well as for the window edge or frame. This model is particularly useful to detail the temperature points for the glass panes (θ_im–θ_em), including the heat flux monitoring points (ϕ_i–ϕ_e) and the flux of heat (q) through the window. The particular resistances for every window depend on various characteristics: the number of panes, material composition and gas encapsulation (air, argon or vacuum). Additionally, materials utilised in the construction of windows present different thermal conductivities that are directly associated to the heat flux through glazing units, therefore these factors are investigated and listed in Table 2.

Experimental setup

An experimental rig was set at the University of Nottingham, analysing the glass samples by using a WIR36-55 environmental chamber (SSJ System Services Ltd, Rhymney, UK), described in Figure 3. This chamber was modified to adapt the glazing samples on a wooden frame with thermawall insulated boards (described in Figure 4a and 4c), this to allow the controlled temperature at the interior of the chamber (0.0 °C) while promoting a contact with the exterior ambient temperatures (~12.0 °C). Eight k-type thermocouples were installed away from thermal bridges and heat sources. Additionally, two heat flux meters HFP01 (Hukseflux Thermal sensors BV, Delft, NL) with sensitivities of Φ_i = 61.54 μV/(W/m²) and Φ_e = 62.77 μV/(W/m²) were attached at the centre of the pane to monitor the heat flow rate. The location of these heat sensors is described in Figure 4b. This test was continuously reporting results at intervals of 30 minutes for a period of 72 hours in the experimental setup detailed in Figure 5.

The data was recorded on a data acquisition system model DT80 (Omni Instruments Ltd, Dundee, UK) and was complemented by radiant energy imagery using an infrared thermal camera FLIR-E6 (FLIR Systems Inc., Wilsonville, US) every 24 hours on the external side of the experimental setup.

Thermal transmittance (U-value) analysis

The results condensed in Table 3 summarise the thermal transmittance calculations for the nine novel windows samples utilised in this research. The single glazing unit (W01 in Figure 6) and double glazing air filled (W02 in Figure 7) recorded U-values of 5.15 W/m²·K and 5.52 W/m²·K, respectively. These values show a contrast to previously described values of 5.80 W/m²·K and 3.0 W/m²·K presented by (Leung et al., 2020) and (Schultz and Jensen, 2008).

Double glazing window with argon filling (W03) recorded a U-value of 3.09 W/m²·K, comparably higher to 2.0 W/m²·K usually reported for similar glazing structures (Cuce, Young and Riffat, 2014). The utilisation of triple glazing units with a double argon gap filling have demonstrated their capacity to reduce the U-factor to coefficients as low as 0.79 W/m²·K (Lolli and Andresen, 2016), representing a promising assembly for insulating purposes when its overall thickness and window mass (which is usually +30 % heavier) are not a restriction for building implementation.

Novel applications of polysaccharide-based aerogels Konjac-glucomannan (W04) and Konjac-glucomannan strengthened with wheat starch (W05) showed U-values of 3.30 W/m²·K and 3.40 W/m²·K, respectively. The reduction of thermal conductivity by using KGCWS described thermal conductivities of 0.046 W/m·K by previous researchers, using the technology for building applications (Wang et al., 2019). KGCWS relies on its porous structure, insulating ability and sustainable manufacturing process for becoming an alternative that could match the insulating capabilities of argon-based double glazing windows.

Within this experimental range, double glazed windows with aerogel granules filling (W06) reported a U-value of 2.07 W/m²·K; other researchers have found that a similar structure could reduce insulating capacity to further 1.00 W/m²·K or to 0.60 W/m²·K using a monolithic aerogel composite contained in gaps of 14 mm (Buratti and Moretti, 2012). Adopting a similar structure can generate a reduction of up to 19 % in energy savings using aerogel solutions; this when compared to triple glazing units with low-e coatings and argon fillings (Schultz and Jensen, 2008). However, the fragility of pure aerogel material applications requires the silicon aerogel to be contained in a consolidated (and usually with a higher thermal conductivity) container. This without taking into account the visual impact it has in the scattering of light, demonstrated by its traditional translucent appearance. Moreover, applications of aerogel in the glazing industry are very promising, especially when privacy and insulating capabilities are required; this demonstrates an optimal performance on this window for providing indirect natural lighting; especially in clerestories, skylights and greenhouse applications.

