1. Introduction
Buildings are responsible for consuming approximately 40% of energy and emitting 36% of CO2 in the EU [1]. Reducing energy consumption in buildings is one of the main areas that require energy efficient interventions for reducing their carbon footprint and achieving the emission targets as set in the Paris Agreement [2]. Thermally insulating building envelopes can help in reducing the space heating energy consumption and achieving energy efficiency in buildings. Windows are often the
thermally poor performing element in the building envelope and can have heat transfer coefficients (U-value) up to 10 times high in comparison of an insulated roof [3]. Window thermal shutters can be used to reduce heat loss, solar shading and glare [4]. Thermal shutters can improve the thermal
performance of a double-glazed window by 25%-30% [5] and can be a non-intrusive option for internally upgrading windows of historic buildings without any change in the external façade. Thermal performance of timber-framed sash windows can be improved cost effectively using thermal
shutters instead of replacing it with standard double glazing. Heat loss through windows can be reduced up to 60% by using window shutters insulated with conventional thermal insulation materials [6]. Further decreases in heat loss would need greater thickness of a shutter’s thermal insulation material, which may not be aesthetically desirable nor ideal for achieving smart windows.
To address this issue, alternative materials are required to be employed.
Previously, Phase Change Materials (PCMs) have been investigated as one of the options in external window shutter application in summer climatic conditions and was found to have reduced the heat gain through windows by 23.29% [7]. Use of PCM in an aluminum hollow blade internal window shutter was investigated by Silva et al. [8] and found to have reduced the heat flux by
10W/m2 in the measurement chamber in summer climate conditions in a Mediterranean region.
Vacuum Insulation Panels (VIPs), an advanced thermal insulation material, have been suggested to improve the performance of thermal window shutters relative to PCMs(?) without any effect on overall thickness [9,10]. VIPs have a thermal conductivity that is potentially 5-8 times lower compared to that of a conventional thermal insulation material [11,12,13]. VIP insulation is a suitable window shutter application due to its low thermal conductivity, thinner section and damage protection inside the outer cover of thermal shutter. However, the issue of thermal bridging for VIP insulated thermal
window shutters has been highlighted to have significant impact on the overall window thermal performance [9]. This paper further investigates the effects of thermal bridging and air gap/cavity
between the window and the shutter and presence of shutter trickle vent on thermal performance of VIP insulated window shutter.
2. Methodology
Static thermal simulations were conducted in VOLTRA, which is a thermal analysis software used for simulating three-dimensional transient heat transfer [14]. A 1000 x1000 mm double-glazed window with 50 mm aluminum frame and an overall U-value of 1.4 W/m2K was modelled. The
simulations were done for a VIP window shutter, with a thermal conductivity of 0.006 W/mK, for the following combination scenarios:
Geometry:
Overall, 18 combination scenarios were simulated. The internal and external temperatures were considered as 20 °C and 0 °C, respectively, to reflect winter conditions in an oceanic climate. Table 1
summarises the simulation conditions and material properties. For the purpose of the ventilation through the trickle vent, a constant inlet/outlet ventilation was considered between outside and the
cavity between the window and the shutter. According to CIBSE Guide A (CIBSE 2015) [15], the ventilation rate for small openings and cracks can be calculated from the equation (1):
qvc = lc k1 (∆p)n
where, qvc is the volumetric flow rate through the crack (L.s–1), lc is the length of the crack/opening (m), ∆p is the pressure difference across the opening (Pa) and k1 is the flow coefficient per unit length
of the opening (L.s–1 .m–1 .Pa–n). The figures for the k1, n and ∆p were defined as 0.8, 0.6 and 0.7 respectively, using Appendix 4.A2 and Table 4.14 of the CIBSE Guide A document [15]. The ventilation rate through a 500mm trickle vent was therefore calculated as: 0.5 x 0.8 x (0.7) 0.6 = 0.32
L.s–1
| Material | Thermal Conductivity (W/mK) | Thickness/Size (mm) | Temperature (°C) |
|---|---|---|---|
| Outside | - | - | 0 |
| Inside | - | - | 20 |
| Brick | 0.90 | 100 | - |
| Cavity Insulation | 0.035 | 80 | - |
| Concrete | 0.85 | 140 | - |
| Gypsum Board | 0.50 | 15 | - |
| Glazing | 0.036* | 20 | - |
| Shutter (VIP) | 0.006 | 50 | - |
| Shutter Frame | 0.5 | 50 | - |
| Air Cavity between Shutter and Window | - | 50, 100, 150, 200 | - |
| Trickle Vent | - | - | 500 |
* Additional inside (0.13 m²K/W) and outside (0.04 m²K/W) surface resistance BS EN ISO 10077-1 BSI (2017) should be considered for U-value calculations.
