ZEMCH 2019 International Conference Proceedings April.2020 | Page 147
The results indicate that there is a direct relationship between heat losses through the window and
the cavity width. The heat loss increased significantly by around 90% from 6.61 W, for the 50 mm air
gap, to 12.44 W, for the 200 mm air gap. This seems to be due to the excessive heat losses through the
surrounding surfaces around the window and the shutter, indicating thermal bridging. Figure 1 and
Figure 2 also illustrate the heat loss and temperature ranges through the windows, shutters and walls.
These figures show higher temperatures corresponding to higher heat‐losses/thermal bridging around
the perimeter of the shutters and windows. The blue colours at the centre of the shutters in Figure 1
(50mm, 100mm) indicate considerably lower losses through shutters, relative to the teal and green
colouring (150mm, 200mm). The figures show lower losses when the air cavity is reduced, in line with
the numerical results shown in Table 1. Overall, the results confirm the findings of earlier studies [4,9]
on the effects of thermal bridging on the performance of the thermal shutters.
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.
Figure 2. Heat‐loss ranges (W/m ) in window systems with insulated cavity wall for (from left to
right): bare window, window & shutter 50mm, 100mm, 150mm and 200mm unventilated air cavity.
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Table 3 summarises the results for different unventilated air cavity sizes when the walls and
surrounding surfaces around the window and shutter were changed to near‐zero conductivity of 0.0001
W/mK. Unlike the above figures, the amount of heat‐losses through the window remained almost
constant regardless of the size of the air gap. According to the BS EN 13125 (BSI 2001) [17], for the “Class
5” airtight shutter an additional R‐Value of 0.17 m2.K.W ‐1 should be considered to recognise the
insulating effect of the air gap between the window and the ‘external’ shutters. The additional thermal
resistance is also considered for internal devices such as internal blinds. The additional thermal
resistance of the air gap seems to be correct in theoretical conditions when thermal bridging is zero or
negligible; however, the presence of thermal bridging significantly deteriorates the performance of the
shutters. Indeed, the main criteria that BS EN 13125 is referring to are the air cavity, between the shutter
and window, and the air permeability of the shutter. No reference has been made to thermal bridging
Thermal Performance of Vacuum Insulated Window Shutter Systems
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