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Energy Efficient Windows: What You Need to Know

April 17, 2012

Old windows lose large amounts of heat to the outside during cold days and gain unwanted heat on hot days. Replacing your old windows with good quality, energy efficient windows can save a significant amount of energy and provide increased comfort in your home.

If you are replacing your windows, don’t let the attraction of low prices sway you to buy inferior windows. Quality windows will last a long time and you will realize great savings over time through increased energy savings. Also, qualifying energy efficient windows are eligible for government tax rebates and incentives (see below).

Understanding Window Energy Ratings

When replacing your windows, there are number of issues you will need to understand. Energy rating labels will allow you to compare the efficiency of various windows. The information provided includes:

  • U-factor  – Solar Heat Loss Coefficient (should be below 0.3)
  • SHGC – Solar Heat Gain Coefficient (expressed in numbers between 0 and 1; in hot climates a lower SHGC is preferable)

Some labels may also include scores for air leakage, visible light transmittance, condensation resistance and the annual heating and cooling coefficients.The National Fenestration Rating Council (NFRC) window label includes a number of energy performance measures, including:

  • Condensation Resistance coefficient (a value between 0 and 100, measuring the resistance of the window to condensation; the higher the coefficient the better).
  • Air Leakage Coefficient (a value between 0 and 1, measuring how much air passes through a square foot of window/door/skylight area (cfm/sq ft) by infiltration through cracks; the lower the AL the better).

Choosing Adequate Glass

Beyond the factors discussed above, the following are important considerations:

  • Clear, single pane glass is highly inefficient and is the reason for huge energy losses as well as excessive sunlight entering your home. Single pane glass has a U-factor of 1.1 which is too  high.
  • Two and three pane glasses are recommended. They usually have a U-factor of less than 0.3, which helps reduce heat loss.
  • Argon-filled glass windows have argon gas between the panes. Argon is a much better insulator than air, making the window more efficient. These are very good quality windows that are recommended for energy efficiency and comfort.
  • Low-E coating glass is the most common type of energy efficient glass. Their lower U-factor means that heat losses are kept to a minimum. Especially recommended in cold climates.
  • Low E2 glass is also called Solar Low-E or Spectrally Selective Glass. Using this glass is highly recommended for homes in hot climates because it blocks unwanted solar gains, reducing the need for air conditioning and lowering utility bills.
  • Visible Transmission glass (VT) has the advantage of being able to control glare and heat. That is important to consider if your home is in the northern hemisphere.
  • Impact resistant glass will improve safety in areas prone to high winds or flying debris or where security concerns are a factor.
  • Tinted windows will improve privacy and help control solar heat gains.

Choose a Reputable Manufacturer

There are several smaller manufacturers that produce and sell windows in local markets across the country. Some may have great windows but it might be more difficult to assess the quality. In this case, choosing windows from a more popular, large manufacturer may be a preferred starting point. Comparing products using a system like Energy Star’s NFRC window rating is a way to have confidence in your decision by comparing energy efficient windows using national standards.

Hire a Qualified Installer

The best windows will not do their job without a good installation. It is not a good idea to install windows yourself, unless you are absolutely sure about what you are doing. Hiring a trained professional who has adequate experience is highly recommended. Risks of condensation, water damage or air leaks are common with “do-it-yourself” installations.

Tax Credit and Incentives

There are several state, regional and municipal incentives available to homeowners for energy efficient windows, as well as the federal tax credit program. The US Federal Tax Credit program includes tax credits for qualifying efficient windows and doors. The program includes exterior windows, skylights, storm windows, exterior doors and storm doors. The incentive covers 30% of the investment, up to $1500. For detailed information, visit the Tax Credits page.

Contact your local RESNET Qualified EnergySmart Contractor about installing energy efficient windows in your home.

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