Master the Certified Energy Auditor Exam 2025 – Power Up Your Energy Audit Skills!

The primary mode of heat transfer through windows on a summer day is radiation. During hot summer days, sunlight passes through the glass of windows and can significantly increase the temperature inside a building. This occurs because sunlight contains infrared radiation, which is absorbed by surfaces within the building, leading to an increase in thermal energy.

While conduction and convection are valid modes of heat transfer, they play a lesser role in this specific context. Conduction refers to heat transfer through direct contact of materials, which is less significant for windows exposed to direct sunlight. On the other hand, convection involves the movement of air and the transfer of heat through fluid motion, but in a still indoor environment on a summer day, it does not account for the primary source of heat entry.

Evaporation is also unrelated to the direct heating effect of sunlight entering through windows. Therefore, among the given options, radiation is the most significant contributor to heat transfer through windows during hot weather, making it the correct answer.