Room & Windows
Window Glass Area by Direction (sq ft, 0 if none)
Total Heat Gain
Room heat gain is the total rate at which heat enters a space from every source. Unlike a quick area-based estimate, a heat gain calculation looks at where the heat actually comes from, and in particular it treats each window direction separately, because the direction a window faces changes its solar contribution dramatically. This is the calculation that explains why one room in a home is always hotter than another of the same size.
| Direction (N. hemisphere) | Peak BTU per sq ft of glass | Peak time | Severity |
|---|---|---|---|
| West | ~130 | Late afternoon | Worst, coincides with hottest air |
| East | ~80 | Morning | Strong but cooler outdoor air |
| South | ~70 | Midday | Steady through the day |
| North | ~40 | Diffuse only | Lowest gain |
Two 14 by 12 ft rooms in a hot climate, each with 20 sq ft of glass, no shading. One faces north, the other west.
This is why the west-facing bedroom in a house is so often the one that never cools down in the afternoon, even with the same AC as the rooms around it.
What is room heat gain?
The total heat in BTU per hour entering a room from all sources: solar through windows, conduction through walls and roof, occupants, equipment, and infiltration. It determines the cooling capacity needed and shows which factors dominate.
How much heat comes through a window?
It depends on direction. In a hot climate an unshaded west window admits 130 or more BTU per sq ft of glass at peak; a north window admits about 40. East peaks in the morning, west in the late afternoon.
Which window direction gains the most heat?
In the northern hemisphere, west, because its peak gain in late afternoon coincides with the hottest outdoor temperature. North gains least. The southern hemisphere swaps north and south.
How can I reduce solar gain through windows?
External shading is most effective, stopping sun before it hits the glass. Solar film and low-e glazing reduce gain through the glass. Internal curtains help but less, since the heat is already inside.
Does heat gain determine my AC size?
Yes. The total gain at peak conditions is the capacity your AC must provide. A 16,000 BTU per hour gain needs an AC rated at or just above that, a 1.5 ton unit. The calculator converts the total automatically.
Why does my room get so hot in the afternoon?
Usually west-facing windows taking direct low-angle sun, combined with a roof or walls radiating heat absorbed all day. If discomfort peaks at 3 to 6 pm, west solar gain and roof heat are the likely causes.
Heat gain estimates are based on standard methods and the conditions you select. Actual solar gain varies with latitude, season, glazing type and local shading.