How to calculate solar heat load

Solar heat load is the increase in temperature for a room or building generated by the sun's rays. Factors that contribute to overall heat load include the climate, season of the year, insulation, appliances in use and the number of people present.

During the colder months, the sun's energy provides a free source of heat. In summer, solar energy affects how much air conditioning you need. Whether you are remodelling an existing heating and cooling system or are trying to save energy, you need to calculate the solar heat load.

According to the National Fenestration Rating Council, windows are a key factor in figuring out solar gain. Bigger windows heat a room more effectively and more quickly than smaller windows. The direction the windows face also is important.

East-facing windows heat a room most in the morning. South and west-facing windows absorb the most heat in the afternoon. North-facing windows absorb the least amount of heat. Awnings, shades and thick or dark-coloured curtains also affect the solar heat gain.

Walls store solar heat and release it slowly over a period of hours. Heat storage depends on the thickness of the wall, the material from which it is made and what kind of insulation is installed. Nearby reflective surfaces also add to the solar heat load.

The roof is exposed to the sun steadily throughout the day. The solar heat load is determined by the thickness of the roof, its insulation and the material in which it is made. A dark-coloured roof will absorb more heat than a light-coloured roof.

Interior ceilings also can influence solar heat load. A layer of air trapped between the ceiling and the roof will hold heat and make the rooms below warmer. Roofs release the absorbed heat slowly, heating rooms overnight after the sun is down.