Environmentally modified housing [EMH; windowless, insulated sidewalls and ceiling, thermostatically controlled ventilation fans) versus conventional housing [CVH; cross-ventilated, insulated ceiling, ceiling fans) improved broiler performance in the summer. The objective of this investigation was to determine whether social behaviors differed between two population densities (0.06 m²/chick [HD] or 0.07 m²/chick [LD]) in these houses. We used a randomized block statistical design, involving houses, population densities, observation times, and bird age. Behaviors were observed weekly, during the morning and the afternoon. Individual observers focused on the group of broilers in one of three defined 26.76 m² areas in each of the four pens in each house. Aggressive encounters, tail and back pecking, feather eating, thermoregulatory, preening, and flock mobility were recorded. Feather pecking, eating and aggressive encounters were expressed at greater rates in HD birds in CVH. A salt-deficient diet caused increased feather pecking and aggressive encounters, which decreased after correction of the mistake. Increased heat indices (HIs), HD, and greater light intensity in CVH influenced behaviors and mortality more severely than in EMH. In CVH and EMH, burrowing/thermoregulatory/resting activity increased with increasing HIs. Afternoon preening was elevated significantly in EMH. It was concluded that broilers reared in EMH were more comfortable and experienced improved welfare compared to those reared in CVH.