What Really Matters in Multi-Story Design? A New Study Looks at It All

Treehugger often talks about window sizes, dumb boxes, mass timber, air tightness, upfront carbon, or even construction cost, but never all in one post. But a new "simultaneous sensitivity study" looks at all these variables at once. It's a wonderful magic box of a study and, in a world where people paid more than lip service to carbon emissions, it would be hugely influential.

The paper, published in the journal Applied Energy, is written by a team led by Hannes Gauch. The team includes engineer Will Hawkins, whose paper on timber and carbon sequestration I quote regularly. The study addresses a subject dear to this Treehugger's heart: the increasing significance of upfront or embodied carbon when operating carbon emissions are reduced, what I call the ironclad rule of carbon.

"Tightened legal requirements, such as in the European Union have already led to a decrease in operational energy consumption of new compared to existing buildings. However, embodied emissions in modern buildings are not only increasing relative to operational emissions, but also in absolute terms. This trend raises the question: Do significant trade-offs between embodied and operational emissions exist in building design?"

There are so many knobs to turn here, with input variables for a building's shape, size, layout, structure, ventilation, windows, insulation, air, and use for residential and office multi-story buildings, across different climates. For embodied carbon, they use "cradle-to-completion," which is our definition of upfront carbon emissions. They modeled a simple building without basement or service cores.

Compactness

We have often quoted architect Michael Eliason on simple, compact forms or "'dumb boxes'—the least expensive, the least carbon-intensive, the most resilient, and have some of the lowest operational costs compared to a more varied and intensive massing." The study confirms that compactness cuts heating and cooling in half and reduces upfront carbon and construction costs.

Structure and Frame Type

Alas, our beloved cross-laminated timber had the lowest upfront carbon but the highest cost. "Cheaper frame types tend to be more carbon intensive and vice versa, pointing to a significant trade-off," states the study. However, the study did not look at light wood frame construction, which is common now in low-rise residential buildings, and would likely have been lower in both upfront carbon and cost.

Lead researcher Hannes Gauch tells Treehugger, "We have not included light timber frames in our study. It is not common in the U.K. to build larger buildings that way, but a comparison might be an interesting study to do!"

Building Height

The Goldilocks spot for upfront carbon is four to six stories, and for cost, six to eight. As you get higher, you pay more to add stability and bracing but less for the roof. The study only went to 15 floors but confirms what Eliason and Treehugger have written. This is an interesting variable that probably needs more study because building codes have such a big impact on the type of structure, the number of stairs, and the fire separations, which, as far as I can tell, are not taken into account here.

Window Size Is More Important Than Wall Insulation

As I keep saying, windows are hard. The study stated: "Decisions concerning windows are most influential for heating and cooling loads, especially the window-to-wall ratio. Whilst higher window-to-wall ratios decrease all three efficiency metrics, windows with lower U-values (triple and quadruple glazing) entail higher costs. This suggests a non-negligible trade-off between energy efficiency and construction costs."

Getting the windows right in size and heat gain is more important than whether they are triple-glazed, and adding more insulation has diminishing returns. This one will be controversial, but that's why I say windows should be designed for well-being and beauty, not watts or lumens.

The researchers also look at cladding (don't use brick!) and ventilation (it matters, and heat recovery is more important than air tightness). They found that a compact building with low window-to-wall ratios could hit Passivhaus requirements at a relatively low cost; get skinny or glassy, and the building could cost 30 to 50% more.

There is also a section on climate that is complex and could be an entire post on its own, so I will not go into it here.

Form and Fenestration

We keep hammering away at increasing energy efficiency and reducing upfront carbon, but don't talk nearly enough about form. In the discussion, the researchers start with a bang:

"Our results show that building size and shape as well as the frame type and layout are amongst the most significant variables determining embodied carbon, construction cost, and heating and cooling loads of a building... We have shown that it is difficult, but possible, to achieve designs with near-zero energy demands for heating and cooling in many climates. Yet only through a combination of mechanical ventilation with heat recovery, compact building forms, limiting the window-to-wall ratio, and small solar heat gain coefficients can designs satisfy, or get close to, Passivhaus standard requirements." 

This is so important. Many cities like New York or Toronto have green building codes, yet keep approving skinny pencil towers that are thermal nightmares because of their form. This is why I have written that the single biggest factor in the carbon footprint of our cities isn't the amount of insulation in our walls, it's the zoning. We need compact six-story buildings everywhere, instead of spiky sprawl.

In an email to Treehugger, Dr. Gauch summarized, "Our results show that to lower both upfront and operational emissions in new multi-story buildings, we should make them compact, design with timber instead of steel or concrete, choose light cladding and modest window areas, and install mechanical ventilation with heat recovery."

In greater detail, the researchers concluded:

• The size and shape of a building have considerable influence on both embodied and operational efficiency, particularly for residential buildings. While planning and designing a new development, the implications of choosing a building size and shape should be thoroughly evaluated. Increasing building compactness was found to considerably lower embodied and operational impacts.
• Choices concerning window-to-wall ratio and window types were found to be of high importance for operational efficiency for nearly all examined cases. Lower window-to-wall ratios and lower SHGC [Solar Heat Gain Coefficient] lead to lower heating and cooling across all building types and climates.

These are all points that we have made before on Treehugger. We quoted Jo Richardson and David Coley of the University of Bath, who called for "a revolution in what architects currently consider acceptable for how houses should look and feel" and acknowledge that "[it's] a tall order—but decarbonizing each component of society will take nothing short of a revolution." Or, as I have written, "If we are ever going to get a handle on our CO2, we are going to see a lot more urban buildings without big windows, without bumps and jogs. Perhaps we might even have to reassess our standards of beauty."

I am not alone in being very impressed with this study. Architect Kelly Alvarez Doran of MASS Design Group and the University of Toronto tells Treehugger: "I found that study super useful. Less is less. Smaller dual-pane windows instead of bigger triple-pane ones is a great illustration of where we need to go."

Twist all the knobs on this wonderful magic box, and you come up with simple forms—not too tall, a wooden structure, and windows designed to frame a view, not make a statement. It's what architect Bronwyn Barry hashtags as #BBB: "Boxy But Beautiful." And no more of this skinny glass tower stuff.