Results for the heat insulation solar glass window (W07) showed a U-value of 1.84 W/m²·K that is well below their argon-based counterparts. Similar studies on thermal transmittance on HISG have shown U-values in the range of 1.30 to 1.70 W/m²·K, for low and high transmittances (Li et al., 2019). Alternatively, PV-integrated solutions provide an new approach to insulation characterisation, reporting U-values as low as 1.10 W/m²·K and marking a record for energy production of 40 W in panels with area of coverage of 0.66 m² (Cuce, Young and Riffat, 2014).

Thermal transmittance for manufactured vacuum (W08) and commercial vacuum (W09) are summarised in Figure 7 and Table 3. Experimental values of 2.52 W/m²·K and 1.12 W/m²·K respectively, are in agreement with the manufacturers’ data that described U-values of 0.7 to 1.4 W/m²·K (Aguilar-Santana et al., 2020). Finally, it was found that the experimental procedure could be optimised by testing larger samples of vacuum glass and the use of additional temperature sensors. Limiting the effect of thermal conduction through the window and the addition of more thermocouples on the glass panes would present a more detailed spectrum of the surface temperatures.

Thermal insulance (R-value) analysis

The specific thermal resistance of the glazing technologies tested defined how efficient are windows to resist the heat flow. This parameter is a reciprocal value of the thermal conductance and it measures the transversal resistance of heat flow of a given window based on its temperature difference (θ_e-θ_i in Formula 2). A detailed outline of the estimated R-values is registered in Table 3. Windows with fewer layers of glass, especially single glazing unit (W01) presented a lower thermal insulance (0.20 m²·K/W) attributable to the soda-lime glass resistance which could be strengthen by the use of additional insulation layers. The use of argon filled solutions on this test reported (0.29 and 0.30) m²·K/W for the bio-aerogels solutions, in samples W04 and W05 respectively.

The comparably higher R-value of the granulated aerogel window (0.48 m²·K/W) when contrasted to argon-based filling solutions, demonstrates the insulating capability of the aerogel material itself. The addition of argon to the filling mixture could significantly increase the overall thermal resistance on this window for future analysis. A similar performace was illustrated by the window W07 (heat insulation solar glass) with 0.54 m²·K/W, demonstrating the insulating properties of this technology in comparison to traditional argon gas double glazing units (0.32 m²·K/W).

Commercial vacuum glazing sample W09 showed a proficient thermal resistance (0.90 m²·K/W), in relation to all the tested window solutions detailed in this paper; a higher vacuum pressure in the gap layer and the use of fewer supporting pillars could have helped for decreasing this coefficient. Alternatively, the sample manufactured at The University of Nottingham (W08), reported a lower thermal resistance (0.4 m²·K/W) due probably to a lower vacuum pressure and sample ageing, illustrating the importance of vacuum quality in evacuated windows.

Thermal conductance (Λ-value) analysis

Thermal conductivity in windows respond to several factors such as the material thicknesses, metal coatings and the interior-to-exterior conditions of testing. The calculations for this value are usually made using Fourier’s law of heat conduction, finding that larger differences in temperature are desirable to reduce the uncertainty during experimentation (Zhao et al., 2019). In this regard, the samples analysed demonstrated a proportional relation from thermal conductance to the thermal transmittance values, varying the temperature difference from 6.0 °C in the single glass sample, to 9.8 °C for the commercial vacuum glazing sample. Larger temperature differences can also be tested using the hot-box method, obtaining more reliable estimations using customisable temperature settings. The use of a laser flash apparatus could show a significant reduction the testing period, this being further detailed in Zhao’s research.

Other factor that significantly affects the overall thermal conduction of samples is the particular thermal conductance of the elements composing the prototypes. The glass utilised in the samples manufactured for this experiment have a thermal conductivity of 1.0 W/m·K and reported thermal conductivities of 0.025 W/m·K in double glazing units filled with air, while 0.016 W/m·K was reported in the samples filled with argon gas. Materials utilised for the edge configuration, such as the warm edge space bars (0.03 W/m·K) and metallic edge soldering materials (86.00 W/m·K) have demonstrated to have a direct impact on the overall thermal conductance of windows.

