Nice Shades: Study Shows Passive Cooling Can Help in a Heat Wave

In a warming world, we will need passive resilience and survivability.

Buildings in Milan have exterior shades and thick walls.
Buildings in Milan have exterior shades and thick walls.

Lloyd Alter

For years on Treehugger, we have been running a series of posts with the prefix "Nice Shades,"  extolling the virtues of keeping the heat out before it got in. In many temperate Mediterranean climates, like in Milan, they don't have air conditioning. Instead, occupants open the shutters and windows at night and flush the fresh cool air through the apartment and close them in the daytime. The thick masonry walls add a frisson of thermal mass to help even it all out. Architect Michael Eliason recently described the virtues of external active solar protection (operating shades), explaining how they eliminate solar gains and keep places cooler in summer and shoulder seasons.

In the northwestern United States and British Columbia, many people don't have air conditioning because the climate has historically been temperate and they didn't need it. But this has changed and there are now regular heat waves that challenge the human ability to survive.

In 2021, it hit 116 degrees in Portland, Oregon. Researchers at the University of Oregon, led by Assistant Professor Alexandra Rempel, have been studying the use of shading and natural ventilation to see if it could help people cope with these heat waves and recently published the paper "Improving the passive survivability of residential buildings during extreme heat events in the Pacific Northwest". There is a lot for a treehugger to love here, given we have been preaching this for years. It confirms what we have been saying: Shading keeps you cooler if you don't have air conditioning and significantly reduces the AC loads when you do. 

The first point to discuss is the use of the words "passive cooling." Much like our discussions of Passive House, it can get confusing, but the study authors explain:

'Passive survivability' describes the ability of a building to provide survivable indoor conditions without grid-dependent mechanical systems, an ability that is becoming essential as heat waves, high winds, floods, ice storms, wildfires, and other phenomena cause widespread power outages with increasing frequency. Measures that improve passive survivability rely on climatic resources and the physical properties of materials; in the realm of cooling, these include cool night air and wind for use in advective and convective cooling, cold night skies for use in radiant cooling, dry air for evaporative cooling, reflective materials for shading, and in many climates, massive materials for heat storage and release. Elements that control access to these resources (e.g., operable windows, operable shades) are then considered 'passive systems' and tailored to the climate and building type."

I will never understand why traditional technologies like opening and closing blinds, windows, and what Eliason calls "active solar protection" are called "passive systems," but that's just me; it is the accepted terminology now.

Apartment plan and shading details
Apartment plan and shading details.

Alexandra Rempel et al

The study simulated the thermal performance of what they call a "typical two-bedroom top-floor apartment" using climate data from the June 2021 heat wave, both with and without additional air conditioning, and using different window types and different forms of both interior and exterior shading. The results were surprising; in homes without air conditioning, temperatures were reduced to tolerable levels. But the results in homes with AC were also remarkable.

The authors wrote, "The extraordinary implication of these results is that simple, well-controlled shading and natural ventilation have the potential to diminish total and peak mechanical cooling loads dramatically – by up to four-fifths during normal weather and by up to two-thirds during historic heat waves – in the Pacific Northwest."

The authors also note, as we have, that heat pumps-which are air conditioners in summer, and AC units on their own are not the best approach for dealing with heat. 

"Air conditioning alone is neither a desirable nor an effective solution: operation requires electricity, which is regularly compromised during heat waves by sagging power lines and extreme demand peaks; additionally, operational efficiency is diminished and unit capacities are often exceeded during heat waves; equipment purchase and maintenance needs create vulnerability to limited funding; and the hot air expelled exacerbates urban heat island effects, which are already intense in Vancouver, Seattle, and Portland."
Natural Ventilation
Thermal Resilience Design Guide, Version 1.0./Public Domain.

Kesik, Ted, Liam O’Brien and Aylin Ozkan.

There are other implications and issues that come out of this study. The unit design chosen for it is singularly bad for cross-ventilation, with windows only on one side. As Ted Kesik demonstrated in his Thermal Resilience Guide, single-sided ventilation in a typical apartment is pretty marginal. This is why we have been promoting designs with single-loaded open exterior walkways—to promote cross-ventilation. The design of the entire building should be taken into account to maximize passive survivability.

Eliason concurred in his comments on the report, telling Treehugger:

"This is an interesting study that I think is timely—we need to be looking at how our buildings perform in a warming world and designing climate adaptive buildings to improve passive survivability. However, this study appears to have a fatal flaw, looking at a specific unit instead of the whole building. The majority of new multifamily construction in the Pacific Northwest is studios and 1BRs in double-loaded corridor buildings, largely situated on arterials in the hottest parts of cities due to the Urban Heat Island effect. Night cooling is not sufficient in these sorts of environments.
Furthermore, due to the double-loaded configuration, cross ventilation isn't feasible. Shading and natural ventilation will not be enough during prolonged events like this heat dome in a warmer climate—even for temperate places like Portland. These issues are exacerbated by zoning codes that induce buildings where homes cannot cross ventilate, and a virtually non-existent active solar protection industry in the U.S.
There needs to be a deeper focus on external shading and not just for multifamily housing. Unsurprisingly, the researchers' modeling also shows status quo construction leads to indoor temperatures of 100 degrees Fahrenheit even on days when it is relatively mild out. We have a lot of work to do."
passive vs grandma
Grandma's House or Passive House?.

Passivhaus Institute

Notwithstanding flaws in this study, it is, as Eliason notes, important and timely. For years, I have been wrestling with the question: Should we be building like Passive House or grandma's house? Should we be designing for passive habitability with natural cross ventilation with tall, tunable windows, porches, and shading, as grandma had, or should we design for Passive House with super-insulating, fancy glazing, and careful sealing to minimize heat gain and loss? This study provides more evidence that I was asking a silly question, it is not one or the other; we need all of the above. As Kesik pointed out in the Thermal Resilience Design Guide:

"Since the beginning of human history, passive habitability has driven the design of buildings. It is only since the Industrial Revolution that widespread access to plentiful and affordable energy caused architecture to put passive habitability on the back burner. Climate change is influencing building designers to rethink building reliance on active systems that became dominant during the 20th century."

And as this study demonstrates, we have a lot of work to do.

View Article Sources
  1. "Portland Records All-Time High Temperature of 116, Setting New Record for Third Day in a Row." Oregon Live, 2021.

  2. Rempel, Alexandra R., et al. "Improving The Passive Survivability Of Residential Buildings During Extreme Heat Events In The Pacific Northwest." Applied Energy, vol. 321, 2022, p. 119323., doi:10.1016/j.apenergy.2022.119323