What You Really Need to Know About Windows, Attic Fans, and Solar Tubes

There are numerous claims in the marketplace regarding the savings and performance of new windows.  Earnest door-to-door sales people and radio ads promote low-E triple pane windows with enticing, but often inaccurate, information about savings.

Home improvement stores and trade shows hype other popular products like solar tubes, solar attic fans, and skylights.  One of these products is a better choice for the Texas Hill Country climate, while the other two are not.

So how do you make sense of all these products and claims?  Let’s review the facts.


First and foremost, new windows retrofitted into an existing home are expensive, and are most likely not going to save you a significant amount on your energy bill.  New windows can provide greater appeal and comfort to a house.  Depending on the quality, they may be better at keeping out noise, dust, and insects.  However,  they will not often have the energy-savings payback that was promised in the brochure.

Before purchasing new windows, perform a blower door test.  This test will depressurize the house and the technician administering the test will walk around the house with a smoke stick identifying where infiltration is occurring.  The existing windows in the house may be very tight and only need minor repairs and/or solar screens added to provide greater comfort.

If you do need to replace windows or are building a new home, understanding how to evaluate a window’s potential performance starts with the rating system.  The National Fenestration Rating Council (NFRC) grades windows on their U-factor and solar heat gain coefficient (SHGC). According to the Building Energy Code Resource Center:

“The U-factor measures how well a product prevents heat from escaping.  The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating value.

“The SHGC measures how well a window blocks heat from sunlight.  The SHGC is the fraction of the heat from the sun that enters through a window.  SHGC is expressed as a number between 0 and 1.  The lower a window’s SHGC, the less solar heat it transmits.”

Government regulations only require U-factor and SHGC minimums, but another performance rating to consider is Air Leakage.  Look for Air Leakage numbers below 0.1.

Look for the window energy rating decal on all new products

For the Texas Hill Country region a U-value below 0.50 and SHGC below 0.30 are minimum standards.

Keep in mind that these numbers do change based on changes to the Energy Code.  The proposed 2012 Energy Code with Austin Amendments was adopted in early 2013.  The minimum Window U-value has been lowered to 0.40 and the minimum SHGC has been lowered to 0.25.  Due to the progressive updates of the Energy Code, it is important to check the current code before making any window purchase.

Now that we understand the numbers, the next question is:  Do we need high performing windows everywhere?   The answer is: Yes and No.  A good U-value is always important, but the SHGC is not a big factor for windows that do not receive direct sun, i.e., windows with north orientation or in a home with good passive solar design.

Another factor to consider is the material and quality of construction.  How do you decide between wood, metal, vinyl, or fiberglass windows?  Is there a major difference in energy efficiency?

High performing, dual pane, thermally broken, metal windows are easily available.  Made of prefinished recyclable aluminum, they provide the benefit of no finishing costs and minimal maintenance.  Fiberglass window frames have high strength as well as low conductivity and are often enhanced with interior wood veneers.  They have the added advantage of the glass and frame responding similarly to temperature and humidity, since they are basically the same material.  Wood windows are also high performing and usually clad with metal or fiberglass on the exterior to improve their durability.     

All of these options produce similar results in terms of energy savings.  Well-built vinyl windows are energy efficient and are generally the least expensive option; however, the material inherently poses environmental concerns.

Of all common plastics, vinyl (polyvinyl chloride) is generally considered the most dangerous.  It is in the same chemical family as the banned pesticide DDT, PCBs, and dioxins, and is considered highly toxic to manufacture.

Vinyl manufacturing creates substantial amounts of dangerous air and water emissions, as well as hazardous waste.  Chemical additives used in vinyl products are themselves toxic, and over time can harm indoor air quality.   In the event of fire, hydrogen chloride and heavy metals are released, and dioxins are created through the combustion process.  In addition to these hazards, vinyl takes as long as 200 years to biodegrade, and toxins in it bioaccumulate in the food chain.

For all these reasons, environmentally conscious consumers should avoid vinyl products whenever possible.

Solar Attic Fans

At first glance, attic fans seem to make sense – cooling down your hot attic means a cooler, more comfortable home, and what could be greener than a solar-powered fan?  Let’s take a closer look.

It’s true that IF the attic is completely sealed from the house and IF the HVAC equipment is not located in the attic, then an attic fan may be a good choice.  However, if the living space beneath the attic is uncomfortable, it’s likely that you have too little insulation and poor sealing between your living space and the attic.  In this case, the better choice would be to add insulation and properly seal the attic floor to prevent heat from transferring through the ceiling.  In the Central Texas climate, aim for R-30 to R-38. Insulation beyond R-38 has a diminishing return on investment.

Be aware that attic fans can cause a few problems if they are installed under less than optimal circumstances.  For example, they can pull conditioned air from inside your living space into the attic if your ceiling is not properly sealed.  Along with wasting energy, depressurizing your home can cause dangerous air quality issues.

Most experts agree that adding insulation and properly sealing between your living space and the attic offer the best payback, both in terms of comfort and energy savings.

Solar Tubes vs. Skylights

Many people enjoy the look of skylights in a home, particularly in rooms with little or no natural light.   However, in Central Texas, having a skylight in a house is much like punching a hole in the roof and letting the sun’s heat pour in.  Although skylights add light to dark interiors, that light comes at the cost of increased cooling bills and lowered comfort.  They also are potential leak points in the roof during our torrential rains and, in winter, they contribute to heat loss.

A recent product called a “solar tube” improves on skylights in significant ways.  Solar tubes are easy to retrofit in an existing house or into spaces without a cathedral ceiling.  Unlike the skylight, the solar tube does not provide direct light.  Solar tubes work by reflecting the light through a tube or a metal duct to the desired room.

Solar tubes are a more efficient alternative than skylights[/caption]

Some solar tube products have an optional light kit that can be installed into the tube, so on a cloudy day or at night the light is still coming from the same place.  This has the added benefit of cutting down on one more recessed light penetrating the attic.

However, it is important to realize that both skylights and solar tubes lose energy compared to a conventional roof without them.  Both let in infrared heat.  Both eliminate ceiling insulation when installed.  Neither one works at night.  Solar tubes are just the more energy-efficient of these two less-efficient options.


There is no one-size-fits-all solution to saving energy and making greener choices.

Windows: Read the label. Be sure the U-Factor is 0.50 or below and the SHGC is 0.30 or below for windows that receive direct sun.

Attic Fans: Seal and insulate the attic before considering a fan.

Solar Tubes: For rooms lacking natural light; easily retrofitted.  Solar tubes are a better choice than skylights because they provide light with less heat gain.

Continue to Mythbusters 2: More Product Information ->

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