Indoor residential environments will be an increasing source of dangerous heat exposure as summer temperatures rise. Although air conditioning is a recognized preventative measure, many vulnerable residents either lack air conditioning or cannot always afford the electricity. Understanding the relative influence of different building characteristics and percentages of tree canopy coverage on indoor air temperatures during extreme heat events can help prioritize intervention strategies. We measured indoor and outdoor temperatures at 140 homes in Detroit (MI), Atlanta (GA), and Phoenix (AZ) and surveyed residents to determine the presence of a working central air-conditioning system and their ability to afford utilities. For each home, we collected information on construction year, interior size, single versus multifamily structure, façade type, and percentage of tree canopy. After air conditioning, the importance of building characteristics versus tree canopy measures varied by city. In Detroit, masonry façades worsened the influence of outdoor temperatures, whereas increased tree canopy coverage moderated the influence. In Atlanta, building characteristics were not significant, but tree canopy moderated indoor temperatures in late afternoon. In Phoenix, some building characteristics moderated the influence of outdoor temperatures in late afternoon. In Detroit, only 35% of respondents had and used central air conditioning as needed compared with 57% in Atlanta and 95% in Phoenix. The influence of tree canopy and building characteristics on indoor temperatures varied by city depending on its background climate. We recommend adding questions about air conditioning and utility poverty to the U.S. Census. For rental properties, municipalities should establish and enforce maximum indoor temperature thresholds. States should include cooling in their Low Income Home Energy Assistance (LIHEAP) programs. Public cooling centers need to anticipate electricity outages. Urban planners must ensure efforts to increase mechanical cooling are powered by renewable energy sources.