One of the first things people look to when designing or renovating a home is good, energy efficient windows. But, with the ever changing window technology and tax incentives, it can be hard to sort out the good from the bad, so after the jump we've highlighted the most important things to look for to save money and energy.

Type of Window: Typically when you are designing a home, you will pick the window that will work best with the overall aesthetic of the building. Most modern houses use awning and/or casement windows, while traditional and historic homes have single or double hung units. There are a number of options and possibilities, but if you are a real stickler about finding an eco-friendly window, you'll want to stick with a casement or awning window. These units open and close like a door using a crank, and when in the closed position, the unit is fully sealed shut, something that double hung windows cannot do.

Frame Material: The most common window frame materials are wood, vinyl, aluminum, fiberglass, steel, composites, or a combination of the above.

• Vinyl: Within the last several decades there has been a huge increase in vinyl building products; however, vinyl is a very toxic material, and should generally be avoided.
• Wood: Wood frames are generally seen and used in historic and high-end homes. Wood is a beautiful and strong material, can be painted or stained, and work well as part of an energy efficient window unit. However, the drawbacks of using wood windows is they require a high amount of yearly maintenance to repaint, stain, or repair any rotting or split sections of wood. One very popular option is to get a wood window that is painted or stained on the interior, but clad on the exterior with aluminum or fiberglass. The exterior won't have the same look, but the window will still perform well and have much less maintenance requirements.
• Fiberglass: One common issue with windows is glass doesn't have same compression and expansion rate of its frame materials. Because of this, fiberglass frames make for a very efficient window unit, and the fiberglass material is generally much stronger and durable than vinyl, aluminum and wood. Windows can be made as all-fiberglass units, or wood units that are fiberglass clad on the exterior.
• Aluminum: Aluminum is a common, durable window material, which can be used as the entire frame or as an exterior cladding for wood or composite windows. If using full aluminum frame windows make sure these have a thermal break so that heat and cold isn't conducted through the frame.
• Steel: Steel windows are traditionally thought of as a more industrial building element, but they have been making a comeback in modern homes with large, sleek expanses of glass. Because steel is so strong, larger windows with low profile frames can be made, which other windows can't do. Similar to aluminum windows, make sure steel windows have a thermal break.

Glass:

• Number of Panes: At a minimum a window should have 2 panes of glass, also known as dual-glazed windows. Depending on where you live and the design of your house, some or all of your windows could be triple-glazed for additional layers of insulation.
• Fillings: The air space between the panes of glass can be filled with a variety of types of gasses, the most common being argon or krypton. These invisible gasses improve the insulating value of the window unit.
• Coatings: The panes of glass can be coated to control the amount of UV rays, infrared light and heat that is transferred to the interior — look for windows that are labeled as 'low-E' (low emissivity). Most coatings are barely visible to the eye, and like gas, help improve the efficiency of the window unit as well as protect the interior of the building from UV damage.

U-Factor, SHGC & NFRC Label: To be labeled as Energy Star and qualify for federal tax incentives, windows must meet U-Factor and SHGC requirements. Third-party organization, National Fenestration Rating Council (NFRC), rates windows on their performance, two of which, U-Factor and Solar Heat Gain Coefficient (SHGC), are used to determine whether a window qualifies for tax rebates. While it varies by region, a good rule of thumb is to look for a window with a U-Factor and SHGC rating equal to or less than 0.30 to get tax incentives (not all Energy Star windows qualify).

• U-Factor: Similar to 'R-values', the U-Factor measures the rate of non-solar heat transfer and is an indicator of how well the window insulates. U-Factor values are generally in the range of 0.25 to 1.25, the lower the number the better.
• SHGC: While U-Factor values represent non-solar heat transfer, SHGC values measure the amount of solar heat and light transmitted through the window. SHGC values range between 0 to 1, the lower the number the better.

Lastly, don't be afraid to use a window company's 'builder's line' collection of windows. Most window companies have several different levels of windows, and 'builder's' level windows are usually lower cost not necessarily because they're lower quality, but actually because there are few sizes, configurations and colors available, but more than likely they'll have what you're looking for.

For More Information:

• US Department of Energy
• Products Eligible for Tax Credits Through 2010
• Energy Star
• The National Fenestration Rating Council

(Images: Celeste Sunderland | Jacob & Caitlin's Green Mountain Manse, Energy Star. Originally published 2010-09-01)