Addressing heat ingress: laying the foundation for zero-energy buildings
In the Middle East, zero-energy buildings have begun to make the transition from concept to feasible objective. Acknowledging this, the Emirates Green Building Council has even defined the concept of nearly zero-energy buildings within the UAE as a “building with a site energy use intensity (EUI) [of] less than 90 kWh per square metre per year, and covers a significant portion of its annual energy use by renewable sources produced on-site or off-site”.
Innovative developers and building owners have now begun evaluating the feasibility of designing as per the net zero standard and have been exploring its implications from both a technical as well as a commercial perspective. As they do so, it is important for them to be mindful of the unique climate of the region as it can render design concepts simply borrowed from the West as inefficient. This is particularly true in the context of façades, which play a major role in the operational energy consumption of buildings.
Before embarking on the quest for net-zero or nearly zero, it is therefore important to understand the specific climatic characteristics of each project location. In the Middle East, winters often entail above average temperatures and a dry climate while summers can prove scorching with varying levels of humidity depending on the specific area. As a result of this, building conditioning is driven by cooling loads and, therefore, façade performance is mainly directed to limiting the ingress of heat into buildings. Understanding and designing for the three main modes of heat ingress can mean the difference between success and failure on the quest for zero-energy.
It should come as no surprise that solar radiation is the main source of heat ingress in the region and, therefore, it is crucial to minimise direct sun exposure on glazed facades. This can be done by applying basic bioclimatic principles such as limiting the percentage ratio of vision glazing within a building. Other factors that can and should be considered are orientation, façade inclination, depth of reveals and the use of shading fins or screens. Selecting appropriate glazing is also essential and selective coatings are available that can filter out the larger part of solar thermal radiation while letting in adequate levels of visible light.
Conduction is the transmission of heat due to temperature gradients and is one of the main transfer mechanisms by which the internal cooling can be lost to the outside, resulting in high operating costs, occupant discomfort and condensation issues.
This contributor to heat ingress can be controlled by using insulative materials and ensuring continuity of the thermal barrier across the building envelope. Insulation on opaque walls must be non-combustible to avoid propagation of fires across the façade. Curtain walls should be designed with double glazed units and appropriate detailing to limit thermal bridging at joints and interfaces.
It is important to consider the position of the vapour and air barriers within the façade build-up in conjunction with the thermal barrier to reduce the risk of interstitial condensation. The optimal position may vary for each building location/use and it is also important to consider ventilation and evaporation rate of any potential condensate.
Also called air leakage, this refers to the unintentional or accidental leakage of air through the envelope of a building, and can be caused by everything from large openings such as doors and windows to minute cracks and crevices. Controlling air infiltration is vital to reduce the energy consumption and the risk of condensation. Moreover, in the region, the air is generally dusty and in its many coastal areas, also very corrosive which raises the value of controlling air infiltration.
The impact of leakage can be limited by specifying appropriate air barriers on walls and pressure-locked openings and by applying appropriate closure details at interfaces. It is also critical to raise awareness among the users on responsible use of AC and openings as very often the AC is running while the windows are open resulting in major energy inefficiency.
By first understanding the factors leading to heat ingress, developers will be able to address some of the main impactors on their zero-energy goals. With these basics firmly in place, they can then go the step further by deploying technologies that actively contribute to reducing the energy footprint of a building by harnessing energy from the environment, such as from solar and wind sources. It is then that they will realize their goals in an efficient and cost-effective manner which will ensure the long-term success of their projects and of course, reduction of the environmental impact of urbanization.