Environmentally Responsive Building Envelopes

Thursday, April 23rd, 2015
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Building envelopes play one of the most important roles in the sustainable built environment.

For the interior to work effectively, the design of the exterior is crucial. Conversely, an environmentally responsive façade is reliant on the efficient design of the guts of a building.

The building envelope is a significant contributor to heat gains and losses for buildings. The actual percentage obviously depends on many factors, including orientation and building location but heat gain or loss can be as high as 30 per cent or more. This simple fact highlights the need for a concerted design of building façades between the architect, façade consultant, ESD consultant and mechanical engineer.

Simple decisions at an early stage have the greatest impact on the overall envelope performance, at the lowest possible cost.

For instance, selecting the optimal building orientation has very little cost implication, if any at all, while having a massive effect on the energy bill of the building. Similarly, adopting an optimal window-to-wall ratio can allow outside views and harnessing of daylight while severely reducing heat transfer, all of it at virtually no cost to the developer. A number of strategies such as these can be implemented, but they all require the involvement of the façade and ESD consultants right from concept stage.

Working with the façade consultant, the ESD consultant may deploy a number of tools, such as whole building energy modelling and ROI computations, to test various combinations of façade materials and performance, shading strategies, window-to-wall ratios and so on, in order to assess the most viable solution for a specific building. Once again, this requires teamwork and close coordination between the architect, façade consultant, ESD consultant and mechanical engineer.

Occupant comfort is another area where the project façade and ESD consultants must work closely together. The ultimate goal is to ensure that the building is liveable and pleasant to its occupants. For residential buildings, this may involve carrying out CFD studies in order to determine the optimal opening sizes and locations so as to harness available wind to cool internal spaces at no or minimal cost.

For commercial spaces, the aim is to achieve thermal and visual comfort, ensuring that building occupants, especially those located close to the façades, are not subjected to excessive radiant heat or glare. Specific tools are available to ESD consultants to carry out such studies.

Over the past 10 to 15 years, the performance of building façades has progressed tremendously. More can still be done, however.

“When looking at how much headway has been made in other industries within the same time frame, I believe that much more can be done,” said Mathieu Meur, managing director of Meinhardt Façade Technology

“Encouragingly, the importance of the building envelope in the heat gains or losses of buildings is such that there currently is abundant research into new building materials and technologies to further improve on presently available materials.

“The advent of thin-film coatings, phase-changing materials and other high-tech strategies is certainly encouraging, and given additional time, I am confident that much higher performance at reduced costs will be achieved.”

CREATE (Campus for Research Excellence And Technological Enterprise) is a precedent-setting scientific research centre located at the National University of Singapore’s University Town campus. It encompasses three mid-rise buildings and a high-rise tower.

CREATE building]

CREATE building

The building itself is orientated along an east-west axis, with long, glazed façades on the north and south elevations to harness daylight with minimal heat penetration, and with short, solid sides along the east and west for minimal heat gain.

Natural lighting and thermal modelling simulations were run to assess the best possible length of sunshades to use on the façade. This achieved an optimal balance between daylight and heat penetration.

Daylight levels within the building were calculated using a uniform design sky.

Annual Sun Path over the building

Annual sun path over the building

As the name suggests, the uniform design sky model represents a sky with a constant value of luminance across the entire hemisphere. Therefore, no matter where in the sky you look, the model will return the same value of luminance.

Design sky values are derived from a statistical analysis of dynamic outdoor sky illumination levels. They represent the horizontal illuminance value that is exceeded 85 per cent of the time between the hours of 9 am and 5 pm throughout the year. Thus they also represent a worst-case scenario that can be designed to, ensuring the building will meet the desired light levels at least 85 per cent of the time.

For the thermal modelling, an annual model simulation was run, which means it takes into account weather conditions, sun positions and sky conditions from every hour of the year.

This showed that there was only a 3.1 per cent increase in total heat gain when reducing the sunshade length to 0.6 metres, and a total 5.6 per cent increase when reducing it to 0.4 metres.

Since the energy input ratio of the HVAC cooling equipment for this project was at most 0.185 (COP = 5.4), a 3.1 per cent increase in heat gain equates to only 0.6 per cent increase in cooling energy use, and a 5.6 per cent increase in heat gain equates to one per cent or less increase in cooling energy use.

Effect of Sunshade Depth

Effect of sunshade depth

This increase in cooling energy is, however, offset by the increased levels of daylight light within the building due to the smaller sunshade. More daylight means less artificial lighting, which also means less lighting heat load in the building. These two effects make up for the 1.2 per cent increase in cooling energy consumption.

The outcome therefore taken was that the sunshades’ length was reduced from the original intent of 1,200 millimetres down to only 400 millimetres. This was better for the envelope performance (smaller frames), preferred by the architect in terms of aesthetics, reduced the amount of building material used and saved the client about S$1 million in capex, with no increase in opex.

The relationship between façade, ESD and building services is intrinsically linked to delivering optimised environmentally responsive building envelopes.

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