The Double TakeAugust 10th, 2020 | Category: Architects' Guide to Glass and Metal
Double-Skin Facades Aid in Energy Efficiency, Occupant Comport
By Jordan Scott
With continued advances in glass and glazing technology, architects have a variety of solutions from which to choose to improve a building’s solar heat gain and energy efficiency. Passive systems, such as insulating glass units, coated glass, static solar shading and some double-skin façades, provide a higher degree of thermal comfort when compared to monolithic glass. They lack the ability to respond to a building’s changing environment, however. Active solutions, such as most double-skin façades, dynamic glazing and operable sunshading devices, can enhance both a building’s performance and occupants’ access to views in different temperatures and sun levels. However, building owners can be a hard sell when it comes to the large investment required for high-performing glazing systems.
Seeing Double – Performance and Aesthetics
“For us, most double-skinned façades would only be considered viable if they are active,” says Robert Heintges, founding principal of New York-based Heintges Consulting Architects & Engineers.
Double-skinned façades allow building operators to control the temperature of the building by exhausting heat in the summer or closing dampers to retain heat in the winter. Sunshades or blinds often are integrated within the system to block light and heat from passing into the occupiable space of a building.
Wellsun, a company based in Delft, Netherlands, manufactures translucent solar panels used within double-skin façade systems that act as a sunshade while collecting solar energy. The company’s Lumiduct panels are situated on a tracking system, which ensures that the solar cells are pointed at the sun throughout the day, generating electricity for the building.
“You make your whole façade available for electricity production, which could be a great benefit, but you also maintain transparency. You maintain a view to the outside for the people who work in the building,” says Wellsun chief technology officer Matthijs Damen.
Technology such as Wellsun’s Lumiduct not only collects solar energy, but blocks light and heat from entering the inner glazing system. The ventilation system within the double-skin façade cavity can then release the excess heat, lessening the work required by the building’s air conditioning system and saving energy costs.
However, double-skin façades are more than a method for achieving excellent energy performance, they allow architects to reach new aesthetic heights.
“The performance benefits can be considerable, but the aesthetic benefits are also really interesting,” says Heintges. “One can achieve a truly transparent building using monolithic clear glass without a coating on the outer skin.”
Damen suggests that potential clients use low-iron glass without any low-E or reflective coatings since the main purpose of the outer façade, from his perspective, is to meet the windload requirements to protect the system within the façade.
“[The outer façade] doesn’t have any insulating properties and it should reflect sunlight as little as possible. Because the panels themselves block all the sunlight, there’s no heat entering the building,” he says. “Behind the panels there is a regular façade that has double or triple glazing where you have your heat insulating properties.
”Cavity depth can have an impact on the performance of a double-skin façade.
According to Benedict Tranel, principal at Gensler, which is headquartered in San Francisco, a deeper cavity allows for more control of the temperature inside the cavity.
“In a sense, the more volume of air and the bigger the space in that cavity, the more ability you have to maintain a temperature that ideally is somewhat close to the interior temperature,” he says. “The change in temperature across the inside glass surface should be as close to zero as possible, which is a static state and very energy efficient.”
A thinner cavity system requires more air circulation to keep the temperature low. An actively ventilated system is needed to have more control over the air flow. Deeper cavities can rely on more passively ventilated systems.
“Then one has to consider the offset of an actively ventilated façade that needs additional maintenance and moving parts. The mechanics of it also take energy, so that can also offset the energy savings,” says Tranel.
The Double-Edged Sword
The energy performance of double-skin façades can be impressive, but cost is a deterrent for many would-be supporters of the technology in the U.S. where energy costs are lower than in Europe.
“The projects that have justified a double-skin façade have focused on other return on investment (ROI) metrics beyond energy, which tend to focus on employee comfort, staff retention, engagement and well-being,” says Tranel. “… If you enhance productivity by even a small percentage, it has a very large return on that investment. If you look at the capital cost of the façade and you look at one less day of sick leave or absenteeism … the cost of those employees will have a very fast ROI compared to the initial cost of that double-skin façade.”
Damen reiterates the importance of regulated daylighting and thermal comfort in increasing productivity among building occupants.
“One thing we really struggle with is that every potential client who considers this product really looks at the energy or electricity production, which is a very limited monetary value. People spend way more money on their employees,” he says. “… Regulating indoor climate not only makes your building energy efficient, but if employees work in an environment they like, and that is healthy, they become more productive.”
Convincing a building owner that a double-skin façade is the best option for a building can be difficult if the owner or developer is not the end-user.
“A developer project that’s being built speculatively for an unknown end-user tends to be difficult to make it work within the constraints of the budget of the project,” says Tranel. “Because it’s speculative, what an end-user would pay in an increase in rent for that higher performing building [is uncertain]. Whereas an end-user that’s building for themselves can make that assessment directly and decide if they feel it’s worth it.”
For that reason, Tranel tends to suggest double-skin façades as an option when working directly with an end-user, especially if they are driven by sustainability and health and wellness aspirations rather than technology for its own sake.
