Interior vs. Exterior Shade Cloth

Mounting shade cloth on the outside of the greenhouse is 20 to 30 percent more effective at reducing interior temperatures than mounting the same cloth inside. The reason is simple physics: external shade blocks solar radiation before it passes through the glazing. Internal shade blocks light from reaching plants but allows heat to build up between the cloth and the glazing, which then re-radiates into the growing space.

External Shade: Maximum Cooling

External shade cloth intercepts sunlight at the first surface — the outside of the greenhouse. The energy is either reflected (aluminet) or absorbed by the cloth and dissipated into the outdoor air by wind. The greenhouse interior never receives the blocked radiation. External shade is the correct choice when heat reduction is the primary goal.

The tradeoff: external cloth is exposed to weather. Wind lifts improperly secured cloth, rain weights it down, and UV degrades it faster than protected interior cloth. Secure external shade with rope or bungee cords at multiple points, and use UV-stabilized horticultural shade cloth rather than general-purpose fabric.

Internal Shade: Easier Installation

Internal shade cloth mounts on wires, hooks, or tracks inside the greenhouse structure. It is protected from wind, rain, and direct UV exposure, lasting 1.5 to 2 times longer than external cloth in the same environment. Retractable internal shade systems allow precise, adjustable shading — deploy for peak sun hours, retract for morning and evening light.

However, heat that penetrates the glazing is trapped between the glass/polycarbonate and the shade cloth, creating a hot air layer at the top of the greenhouse. Even with ventilation, internal shade allows more heat into the growing space than equivalent external shade.

The Best of Both

Many commercial greenhouses use both: permanent external shade cloth for baseline summer cooling and retractable internal curtains for fine-tuned light management. For hobby growers, start with external shade (cheaper, simpler, more effective for cooling) and add internal retractable shade only if you need adjustable shading for different crop zones.

Seasonal Considerations

External shade cloth should be installed in late spring and removed in early to mid-fall. Leaving external shade in place during winter blocks precious low-angle sunlight when light levels are already at their seasonal minimum. Internal retractable shade has no seasonal limitation — retract it during low-light periods and deploy only when needed. This flexibility is one of the key advantages of internal systems for year-round greenhouse operations.

In regions with highly variable summer weather (frequent alternation between sunny and cloudy days), fixed external shade cloth can be too dark on overcast days. Some growers address this by using a lighter shade percentage externally (30 percent) and supplementing with internal retractable shade (deployed on the brightest days only). This hybrid approach provides baseline protection against the most intense sun while preserving light availability during cloudy weather.

Wind is the primary durability concern for external shade cloth. Secure external installations with rope, bungee cords, or clips at 12-inch intervals along all edges. Leave the ridge area partially open to allow wind to pass through rather than catch the cloth like a sail. In areas prone to severe wind events, consider quick-release mounting hardware that allows you to remove the cloth in minutes before a storm. Replacement external shade cloth is an annual budget item for windy locations — plan for it rather than being surprised.

For hobby growers just starting with shade management, exterior shade cloth is the recommended first step. It is the most cost-effective cooling method per degree of temperature reduction, requires no internal hardware, and can be installed in an afternoon with basic tools and hardware. If you later need adjustable or zone-specific shading, add an internal retractable system as a complement rather than a replacement.