Impact of gas molecules in the gap filling

The number of panes and the gas gap had an important effect in the thermal performance of novel glazing windows. Some researchers have pointed out that smaller gaps between panes filled with argon gas result in heat loss by diffusion between panes, while larger gaps promote internal heat convection (Cuce, 2018). In this regard, samples filled with an air mixture (with a thermal conductivity of 0.026 W/m·K) have higher U-values in general; especially when compared to samples filled with argon gas mixture with 98 % in concentration (Λ = 0.018 W/m·K). This is especially noticeable between the sample W02 and W03, where a difference in the gas filling had a reduction of -44 % in the U-value for the argon-filled sample alone. Furthermore, the thermal conductivity of an air mixture reduced the insulation properties of aerogel in prototype W06, where the integration of an argon gas filling is then advised. A comparison of the impact of the gas molecules on the U-value of windows during the 72 hour testing period is illustrated in Figure 8.

Alternatively, the reduction of gas molecules in the gap (vacuum) has demonstrated to have a considerable impact on the thermal transmittance of manufactured and commercial vacuum samples. These results demonstrate the high potential for vacuum glazing windows for the reduction on the heat transfer in window technologies, decreasing the overall glass thickness (to less than 8.1 mm), and additionally improving the thermal transmittance of the window prototypes. These characteristics made possible to reduce the U-values to 2.52 and 1.12 W/m²·K in the manufactured and commercial vacuum glazing, respectively.

Impact of heat transfer through the window contour

Within the experimental range of the thermal transmittance calculation previously presented, three temperature stations were located in all glazing samples during for the 72 hour procedure. This in order to report the U-value of the windows in their weakest point (usually close to the edge of samples). The summary of the average temperatures reported is included in Table 4.

According to the experimental data presented in Table 3, results confirmed that the thermal transmittance of glazing units is higher at the borderline areas. An example of this behaviour is given for the test on the commercial vacuum window in Figure 9, where the top corner station recorded the highest U-value (1.48 W/m²·K), whilst the bottom corner observed 1.33 W/m²·K; this is comparably 32 % and 19 % higher in contrast to the centre U-value (1.12 W/m²·K). Cuce reported a similar trend on the edge insulation, finding 4 % and 11 % increase on top and bottom corner for an argon filled double glazing unit (Cuce, 2018). This results indicate that vicinity of other materials near the window rim (frame, soldering material, sashes), contribute to an increase in the heat transfer rate due to conduction of the composing materials; similarly as for the occurrence of convection motion in those samples that contain a gas mixture in the internal gaps. However, air leakage from the cooling chamber and diaphragm framing structure (see Figure 5) may have contributed to irregular measurements and abnormalities on the rates described.

Limitations of the study

Due to the nature of the testing and the conceptualisation of thermal transmittance, this study has potential limitations. The test conducted focused mainly on the thermal transmittance of glazing systems (at the centre of pane), also described as “U_g” or U-value of glazing and it requires to be differentiated from the thermal transmittance of a whole window “U_w” that includes the effects of window framing, spacers and the thermal properties of each component. As a result, it is reasonable to identify the results contained in this paper as a comparable measure against reports including similar methodologies and that may be an interesting approach for future studies using these innovative technologies.

Suggestions for future research

The results presented in this paper complement the thermal transmittance studies on glazing due to the novelty of the prototypes as well as the window’s area size. However these outcomes could be further analysed using alternative thermal transmittance calculation approaches, such as the calibrated hot-box method (ISO 12567-1:2010) for comparison purposes. Additionally, the analysis of thermal bridging and air leakage in the samples could lead to improvements in the testing method. The utilisation of alternative window technologies, such as triple glazing with argon filling and triple vacuum glazing are advised for enhancing the insulating capacity of building elements. It is important to highlight that innovative window technologies with low U-value coefficients are often required in building codes as an effective method to reduce the energy consumption in buildings.

This study demonstrate the heat flux meter method as a mechanism to calculate U-value of windows utilising an environmental chamber to induce a temperature difference in window panes with satisfactory results. From this analysis, an estimation of the heat flux and ultimately heat transfer in novel window assemblies is presented.