“The main challenge is convincing people that this is a solution that works and is very efficient. But you first require them to put some more effort in the design of their building. It’s a little bit more complex than just one façade and being done,” adds Damen. “So you have to convince the people that this is a smart thing to do and it pays off in the end. What we see now is that people like to keep it simple. Then in the end when the building is done and people start working in it, they notice that there are problems with heat management or with very inefficient use of the building because air conditioning and ventilation need to work real-ly hard to keep the indoor climate comfortable.”
According to Heintges, one major problem with double-skin façades is that most of the technological innovations are happening in Europe where the fire codes are less stringent than in the U.S.
“So getting a true double skin that goes up the whole side of the building that’s not fire separated at every floor is very difficult to do. Even so, where the slab has to come out, or where fire-rated construction has to come out to the outer skin of the double skin, we’ve had to put sprinklers inside the cavities and other things to meet the local building department requirements,” says Heintges. “The concern would be if you build a skin on the outside of the whole building, then the double-skin façade would act as a chimney flue and actually make the fire conditions worse. Fire separation is a serious consideration in doing a really good double-skin façade. In that context you’ve got air in and air out of the façade on a floor-by-floor basis as opposed to air in at the bottom of the cavity and exhausting at the top of the building.”
Architects also must consider the region of the U.S. when designing a double-skin façade. The technology can be challenged by humid, Southern climates because the benefit of direct ventilation from the outdoors into the building is lost when the humidity is too high.
“There are ways of going about it that can work and make sense but it would probably be less of a deeper cavity and more of a shallow cavity that’s just externally ventilated,” says Tranel.
The Blinds Side
Blinds and shades are other active solutions that can protect building occupants from the heat of the sun, but their impacts are different depending on whether they’re set up internally or externally.
“If the blinds or shades are on the interior of the glass, then a double-skin façade with blinds in the cavity is the far superior solution because you are controlling heat gain into the building,” says Tranel. “When you have a blind on the interior surface it’s just capturing all the heat inside the space that’s conditioned and occupied by people … so now you need air conditioning to remove that heat or you’re asking people to sacrifice thermal comfort by sitting next to a window that’s hot.”
The glass itself can also radiate heat into the interior of the building. Exterior blinds solve that issue because they capture the heat on the outside of the building and don’t radiate it into the occupied space.
“It can be very good in that regard, there are just some additional considerations with external blinds, such as maintenance and wind,” says Tranel.
On windy days, blinds may need to be retracted to prevent them from being damaged, meaning occupants lose their solar benefit.
“Most American building owners have been adverse to [external blinds] because of the maintenance issues such as cleaning the blinds or maintaining the motors,” says Heintges.
Blinds and shades often are included within a double-skin façade system, providing protection and allowing for easy access to service the blinds when cleaning or maintenance is required.
“The idea would be that you don’t need blinds all the time, so when there’s no direct sun on the façade you pull the blinds up … it’s normally done with automatic controls so, in a sense, it’s acting similarly to solar control glass or switchable glass,” says Heintges.
A Dynamic Rise
Dynamic glazing technology is still advancing and maturing as companies fabricate glass with more gradual gradients between its tinted and non-tinted areas. The speed with which the glass switches from clear to tinted is also quickening, making dynamic glazing an increasingly desirable option for architects and building owners. Tranel says dynamic glass has yet to be used in a large-scale application because it’s still fairly new to the market, meaning the industry hasn’t tested how robust a solution it is for energy efficiency.
“If you want to compare it [to a double-skin façade] in terms of energy efficiency alone, the solar heat gain is reduced with dynamic glazing so that’s one of the primary benefits of the double-skin façade,” he says. “… The downside of dynamic glazing is that when you darken and tint the glass it distorts the view to the outside. With a double-skin façade, even if you lower the blinds, the blinds don’t have to be at a closed angle. They just have to cut off direct solar heat gains so you can still see out through the blinds and you typically can use very transparent glazing with 80% to 90% visible light transmission.”
Passive but Progressing
Passive solutions for energy efficiency such as coatings, glazing systems and static shading devices may not be able to be altered in reaction to real-time environmental changes, but that doesn’t mean the technologies are not progressing.
Improvements to low-E coatings, spacer technology and thermal bridging prevention are making glazing systems more energy efficient than ever, justifying the use in LEED projects nationwide. This energy-efficient technology is becoming more standard in buildings and, as a result, improves the efficiency of double-skin façades which contain glazing systems.
“Façade technology is this constant incremental improvement and enhancement to products that are offered throughout the supply chain, so it’s hard to say there’s one single evolution that’s absolutely revolutionizing the design of double-skin façades,” says Tranel. “There are a number of more modern incremental enhancements that are always being developed by all the different suppliers involved. That’s where you really drive performance. It’s get-ting down to warm-edge technology in an insulating glass unit, for example, or the thermal break and the quality of that product, and in an aluminum extrusion to prevent thermal bridging. These are the areas—they may not be glamorous but they’re really targeted and focused on the key aspects of performance—that do have a bigger impact on the overall numbers a façade will help the building achieve.”
Jordan Scott is the editor of Architects’ Guide to Glass & Metal. She can be reached at firstname.lastname@example.org
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