Solar Heat Gain and Window Treatments

At Zola, we’re often asked the following question: "What window blinds, internal shutters, or other window coverings should we consider to help reduce solar heat gain?"

Ultimately, interior window treatments (whether insulated or not) don't significantly reduce the amount of heat passing through your insulated glazing units (IGUs, also known as glass).

The heat transmitted through a window or door unit is measured by the Solar Heat Gain Coefficient (SHGC). There are two SHGC values to consider: (1) IGU-only SHGC, and (2) whole unit SHGC. The IGU-only SHGC represents the amount of the sun's energy that passes through the glass alone, while the whole unit SHGC measures the energy that passes through both the glass and the frame.

SHGC values are expressed as decimals and read as percentages. For instance, Zola’s standard triple-pane IGUs have an SHGC of about 0.53, meaning 53% of the sun’s energy will transfer through the IGU into the interior as heat.

To prevent overheating through windows and doors, interior treatments like shutters or blinds are ineffective (whether insulated or not). When used, the sun’s energy transfers from outside to inside, heats the window treatment itself, and then radiates that heat into the room.

Interior window treatments, such as blinds, shutters, or curtains, are excellent for privacy, brightness, or glare control but do very little to combat solar heat gain.

The best defense against overheating is exterior shading solutions. Overhangs, natural shading, and exterior window coverings effectively reduce solar heat gain by blocking the sun before it reaches the windows and doors.

If exterior treatments are not an option, consider lowering the SHGC value of the glass itself. Low SHGC coatings, typically consisting of 2 to 3 sputtered layers of silver, significantly reduce heat transfer through the glass. Zola’s common low SHGC glass has an SHGC of 0.29, meaning 29% of the sun’s energy passes through as heat.

In summary, window treatments are great for privacy, brightness, or glare but they’re not going to help keep interior spaces much cooler - for that option you’re better off using external shades and overhangs or specifying low SHGC glass in your windows.