Solar heat gain and window treatments.

A question we hear constantly: which blinds, internal shutters, or coverings should you add to cut solar heat gain? The honest answer is that interior treatments barely move the needle.

Interior window treatments — insulated or not — don't meaningfully reduce the heat that passes through the glass itself (the insulated glazing unit, or IGU).

How heat gain is measured

The heat transmitted through a window or door is described by its Solar Heat Gain Coefficient (SHGC). There are two values worth knowing: the IGU-only SHGC, which is the share of the sun's energy that passes through the glass alone, and the whole-unit SHGC, which accounts for both glass and frame.

SHGC is written as a decimal and read as a percentage. Zola's standard triple-pane IGUs run around 0.53 — meaning roughly 53% of the sun's energy passes through the glass into the room as heat.

Why interior coverings don't keep you cool

When sunlight reaches an interior blind or shutter, the energy has already crossed the glass. It warms the covering, which then radiates that heat back into the room. Interior treatments are excellent for privacy, brightness, and glare control — but they do very little to fight overheating.

What actually works

The best defense is exterior shading. Overhangs, landscaping, and exterior coverings block the sun before it ever reaches the glass.

Where exterior shading isn't possible, specify a lower-SHGC glass. Low-SHGC coatings — typically two or three sputtered layers of silver — sharply reduce heat transfer through the glazing. Zola's common low-SHGC glass comes in around 0.29, so only about 29% of the sun's energy passes through as heat.

In short: lean on interior treatments for privacy and glare, and rely on exterior shades, overhangs, or low-SHGC glass to keep interior spaces comfortable.