Recently, the Windows and Daylighting Group at Lawrence Berkeley National Laboratory (LBNL) asked me to join an industry panel to provide a perspective on the challenges of achieving low-carbon, high-comfort lighting in buildings. Lighting in buildings can come from electric lighting or daylight, a “free” source of lighting energy. But is comfortable daylight free in terms of a building’s carbon emissions?
Design Around Daylight, Not Electric Light
When considering the decarbonization of lighting, my foundational premise is that the focus must first be on providing daylight rather than electric light. Real, not artificially created, daylight is needed to deliver spaces that are compatible with human health and well-being. Humans have evolved with the need to experience the intensity and dynamic nature of daylight as it changes through the course of the day. Lisa Heschong’s 2021 book “Visual Delight in Architecture: Daylight, Vision and View” provides an excellent summary of the research linking human health, well-being, learning, healing and productivity to exposure to daylight and quality views.
Electric light – no matter how well its output wavelengths are varied to try to simulate daylight (called circadian lighting) – is not a substitute for daylight through windows. Windowless buildings with electric lighting and LED screens showing the view to the outside should not be the goal for decarbonizing buildings. Unfortunately, such buildings are not hypothetical. Charlie Munger’s University of Michigan dormitory building has student bedrooms with no windows. A second Munger-funded project was due to be built at the University of California-Santa Barbara but was abandoned after much negative feedback.
Daylight Must Be Comfortable
Since daylight provision is essential, decarbonizing lighting has big implications for the building envelope. Let’s start with the requirement for daylighting to be comfortable. This requirement also has cascading impacts on both operational and embodied carbon emissions.
Adequate daylight admission into buildings requires careful building massing and orientation and sufficient glazed area with windows of the right size in the right locations. The resulting building envelope (and, by implication, the fenestration) must effectively manage solar heat gain, conductive heat transfer (U-factor) and sunlight glare. If not, occupants will be uncomfortable, especially near the windows, which, of course, is the preferred location to reap the benefits of daylight and view.
Operational Carbon and Daylight
Even though daylight is “free” and can offset electric light usage, there is a significant tension between the fenestration area needed to provide sufficient daylight and views and building energy use and resultant operational carbon emissions:
- Solar heat accompanying daylight increases the load on HVAC systems in summer months, further increasing operational carbon emissions. More window area means more heat gain to manage.
- Conductive heat transfer through windows from hot to cold negatively impacts operational carbon emissions in both winter and summer. More window area typically means lower overall thermal performance of the façade since walls are typically more insulating than windows.
- Glare control using either mechanical blinds or dynamic glazing is often at odds with optimal energy efficiency because of the need to block the direct sun. These strategies typically block much of the available daylight and require supplemental electric light. Rather than being actively managed, manual blinds typically are left down for much of the day, blocking available daylight and views.
Buildings must be massed and oriented effectively to address these additional impacts on operational carbon yet allow sufficient daylight. Fenestration systems must also perform well to provide comfortable daylight and minimize the increase in energy consumption and operational carbon emissions related to heat transfer and glare control.
Embodied Carbon and Daylight
Providing high-performance fenestration has implications for embodied carbon, too: Higher-performance glazing systems typically mean adding material—such as using triple glazing, exterior aluminum sunshades, double-skin curtainwalls, or automated interior or exterior blinds. More material typically means more embodied carbon. Even electrochromic dual-pane insulating glass units (IGUs), perhaps unexpectedly, have multiple times (~5 or more times, depending on the technology) more embodied carbon than regular low-E IGUs.
Service Life is Key
The service life of fenestration assemblies is critical. The larger embodied carbon investment needed to implement higher-performance fenestration in effective daylight designs must be preserved for as long as possible. This means focusing on extending lifespans and ensuring that assemblies are designed to be easily maintained and upgraded during the building’s life.
It is important to be aware that often, the go-to strategy for reducing both embodied and operational carbon is to remove material from assemblies (dematerialize). This can have the unintended consequence of reducing service life. Any product design changes to reduce embodied or operational carbon emissions should be evaluated through the service life lens. Embodied carbon per year of service life is a good yardstick.
High-Performance Fenestration for Comfortable Daylighting is Available
The biggest challenge with implementing high-performance fenestration in well-daylit buildings is not the availability of high-performance fenestration solutions. High-performance fenestration and its components are already available in the U.S. market, often from domestic suppliers.
