By Mark Zeh
Here are a few performance factors critical for creating a great shaded area on a hot day:
- Shade Factor (SF): This is the percentage of visible light which a shade fabric blocks. Higher values mean darker shade areas.
Solar Reflection Index (SRI): This is the average amount of energy from the complete solar spectrum which is reflected by a surface. In the case of shade fabrics, this is the percentage of solar energy which is reflected by the fabric, rather than simply being stopped by it.
This is an important difference, since fabrics with low SRIs become warm and re-radiate Infra-red light to their shaded sides, making the space under a low SRI fabric warm.
Be sure to check how given ratings are measured, since this is sometimes only measured as a percentage of visible light (400 to 700 nanometer wavelength), rather than the full solar spectrum.
Remember that Infrared light (heat) is composed of many wavelengths of light below the sensing threshold of the human eye.
Ultraviolet light, the classification of the family of short wavelengths of light associated with tanning, plant growth, aging of materials, fading of colors, and radical effects on biology, is composed of wavelengths of light shorter than what can be sensed by the human eye. Most shade sail materials will give ratings for UV reflection (UVR) and an overall UV protection factor (UPF).
A good high SRI shade sail fabric should also include reflection of both of these types of invisible light, along with reflection of visible light, in its performance rating.
- Ability to allow airflow: This is usually a tradeoff with the above factors, since it implies a certain open-ness of the weave. In traditional structures shade materials of varying weave patterns and densities are layered with vertical airspace gaps between the layers, to allow for airflow, while maximizing shade.