| Cavity Width/Air Gap (mm) | Heat Loss Through the Window (W) |
|---|---|
| Bare Window - Base Case | 26.65 |
| 50 | 6.61 |
| 100 | 8.75 |
| 150 | 10.92 |
| 200 | 12.44 |
Figure 1. Temperature ranges (°C) in window systems with insulated cavity wall for (from left to
right): bare window, window & shutter 50mm, 100mm, 150mm and 200mm unventilated air cavity
| Cavity Width (mm) | Heat Loss Through the Window (W) |
|---|---|
| Bare Window - Base Case | 26.65 |
| 50 | 4.23 |
| 100 | 4.18 |
| 150 | 4.28 |
| 200 | 4.31 |
Table 4 and Table 5 show the heat losses through the window for the normal and adiabatic walls when a 500mm trickle vent was introduced to the window. As stated above, in the methodology section, a permanent inlet/outlet ventilation of 0.32 L.s–1 was considered to simulate the effects of trickle ventilation on the performance of the shutters. According to the results, the losses through the windows were reduced for all simulated scenarios. This appears to be due to the losses through ventilation, meaning that although the overall heat losses increased, these were reduced through the windows.
| Cavity Width (mm) | Heat Loss Through the Window (W) |
|---|---|
| Bare Window - Base Case | 26.65 |
| 50 | 5.83 |
| 100 | 7.78 |
| 150 | 9.83 |
| 200 | 11.31 |
| Cavity Width (mm) | Heat Loss Through the Window (W) |
|---|---|
| Bare Window - Base Case | 26.65 |
| 50 | 3.67 |
| 100 | 3.60 |
| 150 | 3.68 |
| 200 | 3.70 |
Figure 3 and Figure 4 illustrate the losses through the shutters and windows with a ventilated air cavity. The heat-losses have been reduced significantly through the window when the shutter is deployed. Similar to the above figures (1 &2), the losses are significantly lower for smaller air cavities
indicating the negative effects of thermal bridging.
Figure 3. Temperature ranges (°C) for (from left to right): bare window, window & shutter 50mm, 100mm, 150mm and 200mm ventilated air cavity under adiabatic conditions.
Figure 4. Heat-loss ranges (W/m2
) for (from left to right): bare window, window & shutter 50mm, 100mm, 150mm and 200mm ventilated air cavity under adiabatic conditions.
4. Conclusions
Windows are one of the main areas of heat-loss in buildings. Thermal window shutters could significantly reduce heat-losses through windows; however, the performance of shutters greatly depends on various issues including the type of insulation used, thermal bridging and the ventilation
rate that itself relates to the airtightness of both the windows and shutters as well as the presence of controlled trickle ventilators. This paper studied the effects of air cavity gap and trickle ventilation on the performance of a window shutter made of Vacuum Insulation Panels (VIP). The results indicate that there is a direct relationship between the air cavity sizes (between the window and shutter) and heat losses through the windows. The heat loss through the window was reduced by nearly 50% when the cavity between the shutter and window was reduced from 200 mm to 50 mm.
This indicates the significant effect of cold-bridging on the performance of thermal shutters.
Therefore, the additional heat loss due to cold bridging outweighs the additional thermal resistance due to the increased air gap. The results also indicates that overall thermal performance is slightly
deteriorated when trickle vent was introduced. The significance of trickle vent sizes and ventilation rates require more investigation. No meaningful changes in the performance were observed for
different unventilated cavity sizes when the walls were changed to adiabatic. The results therefore indicate that to achieve the best performance for an airtight shutter, the size of the cavity between the
window and the shutter and/or the thermal bridging through the surrounding walls should be minimised.
Conflicts of Interest: The authors declare no conflict of interest.
References
© 2019 by the authors. Submitted for possible open access publication under the terms
and conditions of the Creative Commons Attribution (CC BY) license
(http://creativecommons.org/licenses/by/4.0/).
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