Greenhouses overheat because solar gain accumulates faster than the structure can release it, and that imbalance pushes leaf temperature, root-zone water demand, and crop stress upward. The most useful metric here is the solar heat gain coefficient (SHGC), because SHGC translates panel behavior into a practical question: how much incoming solar energy becomes interior heat.

Solar control in polycarbonate means spectral filtering, where the sheet is engineered to pass a useful share of visible daylight while reducing the NIR portion that drives excess heat. That distinction matters because a panel can look bright to the eye and still lower thermal load if it uses an IR-reflective coating or an IR-absorbing additive.

Excess IR raises canopy temperature, which can trigger leaf scorch, blossom drop, bolting, weak fruit set, and faster substrate drying. Lettuce reacts quickly to heat stress through bolting and tipburn risk, while fruiting crops often show reduced pollination performance before the grower notices obvious leaf damage.

The core mechanism is spectral selectivity, which means the panel reduces NIR transmission while preserving as much useful daylight as the crop can use. That is why engineered solar control outperforms ordinary tint, because tint often cuts visible light and heat together instead of targeting the heat-heavy wavelengths more precisely.
Temperature reduction claims should be treated as climate-dependent outcomes, not universal guarantees. Some systems report substantial reductions, even up to 15°C in favorable conditions, but real results depend on solar intensity, ventilation rate, humidity, roof geometry, and the exact panel specification.

Polycarbonate can reduce heat loss through insulation, especially in multiwall sheets. Reducing incoming solar heat usually requires solar control glazing plus ventilation, shading, or evaporative cooling.

Use multiwall panels for better insulation, seal air leaks, and manage humidity so condensation does not steal heat performance. Thermal mass and correctly sized heating controls help maintain stable night temperatures.

A well-designed solar control greenhouse does not chase darkness to fight heat. It uses spectral control to admit the light crops need while rejecting the infrared load that turns a productive structure into a stress chamber.