After having dealt with the technical possibilities of thermal optimisation in my last post, this time the focus will be on design aspects: how do changes in facade layout, module size and operable insert units affect the thermal performance of curtain walls?
Reducing operable insert units
The amount of facade profiles (mullions, transoms and frames) affects the Ucw-value in two ways: first, by the U-values of the profiles (which are usually higher than the U-values of the infills/ panels), secondly, by the additional Ψ-values caused by the higher amount of contacting facade components. Therefore, it seems obvious to reduce the amount of operable insert units and replace them with fixed glazing.
The following illustration shows a sequence of our reference curtain wall layout from the previous posts. Below this is the same layout, but this time without operable window units.
As already described in the last post, the U-values of the reference curtain wall layout (i. e. with operable insert units) are as follows (1):
- with standard components:
Ucw = (10.434 W/K + 4.534 W/K) / 12.00 m²
Ucw = 1.2 W/(m²K) (1.247)
- with thermally optimised components:
Ucw = (ΣA×U + ΣΨ×l) / Acw
Ucw = (5.742W/K + 1.012 W/K) / 12.00 m²
Ucw = 0.6 W/(m²K) (0.563)
The U-values of the same curtain wall without operable insert units are:
- with standard components:
Ucw = (10.024 W/K + 4.072 W/K) / 12.00 m²
Ucw = 1.2 W/(m²K) (1.175)
- with thermally optimised components:
Ucw = (5.540 W/K + 0.848 W/K) / 12.00 m²
Ucw = 0.5 W/(m²K) (0.525)
The use of fixed glazing instead of operable insert units improves the Ucw-value by 6% (standard) and 7% (thermally optimised). By rounding the value to one decimal place in compliance with the standard, the Ucw-value of the thermally optimised version can be lowered from 0.6 to 0.5 W/(m²K).
It should be noted that - despite these improvements in U-values - natural ventilation by means of operable windows generally has a positive effect on thermal comfort and occupant satisfaction (2).
Wider facade module
Another way of reducing the percentage of facade profiles in the total area of the curtain wall is to use a wider facade module.
Changing the facade module from 1.5 m to 2.0 m - as shown above - yields the following results:
- with standard components:
Ucw = (13.655 W/K + 5.224 W/K) / 16.00 m²
Ucw = 1.2 W/(m²K) (1.180)
- with thermally optimised components:
Ucw = (7.550 W/K + 1.157 W/K) / 16.00 m²
Ucw = 0.5 W/(m²K) (0.544)
This means that a facade module increased by 33 % like in this case improves the Ucw-value by 6% (standard) or 3% (thermally optimised).
For example, the rather large insulated aluminium panel could be replaced by a smaller opaque spandrel panel and an additional glazing unit with a transom at parapet height (see above). In this case, the Ucw-values would be as follows:
Additional transoms
The horizontal division of a curtain wall can be modified, too. Often, transoms are added for aesthetic reasons.For example, the rather large insulated aluminium panel could be replaced by a smaller opaque spandrel panel and an additional glazing unit with a transom at parapet height (see above). In this case, the Ucw-values would be as follows:
- with standard components:
Ucw = (13.322 W/K + 3.994 W/K) / 12.00 m²
Ucw = 1.4 W/(m²K) (1.443)
- with thermally optimised components:
Ucw = (7.111 W/K + 1.248 W/K) / 12.00 m²
Ucw = 0.7 W/(m²K) (0.697)
The modified curtain wall layout has a Ucw-value worse by 14 % (standard) and 19 % (optimised) than the reference layout. However, this deterioration in Ucw-value results not only from the additional transoms but also from the higher percentage of glazed areas (opaque insulated panels usually have lower U-values than glazed areas).
Thus, the reference curtain wall layout with less transom and glass areas has a Ucw-value that is much better than the modified layout.
More glass
In this example, the percentage of opaque panels was decreased in favour of bigger glazed areas. This leads to the following U-values:
- with standard components:
Ucw = (13.299 W/K + 3.532 W/K) / 12.00 m²
Ucw = 1.4 W/(m²K) (1.403)
- with thermally optimised components:
Ucw = (7.169 W/K + 1.079 W/K) / 12.00 m²
Ucw = 0.7 W/(m²K) (0.687)
Although there are no additional transoms like in the previous example, yet the Ucw-values have deteriorated similarly. The main reason for this is the increased frame area (Af) due to the bigger size of the operable window.
Here, too, the reference curtain wall layout has a better thermal performance than the modified layout.
Conclusion
The geometrical optimisation too, has a positive effect on the thermal transmittance of curtain walls. However, the geometrical modifications described here have fewer positive effects on the Ucw-value than the technical optimisation of the individual facade components (as described in the previous post, the Ucw-value could be reduced by half by means of technical optimisation).Only two of the four modifications examined here led to an improvement in Ucw value. This suggests that the reference curtain wall layout already has relatively favourable thermal insulation properties due to its simple layout.
In any case, the examined cases allow the conclusion that the following design measures have a positive effect on Ucw-values:
- fewer facade profiles (mullions, transoms, frames)
- larger facade modules
- fewer operable window units
- fewer glazed areas and more opaque insulated panels
It should be mentioned that when dealing with very low U-values, rounding the value to one decimal place can lead to excessive generalisations. For example, results ranging from 0.550 to 0.649 W/(m²K) yield a Ucw-value of 0.6 W/(m²K), although the difference between the two limit values is 15 %.
References
(1) All calculations of thermal transmittance were performed acc. to ISO 12631 (first edition 2012-10-01), Thermal performance of curtain
walling - Calculation of thermal transmittance (Reference number: ISO
12631: 2012(E))
(2) Cf. Brager, Gail S.; de Dear, Richard: Climate, Comfort and Natural Ventilation, Berkeley 2001, p. 5 et sqq.
(2) Cf. Brager, Gail S.; de Dear, Richard: Climate, Comfort and Natural Ventilation, Berkeley 2001, p. 5 et sqq.
