The hottest direction of a residence in the United States during the summer is typically the southern side. This is due to the angle of the sun's path, which is at its highest in the sky during the summer solstice, around June 21st. The sun's rays are more direct and intense on the southern side of a building, leading to higher temperatures in that area. According to the U.S. Environmental Protection Agency (EPA), the southern exposure can result in up to 10-15 degrees Fahrenheit warmer temperatures inside a home compared to the northern side.

The reason for this lies in the way solar radiation interacts with the building's structure. During the summer months, the sun is higher in the sky, and its rays strike the southern side of a building more directly. This direct sunlight heats up the walls and roof, which then radiate heat into the living spaces. Additionally, the southern side of a building often receives more sunlight throughout the day, leading to prolonged exposure and increased heat gain.

Data from the National Renewable Energy Laboratory (NREL) supports this observation. A study conducted by NREL found that the southern exposure of a building can lead to higher indoor temperatures, especially during the afternoon and early evening when the sun is at its highest point in the sky. The study also noted that the impact of solar gain can be mitigated by using reflective materials on the southern side of the building, such as white or light-colored siding and roofing materials.

Furthermore, the orientation of a residence can have a significant impact on its energy efficiency. A home that faces south is often better suited for passive solar heating, which can reduce the need for heating during the winter months. However, during the summer, this same orientation can lead to increased cooling costs as the home absorbs more heat from the sun.

In conclusion, the southern side of a residence is the hottest in the summer due to the angle of the sun's path and the direct exposure to sunlight. This can lead to higher indoor temperatures and increased energy costs for cooling. To mitigate these effects, it is important to use reflective materials and proper insulation on the southern side of the building.