Information on all the Energy Terms: VT, U Factor, Glazing, and SHGC
Energy efficiency talk has become so popular lately in the window and door industry that we have decided to plainly lay out all the notable terms so that every aspect of every element in your future sunroom or patio room can be easily understood.
First of all, sunrooms and patio rooms may seem much more to handle than a simple window or door installation, but in fact, the only difference is that these rooms simply combine all the elements of walls, doors, and windows, and so the energy standards do not change as more and more elements are added to construct a sunroom such as the one you may be considering.
Here are all the numbers laid out: (scroll down for terms explained)
Windows and Skylights: U factor less than or equal to .3, SHGC (Solar Heat Gain Coefficient) less than or equal to .3
Doors: The requirements mimic those of windows, but must also meet IECC requirements
Roofs: Most metal and reflective asphalt shingles qualify for the ENERGY STAR Tax Credit, but you can check this out for more details.
Most common terms and what they correspond to:
U Factor: The measure of the rate heat loss/gain through a material over time, calculated as a fraction between 0 and 1. As the U Factor approaches zero, energy efficiency increases.
SHGC (Solar Heat Gain Coefficient): The fraction of solar radiation admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward. The solar heat gain coefficient has replaced the shading coefficient as the standard indicator of a window's shading ability. It is expressed as a number between 0 and 1. Again, as the value approaches zero, the more energy efficient the specific material is.
VT (Visible Transmittance): The percentage of light visible to the human eye that a material allows to cross from its full potential, to a lesser amount controlled by window glazing. Simply, the lower the value, the less light is permitted to travel into a specific area through a window or skylight.
Energy Efficiency: This is an objective qualifier that can be made after piecing all these values together and comparing the result to the ENERGY STAR Standards listed above. An efficient window, door, or skylight in your sunroom will not only allow the maximum amount of light to brighten up your space, but also keep the heat in or out depending on your preference. The efficiency varies as the materials change: glazing, frames, spacers, and gas between the panes.
Note: This information provided is intended to simply lay out all the information about energy efficiency and direct you, the consumer, to a smart and well-informed purchase that could end up saving you thousands with today's latest ENERGY STAR Tax Credit Policy.
See these sites for additional information:
In depth anaylsis of the interplay with the U Factor, VT, SHGC, and different glazing
The US Department of Energy's Guide to Energy Savings