- R5 architectural windows (AW) and curtainwall using proven polyamide thermal barrier technology are available from several domestic manufacturers.
- Mechanical shading systems and double-skin curtainwall are also available and have long track records.
- Warm edge insulating glass spacers appropriate for non-residential applications, such as plastic hybrid stainless-steel spacers, have a track record of more than 25 years and are available from several manufacturers.
- Room-side low-E coatings are offered by several manufacturers and have been used for over a decade. When used in a dual-pane IGU, they can deliver R5 (IP) center-of-glass performance.
- Triple-pane insulating glass has a track record of over 20 years and is available from many manufacturers. Using the vast selection of low-E and solar control coatings developed over 30 years and offered by several manufacturers, an R8 (IP) center-of-glass thermal performance can be achieved.
- Vacuum insulating glazing (VIG) has a 20-year track record, with several manufacturers coming to market with higher-performance offerings. When used in an IGU as the room-side lite (a hybrid VIG), it can deliver R16+ (IP) center-of-glass performance.
- Electrochromic glazing has been commercially available for over 20 years with three domestic manufacturers.
Using existing technologies, low U-factor fenestration with dynamic solar control and daylight-harvesting integrated façades can be specified, achieving the Department of Energy’s net-zero window performance and enabling comfortable daylighting.
The Biggest Challenge: Codes Require Neither Comfortable nor Daylit Spaces
The biggest challenge is that our codes do not require buildings to be either “comfortable” or well “daylit.” The model codes address acute life-safety issues, like preventing building collapse, resisting the spread of fire, ensuring glass breaks safely upon human impact or managing impact from windborne debris. Our codes do not address the quality of space and chronic impacts on human well-being. Also, U.S. model energy codes do not address reducing operational or embodied carbon.
As a result, designing and constructing building façades well—that is, delivering comfortable, energy (carbon) efficient daylighting—is not business as usual in the U.S. Low-carbon, high-comfort daylighting is difficult and expensive.
- It is difficult because of a lack of familiarity. There are too few people with the appropriate competencies (lack of capacity) in the design and construction community to design, install and commission such systems. New methods and systems increase perceived risk and reduce the willingness of parties to undertake the work and drive to “rinse-repeat” designs.
- It is expensive because economies of scale have not been achieved to drive costs down through volume and experience since it is not done every day. Since building codes set the baseline for cost, anything higher performance than the code will have a price premium and appear “expensive” relative to the baseline. Perceived risk also adds to the price premium.
Arguably, it may be easier and cheaper to deliver low-operational and lower embodied carbon buildings with a window-less or low-window area approach, but the long-term negative impacts on building occupants should not be ignored.
As building occupants ourselves, we have spent time in uncomfortable buildings—whether they are under or over-daylit or the area near the windows is too hot or too cold. We already know the impact of having no guardrails to prevent poor human-centric design.
For low-carbon, high-comfort daylighting to become business-as-usual, building codes need to require:
- a minimum quantity of daylight admission,
- a minimum threshold for occupant thermal and visual comfort in the perimeter zone (near fenestration) and
- a minimum thermal performance for the building envelope to prevent building energy targets from being met primarily through unnecessarily oversized HVAC systems.
To remove additional barriers, additional strategies outside building codes will be needed to support market education, capacity building, risk mitigation and code compliance verification.
As Duane Jonlin from Seattle stated, “The only places you’ll find high-performance buildings being consistently constructed at massive scale are those places where it’s the only way to get a building permit.”
As long as codes do not require the delivery of comfortable, well-daylit, low-carbon buildings, such high-performance human-centric buildings (and the fenestration systems that go in them) will remain on the fringes of the market, designed and constructed by a small, knowledgeable professional community. Capacity building and economies of scale in the market that reduce the cost and risk will remain elusive. And, as building energy codes drive down the total energy use intensity of buildings, we will continue to see the tension between window area and energy use grow.
Minimum daylight and occupant comfort requirements in building codes, as well as stringent envelope-first focused energy codes, are critical to achieving the appropriate balance of human-centric and low-carbon design.
Thank you for writing this. There are many moving parts to designing an “effective” building envelope, especially as targets become stricter over time. For us at Walker, that means the job of communicating how glass performs is becoming increasingly complex and necessary. This article neatly brings competing concerns into focus and gives much-needed perspective on how they’ll have to work together.