Question

Can exterior sun blocking shades on south, west windows help reduce summer heat gain in my home enough to warrant their cost?

Asked by Marcia Brink
Ames, IA

I've looked at several manufacturers online, and exterior sun-blocking shades are all quite expensive and require professional installation, so I want an unbiased opinion before I seriously consider making such an investment. The windows involved are tall (5 ft). Inside, the window treatments are cafe shutters with 2-1/2 in. louvers on the bottom half of the windows only, nothing on the top half. I like the way I can use the louvers to adjust the amount of light and privacy but am considering blocking more sun in the summer by adding a shade to the top half (interior) or a full, sun-blocking shade on the full window outside.

Answer

David Willson

Answered by David Willson

Sebastopol, CA

Advanced Home Performance

September 23, 2013

Marcia,

I wish there was an easy "yes, do this and your problems will be solved" answer to your questions about solar shades, but there's not.

  • Here's part of the answer: solar shades can and likely will help reduce your summer heat gain through the windows.
  • Here's another part of the answer: solar shades may not be the best or most cost effective way to reduce summer heat gain through your windows.
  • Both are correct.

Reducing heat gain

There are four ways to reduce solar heat gain through an existing window.

  • an exterior sun shade that physically blocks the suns rays from hitting the glass,
  • a window film applied by a reputable installer,
  • interior shades such as what you already have, or
  • replacing the glass with new glass that has a lower SHGC (Solar Heat Gain Coefficient).

Let's assume you don't want the cost of replacing glass, then you are left with the other three methods.

Exterior sun shade

An exterior sun shade has to be designed so that it blocks the sun during the hottest part of the day, late morning through late afternoon, and then only during the hottest season, early summer through fall. Just putting up an awning without considering the angle of the sun's rays that you need to block may give you an unsatisfying result.

If you decide to go this route, talk to the contractor/installer about how well his product takes into account the angle of the sun's rays.

  • The pluses: an exterior shade can block ALL of the sun's rays and offers the most heat rejection.
  • The negatives: susceptible to weather damage, not long lasting unless made of metal, may greatly reduce the view out of your windows.

Window film

An applied window film on the inside of your existing windows can stop a lot of the sun's heat.

Clear glass is mostly transparent to the wavelength of light that carries the most heat - infrared. The infrared passes from the sun through your window glass and hits the floor or furniture, heating whatever surface it hits and warming the inside of your home.

A window film stops the majority of infrared at the glass so your floor and furniture stay cooler, with less sun damage. Some of this heat gets re-radiated into your home but much of it is sent back outside, with the net result that your home stays much cooler while you can still see out your windows.

  • The pluses: does not alter the exterior look of your home, retains the full view out of your windows, possibly cheaper than an exterior sun shade.
  • The negatives: also reduces the visible light coming in through your windows similar to a pair of sunglasses, washing your windows with an ammonia based window cleaner may void the warranty of the window film and cause it to bubble and separate.

You are already familiar with interior window shades so you can imagine, and decide for yourself, whether an interior shade treatment is your preference.

Whichever you choose, be sure to talk to at least two companies before you settle on one.

 

For more information:

Read "In a west-facing room, what is the most effective way to stay comfortable in summer and winter: replace windows or add shades?" a Q&A answered by Polly Osborne.

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