Solar shading meets thermal performance

When specifying solar control, not just any curtain walling bracket will do if the thermal performance of the building facade is not to be compromised, says solar shading and screening solutions provider Levolux.

While glass continues to be the predominant cladding material for commercial buildings, its thermal performance is now being placed under closer scrutiny.

A glazed facade, formed from a modern curtain walling system, will still be susceptible to excessive solar heat gain when it is exposed to direct sunlight for long periods. This can have a significant impact on the cooling loads of a building. While a degree of solar gain is appreciated in winter, if left unchecked, it will make a building overly dependent on air conditioning during the summer.

As energy costs have soared over recent years, this has changed the focus of architects not only on meeting the required building regulations and satisfying a client’s brief, but also on maximising the thermal performance of buildings.

Buildings are increasingly designed with greater emphasis on passive cooling techniques. This demands a more considered approach to curtain walling, as there is often very little flexibility for it to accommodate external solar control devices.

Shading performance for devices can vary enormously and this is defined by a Solar Heat Gain Coefficient (SHGC) rating. For instance, a double glazed opening without any form of solar control will have a SHGC rating of around 0.65. The same double glazed opening fitted with moveable horizontal fins will have a SHGC rating of around 0.1.

While it is important for an architect to consider the SHGC rating of a shading device, attention should also be given to how the device is secured to the curtain walling. Any savings that may be achieved by incorporating a shading device with a high SHGC rating can very easily be undermined if the bracket used does not incorporate an effective thermal break.

Typical curtain walling brackets may satisfy load-bearing requirements and may be compatible with a range of curtain walling systems, but perhaps the more important question is how they penetrate the curtain walling mullion. If the bracket does not incorporate a comprehensive thermal break, then it will be susceptible to cold bridging, acoustic and vibration transmission and interstitial condensation.

Under Part L of the building regulations, a new commercial building must have a U-value of less than 2.2 W/m2K. One of the key considerations for engineers when targeting a low U- value, is the thermal transmittance rating, or ‘χ‘, of materials used to form the building envelope. If a curtain walling bracket compromises the thermal break of the curtain walling, this will raise the U-value of the entire building envelope, possibly by as much as 10 per cent.

It is therefore critical to assess a number of curtain walling brackets from a variety of manufacturers and demand evidence of the thermal transmittance rating or ‘χ’ for each bracket.

As a guide, a curtain walling bracket with a good thermal break should deliver a thermal transmittance rating of around 0.14 W/K, while a bracket without a thermal break would achieve around 0.42 W/K. It is also advisable to ask if testing was carried out by an independent body, such as the Centre for Window and Cladding Technology (CWCT).

Upon close inspection, it is relatively easy to assess the effectiveness of a ‘thermal break’ as a feature of a curtain walling bracket.

A thermal break can be defined as “an element of low thermal conductivity placed in an assembly to reduce or prevent the flow of thermal energy between conductive materials”.

In the case of curtain walling, the most conductive element is the grid of aluminium mullions and transoms that are used to secure glazing. The more isolated a conductive material is, such as aluminium, the more effective the thermal break and the lower the U-value achieved by the curtain walling.

Many curtain walling brackets purport to feature a thermal break, but when examined in detail, it is clear that while an insulating material is present in some areas, this is not comprehensive enough to prevent the curtain walling mullion from being compromised and cold bridging from occurring. This increases the risk of interstitial condensation developing inside the curtain walling structure. Over time, this can weaken the curtain walling structure and lead to mould growth forming.

The optimal design for a curtain walling bracket is for it to fully isolate the mullion at the point of penetration. A low conductivity, insulating element should be used throughout the bracket, perhaps in the form of insulated bolt sleeves and screw caps to ensure the thermal break is comprehensive. When this is achieved, the thermal performance of a curtain walling system can be maximised. A U-value of less than 0.9 W/m2K is then a real possibility, which is around 50 per cent lower than an average commercial building facade.

As an additional benefit, the inclusion of an insulating element can help to prevent noise and vibrations being transmitted into a building.

When it comes to specifying solar control for a building, it is therefore not sufficient to accept any curtain walling bracket, but demand one that is fully isolated, with a comprehensive thermal break. This will ensure that the thermal performance of a building’s facade is not compromised.