A window’s, door’s, or skylight’s ability to transmit sunlight into a home can be measured and rated according to certain energy performance characteristics.  Transmitting sunlight into your home is called “daylighting”, and it can help you reduce the amount of artificial lighting you need to illuminate your interior spaces.  This, in turn, reduces your electricity use.

The ability of windows, doors and skylights to transmit sunlight is measured using the following criteria:

·         Visible transmittance (VT)

This is a fraction of the visible spectrum of sunlight, weighted by the sensitivity of the human eye, that is transmitted through a window’s, door’s, or skylight’s glazing.  A product with a higher VT transmits more visible light.  VT is expressed as a number between 0 and 1.  The VT you need for a window, door, or skylight should be determined by your home’s daylighting requirements and/or whether you need to reduce interior glare in a space.

·         Light-to-solar gain (LSG)

This is the ratio between the Solar Heat Gain Coefficient (SHGC ) and the VT.  It provides a gauge of how efficient the glass or glazing is in transmitting daylight while blocking heat gains.  The higher the number, the more light is transmitted without adding excessive amounts of heat.  Consumers shopping for windows and doors should be aware that this energy performance rating isn’t always provided.

Today’s highly energy-efficient windows, as well as advances in lighting design, allow efficient use of windows to reduce the need for artificial lighting during daylight hours without causing heating or cooling problems.  The best way to incorporate daylighting in your home depends on your climate and home’s design.

For example, south-facing windows are most advantageous for daylighting and for moderating seasonal temperatures.  They allow sunlight into the home during cooler weather but little direct sun during the summer, especially when properly shaded.

North-facing windows are also advantageous for daylighting. They admit relatively even, natural light, producing little glare and almost no unwanted summer heat gain.

Love the morning sun through your east-facing windows?  Keep in mind that, although east- and west-facing windows provide good daylight penetration in the morning and evening, they should be limited. They may cause glare, admit a lot of heat during the summer when it is usually not wanted, and they also contribute little to solar heating during the winter.

If you’re constructing a new house, you want to consider daylighting as part of your whole house design.  It’s an effective strategy for building an energy-efficient home.

Our goal is to help educate our customers about energy and home comfort issues (specific to HVAC systems).  For more information about daylighting with energy efficient windows and doors and other HVAC topics, click here to download our free Home Comfort Resource guide.

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