Low E technology & double-glazed windows…

January 19, 2009 · Filed Under Buildings & Equipment · Comment 

Low E (Emissivity) technology uses microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor (explained below) by suppressing radiative heat flow. A typical low E coating is transparent to the solar spectrum (visible light and short-wave infrared radiation) and reflective of longwave infrared radiation.

The National Fenestration Rating Council (NFRC) label, which appears on all window, door, and skylight products rated by Energy Star, summarizes a window’s performance in terms several factors listed below.

Reading the NFRC Certified Window Label

U-value (U-factor) is a measure of the rate of non-solar heat loss or gain through a material or assembly. It is expressed in units of Btu/hr-sq ft-ºF (W/sq m-ºC), where sq ft (sq m) represents per square foot (square meter) of window area and ºF (ºC) represents per degree temperature difference between indoor and outdoor temperatures. Values are normally given for NFRC/ASHRAE winter conditions of 0ºF (18º C) outdoor temperature, 70º F (21º C) indoor temperature, 15 mph wind and no solar load. The U-factor may be expressed for the glass alone or the entire window, which includes the effect of the frame and the spacer materials. The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating value.

Solar Heat Gain Coefficient (SHGC) measures the fraction of incident solar radiation admitted through a window (both directly transmitted and absorbed) and subsequently released inward.

Visible Transmittance (VT) measures how much visible light is transmitted on a scale from 0 and 1.

Air Leakage (AL) measures the rate at which air passes through cracks in the window, in terms of cubic feet of air per minute per square foot of window area (cfm/ft^2). Most industry standards and building codes require an AL of 0.3 cfm/ft^2.

In most cases the U-factor, which is found in the top left box on the NFRC label, is the most important number to consider when evaluating windows for thermal insulation and energy efficiency.

Double-Glazed Windows

The figure on the left illustrates the characteristics of a double-glazed window with high solar gain and low E glass. The term “double glazed” means two layers of glass separated by a spacer and sealed at the edges. You can find different types of low E windows with either high solar gain or low solar gain. High solar gain is best suited for buildings located in heating-dominated climates that experience cold winters. High solar gain admits heat from the sun, while a low U-factor due to the low emissivity prevents the escape of heat. This type of window is good for passive solar design relative to other types of windows with low E coating and low solar gain. A double-glazed window with low solar gain and low E glass is suitable for cooling-dominated climates and hot summers.

Related Links

Efficient Windows Collaborative: Window Selection Tool

Efficient Windows Collaborative: Window Operating Types

National Fenestration Rating Council: The NFRC Label

National Fenestration Rating Council: The Facts About Windows & Heat Loss

PG&E: Energy Efficient Window Glazing Systems for Commercial Facilities

NPR: Energy-Saving Windows A Legacy Of ’70s Oil Crisis

Alside: Glossary of Window Industry Terms

Kansas State Energy Extension Service: Building Envelope: WIndows and Doors

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