Understanding the visual light transmittance requirements in building envelop

Visual Light Transmittance (VLT) is a measure of the amount of visible light that passes through a fenestration product such as windows, skylights, and doors. VLT is expressed as a percentage of the total visible light that strikes the surface of the glass, and it ranges from 0% (completely opaque) to 100% (completely transparent). VLT plays a significant role in determining the energy efficiency of a building as it can affect the amount of natural light that enters the building and the amount of heat that enters or escapes through the fenestration product.

The VLT of a fenestration product is determined in accordance with ISO-15099 by an accredited independent laboratory, and labeled or certified by the manufacturer. For unrated products, the default table in Appendix A can be used. The VLT calculation is an important part of building design, as it can help to ensure that the building’s energy use is optimized.

Here is a five-step method to calculate VLT using the ECBC code:

Step 1: Determine the total solar energy transmittance (TSET) Total solar energy transmittance (TSET) is the ratio of the solar energy that passes through the fenestration product to the total solar energy that strikes the surface of the glass. TSET is calculated using the following formula:

TSET = Tsol × VF + 0.06 × VNF

Where: Tsol is the solar heat gain coefficient (SHGC) of the glazing, as determined by ISO-15099 VF is the visible transmittance of the glazing, as determined by ISO-9050 VNF is the visible non-fading factor of the glazing, as determined by ISO-9050

Step 2: Determine the visible light transmittance (VLT) of the glazing The VLT of the glazing is calculated using the following formula:

VLT = VF × (1 – R)

Where: VF is the visible transmittance of the glazing, as determined by ISO-9050 R is the reflectance of the glazing, as determined by ISO-9050

Step 3: Determine the visible light transmittance (VLT) of the fenestration product The VLT of the fenestration product is calculated using the following formula:

VLT = VLTglazing × (1 – VFframe) × (1 – VFspacer)

Where: VLTglazing is the visible light transmittance of the glazing, as determined in Step 2 VFframe is the visible transmittance of the frame, as determined by ISO-9050 VFspacer is the visible transmittance of the spacer, as determined by ISO-9050

Step 4: Adjust the VLT for shading devices If the fenestration product is equipped with shading devices such as overhangs or screens, the VLT needs to be adjusted. The adjustment is calculated using the following formula:

VLTadjusted = VLTunshaded × (1 – PF) + VLTshaded × PF

Where: VLTunshaded is the unshaded VLT, as determined in Step 3 VLTshaded is the VLT of the shaded fenestration product, as determined by the manufacturer or calculated using a simulation program PF is the projection factor, which is the ratio of the horizontal depth of the shading device to the height of the fenestration product

Step 5: Determine compliance with the VLT requirements of the ECBC Compare the VLTadjusted value with the VLT requirements of the ECBC. If the VLTadjusted value is greater than or equal to the required VLT