All wrapped up

Highly glazed facades are an increasingly common feature in today’s urban landscape. For designers this trend introduces several challenges as they attempt to balance the visual and aesthetic benefits that glass provides against the environmental performance of the building envelope. As the culmination of a two-year Partners-in-Innovation project, sponsored by the Department of Trade and Industry to investigate the overall environmental performance of highly glazed facades, CIBSE has published TM35: Environmental performance toolkit for glazed facades. Computer simulation techniques were used to analyse various criteria that were identified as being indicative of the facades’ performance. These include building heating and cooling requirements, solar heat gain penetration, condensation risks, daylight penetration, view out, acoustic attenuation and capital costs.

While the publication provides readers with a good understanding of the factors affecting glazed facade performance, the software provided on the accompanying CD-Rom, the Facade Selector, allows users to access, interrogate and compare these simulation results to identify suitable facade options for the users’ design purposes. Users specify basic facade requirements such as peak heating/cooling loads, daylight availability, facade orientations and costs to identify the facade systems that will meet these requirements, from a database of 37 different facade configurations. Although it has been noted that there are countless possible types of glazed facades, it was agreed amongst the project partners that these 37 options were representative of the industry’s preferences today and into the near future.

The computer simulations carried out to obtain the performance data used in the Facade Selector are based on a typical commercial office perimeter zone 9 m wide and 6 m deep located in south east England. The design conditions assumed are based on British Council of Offices best practice recommendations for office specifications, simulated over external conditions as given by CIBSE London test reference year annual weather data. Even though these assumptions may not always be similar to those specified by the users, the Facade Selector will still be extremely helpful as a comparative tool for the environmental performance of different glazed facade options, independent of design conditions. This is especially useful during initial design stages in setting the direction of the design process, without excessive time and effort being expended on repetitive calculations and thermal simulations on the different facade options.

The following describes how the Facade Selector works. The main selection window (figure 1) allows users to specify the orientation of their facade, and whether they would like to limit the search to specific glazing types only (such as double glazed units, mechanically ventilated double skin facades etc), or facades that do not use any shading devices such as external louvres or internal blinds. Users then tick off the various performance criteria that they are designing for. These criteria include:

• Peak heating and cooling loads (in W/m

• Glazing costs (in £/m

For instance, figure 2 shows the selections made on the specification window for the following performance requirements:

• South facing glazed facade.
• Peak cooling loads of 80 W/m

This is equivalent to designing for an office space that is both sufficiently day-lit and that can be cooled with a passive chilled beam/ceiling system.

After the requirements are set, clicking on the ‘Show facade options’ button will reveal the facade systems that will meet these requirements. In the above example, five facade options have been identified. The user can now obtain more information regarding any individual facade option, by highlighting one of the options in the list box, as shown, and clicking the ‘Show summary sheet’ button. This will open the pdf file relating to the selected option. This is a nine-page summary that provides the following information:

• Facade configurations and dimensions (figure 2).
• Overall performance summary.
• Detailed heating, cooling and solar information, including peak loads, and monthly and annual demands.
• Seasonal heating, cooling and solar gain demand charts.
• Detailed heating, cooling and lighting energy savings information, for cases with idealised lighting controls (ie when daylight penetration is sufficient to negate the need for electric lighting).
• Condensation risk prediction.
• Daylight and view factor analyses (figure 3).

To compare the options that have been identified, the user clicks the ‘Show comparison charts’ button. This opens a new window showing a chart comparing the peak heating and cooling loads for the different facade options. Other comparison charts available via the ‘Analysis’ section of the menu bar include solar gains, annual heating and cooling demands, daylight factors and facade costs/cost indices.

• Kai Lim is principal author of TM 35 and an engineer with the sustainable development group at Faber Maunsell.
• To obtain a copy of TM35: Environmental Performance Toolkit for Glazed Facades, please contact CIBSE. Also, for more information on the Society of Facade Engineering