At the Xiaomi YU7 launch event, CEO Lei Jun mentioned that the Xiaomi YU7' s EC smart dimming sunroof, when in shading mode, provides better heat insulation than a physical sunshade.
This may lead many consumers to ask: "A sunshade is nearly opaque, So how can an electrochromic (EC) dimming sunroof, which blocks 99.85% of light, outperform it in heat insulation?”
To understand this, we first need to clarify what "heat" is. Sunlight is primarily composed of three types of radiation:
● Ultraviolet (UV): ~7% of solar radiation
● Visible light: ~43%
● Infrared (IR): ~49%
UV radiation causes the sensation of being "sunburned", while visible and infrared light cause objects to heat up. Therefore, controlling the amount of visible and infrared light entering the car is key to heat insulation.
Heat in a Car Comes from More Than Direct Sunlight
When sunlight hits the surface of the sunroof, its energy divides into four parts:
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Some is reflected away,
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Some is absorbed directly by the sunroof.
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Some raises the sunroof' s surface temperature, turning it into a secondary heat source.
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The rest passes directly into the cabin.
The “heat” we feel inside the car comes not only from transmitted light but also from secondary heat transfer—heat absorbed by the sunroof and then radiated or conducted inward.
To understand this, recall the three primary modes of heat transfer:
● Radiation: Standing near a sun-baked car feels like a furnace, as hot surfaces radiate energy outward.
● Conduction: Touching a hot sunroof burns your hand, but the sensation vanishes once contact stops.
● Convection: Opening the windows or turning on the AC creates airflow that helps carry heat away.
"EC Dimming Sunroof" vs. "Sunroof + Sunshade": How Does Heat Transfer Differ?
In real-world summer driving with the AC on, both radiation and convection matter. Let' s compare the two systems:
● EC dimming sunroof: Most light and heat are blocked before entering the cabin. The small amount that passes through adds minimal direct heat, while the rest is secondary heat transfer from the sunroof itself (radiation + convection).
● Sunroof + sunshade: This setup creates three zones—outside, between the glass and the sunshade, and inside. The sunroof heats up and transfers energy to the sunshade through radiation and convection. Although the sunshade reflects some light, it still absorbs heat, which then accumulates in the cabin as secondary heat transfer.
The key distinction is where the heat is stopped:
● With EC glass, heat is blocked at the exterior surface.
● With a sunshade, heat has already penetrated the glass and is trapped inside, similar to closing curtains at home after sunlight has already entered through the window.
Official test results from Xiaomi confirm this: after 1.5 hours under 38°C (100°F) sunlight, the EC smart dimming sunroof in shading mode provided better heat insulation than a competitor’s physical sunshade.