Replace Windows to Conserve Energy

Installing new windows can cut your energy use by up to 30 percent, and you can improve the energy efficiency of old windows with a few basic steps.

| July 13, 2009

  • Window
    Sun-sational: Energy Star-rated windows are twice as efficient as typical models sold just 10 years ago. 

  • Window

Windows let the sunshine in, but in many cases, they also let the heat in (or out, in the winter). According to the U.S. Department of Energy, heat transfer through windows can account for 10 to 25 percent of your heating and air conditioning costs. Older, single-paned windows are the biggest energy wasters.

Replacing older windows with energy-efficient ones can be expensive, but doing so will save you money in the long run by reducing your energy use as much as 30 percent. Energy Star-rated windows are twice as efficient as typical models sold just 10 years ago. A variety of factors determine a window’s energy efficiency:

  • Solar heat. A window’s solar heat gain coefficient (SHGC), a value ranging from 0 to 1, indicates the fraction of incoming solar radiation admitted through a window. The higher the number, the more heat will be transmitted, so you want a high value in colder climates (to take advantage of free heat provided by sunlight) and a low value in warmer climates (to keep heat out and lower cooling costs).
  • Heat transfer. The rate of heat transfer between the inside and outside of a home (unrelated to solar radiation) is known as a window’s U-factor, which generally ranges from 0.2 to 1.2. The lower the value, the less heat is lost from your home — especially helpful during the winter. Some double- or triple-paned windows also contain argon, an inert gas, between the panes to minimize heat transfer.
  • Glazing. Most energy-efficient windows are coated to help reduce heat transfer. Low-emissivity (or “Low-E”) coatings, composed of microscopic metal particles, reduce heat transfer by 40 to 70 percent while still allowing most light through. Tinted and reflective glass are also available, but they block some incoming sunlight as well.
  • Framing. Aluminum is a poor choice for window frames because it conducts heat readily.

Cost Considerations

Windows purchased in 2009 or 2010 that meet specific efficiency criteria are eligible for a federal tax credit equal to 30 percent of the purchase price (up to a maximum of $1,500 for all qualifying home improvements).

How to Make Old Windows More Efficient

If you can’t replace your old windows now, there are other steps you can take:

  • Seal air leaks around windows with caulk or weatherstripping.
  • Affix Low-E coated film directly to windows to help reduce heat loss.
  • Install storm windows to reduce heat loss from single-pane windows by 25 to 50 percent. Low-E storm windows can cost less than a new energy-efficient window.
  • Use insulating window treatments including shades, curtains, blinds or awnings to block incoming sunlight in summer and keep heat in during the winter.

The Union of Concerned Scientists is the leading science-based nonprofit organization working for a healthy environment and a safer world. Founded in 1969, UCS is headquartered in Cambridge, Mass., and has offices in Berkeley, Calif., Chicago and Washington, D.C.
8/23/2009 10:42:42 AM

New subscriber- 1st article read-disappointed in the quality of info. Thank goodness I read the comments where more useful info is contained. Thank you Roy Pfz for your most helpful advice and content. More of what I had expected from Mother Earth the mag.

8/21/2009 7:26:37 PM

Thank you for the link in your comment Laura. We have a 100 year old house with the original wooden, leaded glass windows. They are of an unusual size--long and thin and beautiful. I really didn't want to replace them. The ariticle that you sited gave me suggestions so that I wouldn't have to. We insulated the walls in the house last year and I think that made more of a difference than anything. We will be adding to the insulation in the attic this year. We noticed a huge reduction in the amount of drafts last winter. And Dear Mother: I would prefer articles that help me make the best use of what I already have. Our societies are much too wasteful. That is why we purposely chose to buy an existing house and refurbish it rather than some new mini-Mc-mansion. It is cheaper and more environmentally responsible. And the surprising benefit is that our forebearers knew how to build houses that take advantage of natural environment around us. We didn't realize just what a gem we had until we move in. The house was built south facing with lots of windows to let the light in. It cools itself so well in the summer that we hardly have to use air conditioning at all. There is a working cellar under one of the out buildings. The trees in the yard are mature and well placed--not to mention all the little gems we find in our naturalized woods--mature fruit and nut trees, wildlife, and crystal clear creeks. The soil is fertile and great for growing things. We opened the fireplace back up, lined it and popped in a woodstove insert, now we have free heat all winter long. Yes, the house is small, the kitchen is small, the bedrooms are small, and the bathroom is an addition that was added in the 90's (when they got rid of the outhouse)--but I love it and I wouldn't trade it for anything. It is perfect--the air is clean, the garden is wild and we will be adding poultry next year! Viva the more self-sufficent good life!

Roy Pfz
8/21/2009 12:33:00 PM

I would agree with the previous poster. I am an energy auditor and that LAST thing that we recommend replacing is the windows. The payback for most cases is 30+ years, your money is better spent else where. The insulating blinds you can buy have the same or greater R value than most new windows and are much cheaper. I have see cases in homes where they have large south facing windows that they have replaced and their heating bills go up because they lost the solar gain and the single paned windows that they had were fine. Now if you have aluminum framed windows or the frames are rotted out on your windows I would then think about replacing them, but otherwise sealing air leaks around the windows or installing storm windows or plastic over the windows in the winter is a much cheaper and more energy efficient option.

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