Inside the “concrete battery”: CED researcher Stefano Schiavon exposes lethal heat in Florida prisons
A study by Professor of Architecture and Civil and Environmental Engineering Stefano Schiavon found that temperatures inside a Miami-area prison routinely reach dangerous levels for extended periods. His report is central to a lawsuit claiming that excessive heat in the prison complex violates the Eighth Amendment’s prohibition against cruel and unusual punishment.
The Florida Justice Institute, a public interest law firm, commissioned the study as part of a class-action lawsuit filed on behalf of prison inmates arguing that persistent and excessive heat creates inhumane conditions. The suit aims to require the Florida Department of Corrections to maintain the indoor heat index below 88ºF, a threshold associated with heat-related stress and illness in previous lawsuits.
The associate director of research at the Center for the Built Environment, Schiavon is an expert in sustainable architecture and thermal comfort, studying how indoor environments affect human health, productivity, and sleep.
He advises on issues related to thermal comfort and building construction as a member of the Extreme Heat Work Group of the International Code Council, the World Health Organization’s Technical Advisory Group on indoor heat risks to human health, and ASHRAE, the international society of heating, refrigerating, and air-conditioning professionals.
The Florida Department of Corrections currently relies on outdoor weather data to trigger heat-safety protocols, such as providing ice or limiting outdoor exercise. “The Florida Justice Institute lawyers convinced a judge to allow us to measure the environment inside the prisons,” Schiavon says. “I helped them make a scientific case that indoor conditions can be warmer than outside.”
After collecting data for the summer 2025, Schiavon found that the indoor heat index was above 88ºF for 97% of the time and frequently rose close to or over 100ºF for lengthy periods, putting the prison population in danger of serious health issues and even death. The risks are especially acute for older prisoners; The Miami Herald reports that more than 50 percent of the prisoners are over age 50 and 24 percent are over 65.
“I was shocked by what I saw,” Schiavon says of his visit to the Dade Correctional Institution, his first time inside a U.S. prison. “I had a mental image of prisoners as young, but many are elderly.”
Calculating heat index in a high-security prison
Schiavon has collected data about the thermal performance of buildings for decades. But this was his first time working inside a prison. He had to devise a rigorous, as well as legally defensible, data-collection strategy within the limitations of the high-security environment.
After a long negotiation with the Department of Corrections over security and logistics, 31 sensors were installed throughout the prison, which logged air temperature and relative humidity every five minutes. Some sensors also measured CO2 levels, which helped Schiavon analyze ventilation and the overall thermal environment. He targeted two distinct areas: open bays housing roughly 80 people in bunk beds, and high-risk wings where some inmates are often confined to small cells for 23 hours a day.
He was not allowed to place sensors in individual occupied cells or collect data remotely: data had to be downloaded manually at regular intervals by FJI staff.
Schiavon used the collected environmental data to calculate the heat index, a metric combining temperature and humidity that measures how the body actually experiences heat stress.
“In some of the prison wings, the heat index was above the safety threshold 97% of the time,” Schiavon says. “The median value was 98°F, and we recorded peaks as high as about 120°F. At every location, it was hotter inside the prison than it was outside. For the entire period we measured — from May to October — it was almost always above 88°F.”
For Schiavon, this data confirms the necessity of measuring heat index conditions inside prison walls rather than relying on outdoor data for policymaking.
The “Concrete Battery” Effect
In many climates, the most hazardous time for heat exposure is the mid-afternoon. However, Schiavon’s data revealed that the most extreme conditions in the prison often occurred in the middle of the night.
“A prison building constructed of massive amounts of bricks and concrete acts like a battery,” Schiavon explains. “In a place like Berkeley, that’s great — it absorbs heat during the day and releases it at night when it’s cool. But in Miami, it doesn’t cool off at night. The building traps the heat, and it becomes a machine that collects energy and keeps it inside.”
Due to climate change, Florida is experiencing higher temperatures and for longer periods than in the past. Fans used to increase air movement indoors are not sufficient to maintain proper thermal conditions. And, Schiavon notes in his report, the buildings are not protected from the sun’s radiation by trees or other shading devices.
“Heat waves are dangerous not just because of the peak, but because of the duration,” Schiavon notes. “If it stays hot for 24 hours straight, that continuous stress on the body is where you see the highest risk.”
Inmates reported being unable to breathe and sleeping on the floor to stay in contact with concrete in a desperate attempt to shed body heat. The FJI lawyers claim at least four deaths at the prison can be attributed to the inability of prisoners to lower their body temperatures.
Using data to end “cruel and unusual” heat in prisons
The FJI submitted Schiavon’s report as part of its lawsuit against the Florida Department of Corrections. FJI lawyers wrote in a statement cited in the Miami Herald, “We’re hopeful that this information [in Schiavon’s report] can be used to protect the lives and health of people incarcerated [at Dade Correctional Institute].”
The goal of the lawsuit, scheduled for trial in summer 2026, is to show that the conditions inside the prison constitute “cruel and unusual punishment” and to force the installation of air conditioning.
“Usually, we look for alternatives to air conditioning to save energy,” Schiavon says. “But there is a limit. Above a certain point, particularly when you have comorbid health conditions, safety becomes the priority and energy becomes secondary. At this Florida prison, only the use of air conditioning could reliably and consistently keep the heat index inside below 88°F.”
Protecting vulnerable populations from extreme heat
Schiavon’s report for the FJI lawsuit is part of a larger effort at CBE to protect marginalized populations as weather across the globe becomes more extreme. Schiavon is currently co-leading a three-year, grant-funded study in Singapore looking at the sleeping conditions of migrant workers to understand how nighttime cooling affects worker safety and recovery. For that study, Schiavon is deploying wearable and ingestible technologies, such as “smart pills” that measure core temperature, skin temperature sensors, heart-rate monitors, and watches that measure sleep.
And in the wake of the Florida study, Schiavon has been contacted by nonprofits and state officials regarding conditions in California prisons, where an aging population combined with an increasing number of heatwaves in the Central Valley presents a similar looming tragedy.
“Our body is not very good at being aware of the problem coming; you are walking and then you suddenly collapse,” Schiavon says. “Data is the only way we can prove these environments are unsafe before it’s too late.”