Opening windows at night and closing blinds during the sunniest part of the afternoon can prevent homes from becoming dangerously hot during extreme heat waves. In other words, passive cooling of homes is efficient.

New research from the University of Oregon (UO), USA, measures the impact of various strategies for passively cooling homes, particularly in the Pacific Northwest.

In the simulations, which use weather data from a severe heat wave in 2021, a combination of shading and natural ventilation kept apartment temperatures out of the danger zone throughout the three-day event, even without air conditioning. In other cases, it reduced air conditioning use by up to 80%.

Passive cooling of homes works, but access to it needs to be increased

The findings could help create building strategies to protect tenants from the effects of severe heat: Cities could require apartments to have operable windows that can be safely left open overnight, as well as functional blinds, they write. Tech Xplore.

“In the Pacific Northwest, where we have such cold air at night, we have an amazing climate for passive cooling. And we should take advantage of that,” said Alexandra Rempel, researcher at the UO.

Rempel and her research group will publish their results in the September issue of the journal Applied Energy.

Data collected in the summer of 2021

In June 2021, an extreme heat wave hit Oregon and Washington. Temperatures reached 47 degrees Celsius in Portland and 44 degrees Celsius in Eugene, breaking previous records. The prolonged heat was deadly, and the impact was particularly severe on people living in apartments in dense urban areas.

Such extreme heat events are expected to become more frequent due to climate change. Therefore, finding ways to make housing livable during the intense summer heat is becoming an increasingly urgent problem.

But buildings in the Pacific Northwest are typically designed to retain heat. Many homes have no air conditioning at all, given the usually mild summer weather, or only have portable, window units.

While strategies such as drawing blinds and opening windows are time-tested ways to passively cool homes, there hasn’t been strong evidence to show whether they can make a significant difference in very high temperatures, Rempel said.

What did the researchers discover?

Armed with weather data collected from cities like Eugene, Portland and Seattle during the 2021 heat wave, the researchers used a computer program to simulate conditions inside a hypothetical west-facing two-bedroom apartment with different cooling strategies.

“Without blinds or ventilation, you’re going to be in the danger zone quickly,” said graduate student Jackson Danis, co-author of the study.

But even a small window opening reduced the amount of time the apartment was dangerously hot. And the strategic use of a combination of passive cooling techniques could make the apartment surprisingly livable, even in the face of extreme outside temperatures.

Opening the windows made the biggest difference at night and early morning, when the outside air is coldest, the researchers found. Meanwhile, using blinds or shutters helped the most during the late afternoon when the sun was shining directly on the windows.

Thick exterior shutters were most effective, but standard interior blinds or drapes, which tenants are more likely to have, still showed a difference, especially if their edges were sealed with side rails.

The impact was even greater with a fan in the window to help circulate the air.

Passive cooling of homes, “lifeline” for those without air conditioning

While the advice seems intuitive, “the magnitude of the improvement is something we didn’t expect,” said Alan Rempel, a mathematician and co-author of the study on passive cooling of homes.

Passive cooling strategies can be a “lifeline” for people without air conditioning. But even people with AC could use these techniques to lower their energy bills during the summer, added Michael Fowler, a building researcher at Mithun Inc. of Seattle, who was also involved in the study.

Reducing the use of air conditioning relieves stress on the power grid, reducing the risk of power outages during heat waves. It’s also good for the environment, added Alexandra Rempel.

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