Glass buildings have a seductive visual lure that attracts its designers and owners alike. Developers value its marketability appeal just as much as architects do the seductive nature of a rendered image of glass towers, shining in all their glory at dusk against the city skyline backdrop. As the construction industry matures in India, architects and designers (with or without our conscience in tow) will find ourselves designing a fully glazed facade in a climate where we may be better off without one. All is not lost though. With some key design strategies and selection of appropriate material, designers can find the right balance between the poetics of pristine glass and performative aspects of the building.
Our team had the opportunity to design such a project in the hot and humid climate of southern India. As we all know, designing sustainable buildings in such a climate has never been easy, especially when relying heavily on passive design strategies. This is one zone where it’s nearly impossible to reduce the EUI (Energy Use Index) passively without subjecting its occupants to a (dis)comfort level much beyond the limits of even adaptive comfort zone.
While designing the building, the first useful design strategy we utilized was selectively applying high-cost glass façade to specific and limited orientations. This allowed us to design fully glazed facades on prominent selected orientations only, thereby reducing the overall window wall ratios (WWRs). Since our performance goal for the building envelope, specifically in this climate is to reduce direct solar gains transmitted through the facade, reduction in WWRs will have a significant impact. Typically, if only thinking of design aesthetics, I would be opposed to ignoring the ‘back and side’ of the buildings as is common practice in India. What we decided to do in our process was to continue the façade design language to the under-exposed ‘back and side’ of buildings, albeit using cheaper material and finishes. This design strategy allowed us not only to minimize cumulative glazed surfaces to a little over 50% WWR, but also provide cost savings for the client.
With the Indian construction industry expanding exponentially, the availability of various products and materials has also grown. Many advanced products are now being manufactured in the country. For the glazed curtain wall system on our project, we were able to find high performing double glazed units (DGUs) with desired properties and manufactured within the country. Using a spectrally selective low-E coating, the DGUs have a Solar Heat Gain Coefficient (SHGC) value of 0.25 and u-value of 1.6 W/sq.m.K. Beyond using just a low-E coated double glazed glass, there are other products that can further enhance the performance of the glass. One such product is the ceramic frit, which can further block radiation and diffuse natural daylight. For our project, we designed a custom frit pattern, which was printed within the DGU cavity by the manufacturer. The idea of customizing ceramic frit begins to literally fuse the poetics and the performance aspects of the building. Apart from ceramic frit, there are other options also available in the market, such as PVB (Polyvinyl butyral) films on which almost any pattern or image can be printed. Many new products are now inserting a variety of materials in the DGU cavity including metal and wood mesh. When proposing custom treatment of DGUs, the designer must be aware of the implications of surface location of coatings, prints and ceramics so as to avoid the heat entrapment in the cavity and allowing for conductive transfer inside the occupied space. I realize that much of what I’m writing sounds like technical jargon, but the more proficient we are in technical knowledge, the better we’ll be able design whilst achieving our performance goals.
In our project, we went a step beyond the building envelope and designed baffle or shading panels using channel glass. This added layer of channel glass helps reduce some additional solar gains, diffuses direct light and also reduces sudden drafts through operable windows. While it performs, the channel glass with frosted finish maintains a light translucent shimmering glass aesthetic on the façade.
Of course, all our efforts must be quantified and assessed for its performance. Another hurdle we often face, particularly in smaller projects are that there are no energy analysts on the team to run full energy models. In the absence of full energy model, we can find other segmented environmental studies that can be used to extrapolate results. In our case, since we understand that direct insolation is the largest contributor to internal gains, our team decided to study solar radiation transmitted through the proposed building façade and compared it to ASHRAE 90.1 compliant base case building envelope. What we found was that the proposed design case resulted in transmitted solar radiation averaging approximately 55% less than the ASHRAE compliant base case. Considering that solar radiation can amount to around 38% of building envelope loads in this climate, a significant reduction in solar gains through the envelope will certainly have a considerable impact not only in the overall energy consumption of the building, but also improve occupant comfort, especially at the perimeter of the building.