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As the planet warms, heat-vulnerable communities in cities face increased heat-related risks including lost productivity, reduced learning outcomes, illness, and death. Despite the growing threat of heat, effective approaches to alleviate urban heat are available. Tree planting has received investment in a growing number of cities around the world, but there are significant gaps in our understanding of the cooling potential of trees in the urban context, particularly the impacts on indoor spaces where urban dwellers spend most of their time. Our study engaged community scientists in Los Angeles County, USA to collect data on the impacts of trees on indoor and outdoor thermal conditions in residential sites. Participants created a thermal sensor network that contributed continuous readings for the study period. We mimicked an experimental research design using a difference-in-differences approach where “treehouses” with more trees and “non-treehouses” with fewer trees were compared on hot days (>90°F or 32°C) and non-hot days. We found that on hot days indoor temperatures in treehouses warm less than in non-treehouses, but that trees provide relatively less benefit at night. We also found that exposure to extreme heat reaches dangerous levels in older residences without trees or air conditioning. underscoring the need for swift action to cool heat-vulnerable communities.