Energy Efficient Windows

Installation of energy efficient windows is one of the most effective energy upgrades that can be done for not only energy efficient homes but conventional homes as well.

Although windows provide light, ventilation and views to the outside, they can make heating and cooling the home more difficult. On a hot day, more energy enters through a small patch of clear glass than through an entire insulated wall. Up to half of the total energy loss in a well-insulated structure occurs through and around the windows and other openings.

In hot climates, preventing sunlight from entering the house is the best and most environmentally friendly way to keep the house cool. The most effective method of preventing solar energy from entering windows is by shading from the outside (See eco homes).

A deep overhang on the roof of the house can protect the windows from high summer sun, allowing lower winter sun to enter. Exterior shutters, bamboo shades, blinds or solar screens, fabric or aluminum awnings also work well to provide shading. Shading from trees and trellises is also very effective.

Solar heat entering a house through the windows does so by many different means. These are -

  • Conduction: Heat passes directly through the window panes from the outside to the inside or vice versa.
  • Convection: The window absorbs heat from the surrounding air, causing the cool air to sink to the ground and warmer air to move towards the glass. The heat is absorbed from this warm air.
  • Radiation: Heat passes through the windows as infrared energy.
  • Leakage: Air slips through the cracks around the windows and heat transfer takes place.

There are three fundamental approaches to improving the energy performance of windows.

  1. The glazing material itself can be altered by changing its chemical composition or physical characteristics, for example tinted glazing.
  2. A coating is applied to the glazing surface to reduce heat gain and glare. Films, reflective and low-E coatings are the examples.
  3. Low-conductance gas is filled between two or more panels and thermally improved edge spacers are used.

Heat loss/gain is reduced in energy efficient windows by many methods. The most common method is having double glazed windows with a layer of gas sealed between them (generally argon, a non-toxic, inert gas having low heat conductivity). The resulting glazing panels are known as "insulating glass".

A "Low-E" coating is applied on the outside of the glazed panel. The "low-E" coating is designed to reflect the heat of the sun off the window but allowing the light to pass through. The gas sealed between the double glazing reduces the amount of heat transfer through the window.

Low-e or low emissivity, coating is a thin, nearly invisible coating applied to the glazing panel. Low-e coatings can also be applied to existing windows to improve energy efficiency without replacing the entire windows.

Windows with low-e coatings can save up to 30-50 percent more energy than those without it.

The energy efficient windows design will ultimately depend on the climate where your house is located.

There are many factors which affect the treatment for energy efficient windows -

  • Consider how much sun enters each window. For summer, east and west windows need greater protection from direct sun than north and south windows. In winter, south windows can capture solar heat and reduce heating loads.
  • Color affects heat absorption. Dark colors absorb more heat than light colors. Shiny surfaces reflect more light than flat or dull surfaces.
  • Loosely woven fabrics allow more heat to enter during the summer and more heat to be lost during the winter.
  • Other factors affecting the amount of heat in a room including warm air entering through the air gaps, heat transferred through walls and ceilings, heat generated by lighting, equipment, the number of people and activities in the room and the humidity level in the house need to be controlled.

Various benefits of energy efficient windows are -

  • Savings in energy and subsequently money by less heating and cooling.
  • Better insulation of the home, keeping it warm in the winter and cool in the summer.

  • Higher impact resistance.
  • Cutting-off outside noise.
  • Protecting interior furnishings.

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Sushil Wadhwa

Sushil Wadhwa
Architect, Interior Designer, creator of Homes and Interiors... more.