Shopping for replacement windows: a guide to frame materials

You may or may not be able to judge a book by its cover, but you can tell a lot about the energy-efficiency of replacement windows by their window frames -- what they're made of, how they're engineered to prevent heat loss in cold weather, and even the style.

Two factors determine the energy-efficiency of home windows: the glass and the frames. When shopping for new home windows. look at how their ratings compare to the latest Energy Star requirements for your climate zone. If you want to keep your home's temperature as comfortable as possible while reining in your heating and cooling costs, look for replacement windows that meet or exceed the energy-efficiency codes for new windows established by Energy Star as of 2015 and 2016.

In particular, you want to pay attention to U-factor, also known as U-value. This number represents how well the window resists heat transfer out of the home. For homes in the Northern climate zones, choose a window with a rating of .30 or lower. You can find the U-value rating on the National Fenestration Ratings Council label for the windows you are considering.

How to pick energy-efficient home windows: frame materials

In addition to the thermal resistance of the glass, the type of window frame also contributes to the window's U-factor rating. Unless you have an historic home and must maintain the integrity of the original construction materials, you are not limited to wood and metal frame options. Advances in material engineering during the latter half of the 20th century have given us manufactured frame materials with superior energy-efficiency over traditional frame materials.

The following guide to window frame types can give you an idea of which frames provide enhanced energy-efficiency and what are the advantages and disadvantages of each option:

Wood

Traditionalists prefer wood for its appearance and because it is a natural material that's recyclable.

Pros: Wood is inherently insulating; however, newer materials are more energy-efficient.

Cons: Wood frame windows are typically expensive and require frequent maintenance such as painting or staining. They are susceptible to insect damage, dry rot, and expansion and contraction that can cause sticking and warping. Aluminum, vinyl, or fiberglass cladding eliminates these maintenance issues, but you lose the appearance of wood grain and do nothing to improve the energy-efficiency.

Metal

Of all window frame materials, metals are easiest to custom design, making them a popular choice for special architectural remodeling and historical restoration projects. Metals are typically the least energy-efficient for colder climates, however, because they conduct heat quickly out of the house. Aluminum is more appropriate for hot climates where heat loss is not a concern. Continuous plastic strips may be placed between the inside and outside aluminum of the frames to create a thermal break and help improve energy efficiency in colder climates.

Pros: Metal window frames are often chosen for their strength. Aluminum is considered strong; however, steel is three times stronger. Metals are durable; they can be factory-coated to prevent corrosion and are virtually maintenance-free. Steel can last for generations. Aesthetically, metal is often chosen for its narrow sightlines. Steel-arte, available from Dynamic® Architectural Windows and Doors, is a customized frame solution for luxury and restoration clientele with "superior energy efficiency," according to their website.

Cons: Poor energy-efficiency and condensation are the biggest drawbacks to metal window frames. Custom steel window frames are high-priced.

Vinyl

Vinyl window frames are made from polyvinyl chloride (PVC), and there is much debate about their eco-friendliness despite contributing toward LEED building credit, one of today's hallmarks of green construction. However, there is no debate about their energy-efficiency or their popularity. According to the American Architectural Manufacturers Association (AAMA), in 2012 vinyl windows accounted for almost three quarters of the replacement windows market share. Left empty, the hollow compartments inside vinyl window frames resist thermal transfer very well but can be filled with insulation for even better energy savings.

Pros: Vinyl windows are one of the least expensive home windows and one of the most energy-efficient frame choices. They require virtually no maintenance and won't rot or corrode. Vinyl can be melted and recycled.

Cons: Vinyl is a weak material, so the size of the windows is limited. Years of heat and cold can cause vinyl frames to sag, crack, twist, bow, and discolor. Color choices from some manufacturers are extensive, but you are usually stuck with that color once you commit to it. They do not take paint well, and some primers weaken the vinyl further.

Fiberglass

Fiberglass is a relative newcomer to the window frame arena. Like vinyl, the hollow cavities of a fiberglass frame can be insulated, giving it superior energy efficiency to wood and uninsulated vinyl.

Pros: Besides providing excellent energy-efficiency, fiberglass frames are structurally strong. They expand and contract very little; do not warp like wood; do not bow like vinyl; and as with the other manufactured materials, do not rot or corrode. They can be painted multiple times if you want a change of color but do not require repainting. Best of all, they look a lot like wood. Fiberglass does not outgas, it doesn't leach toxic chemicals into landfill, and it can be recycled.

Cons: Cost is the only real deterrent to purchasing fiberglass, and it's been coming down.

Composite

One of the latest entrants into the window frame market, composite wood is made from polymers and sawdust that can be extruded into lineal shapes.

Pros: The material is strong, thermal- and rot-resistant, and it can be textured so it can be stained like wood, or painted.

Cons: "Composite" can refer to the wood-based product or a window made from both fiberglass and the wood composite product, so what one manufacturer calls a "composite window" may not be the same as that of another manufacturer. Availability is still limited. Cost is high and quality varies.

In addition to materials, window types also contribute to energy-efficiency. Because of their low air leakage rates, the best types to minimize heat loss are hinged windows such as casements, awning windows, hoppers, and fixed windows that do not open at all.

Choosing from the various window frame materials and types might make your eyes glaze over, but with so many options for improved comfort and energy savings, it makes sense to look at window frame choices as carefully as you do glass options -- until it all becomes clear.

Photo credit to Kevin Irby