What Windows Should I Buy?
There are a lot of options on windows that make buying windows difficult. In terms of efficiency, there are really only a few things that are important about the construction and science behind windows.
There are two types of glass for windows. Lo-E coated and standard or uncoated glass. Lo-E is simply a coating that reflects the short wave UV rays (the rays bouncing off of other objects before hitting the window) while still allowing long wave UV rays to penetrate into the house. The science behind this is since the sun in high in the sky during the summer and lower in the sky in the winter, the Lo-E coating helps keep the heat out during the summer and allows the heat to come in during the winter. There is also less carpet and furniture fading because of Lo-E.
Lo-E is a tin or titanium oxide that is spray applied in many different directions on a pane of glass while the glass is still hot from the oven. Tin is less effective but more common of a Lo-E coating. Lo-E is a smart option when buying windows and can save hundreds over time.
Window spacers all have their downfalls. None of them are good. Silicone tends to warp and displace allowing the gas in between panes of glass to escape. Aluminum spacers tend to get cold and can render condensation prematurely on the inside of a window.
The best performing spacer still seems to be the aluminum spacer, however. There is no reason except for it's durability. It tends to outlast it's more trendy, and frankly cheaper silicon spacer.
When talking about efficiency, form and function are part of this design, so I am going to break these down into types;
Vinyl Windows These are currently the most popular because of their inexpensive and maintenance free aspects. Vinyl itself is one of the most efficient materials on the market today, only losing 1 BTU/HR/IN. The downside is that it is weak and glass is heavy. Casement and slider vinyl windows are good purchases (with the right glass, spacer, seals), but NEVER buy a double or single hung vinyl window over 24 inches wide. They simply do not last. What happens is during the summer time the vinyl gets weaker in the sun and causes the sash to bow under the weight of the glass. The average price is $285 to $400 for your average window.
Wood Windows With Cladding Wood framed windows have a vinyl or aluminum cladding on the exterior to protect the wood from the elements and to have a maintenance free exterior. These windows are the most attractive and usually the most efficient, frame wise. They lose a about .93 BTU/HR/IN. Strength wise, they are second only to aluminum windows. The biggest downfall with wood windows is wood rot, and maintenance. Two things that are inevitable with wood windows. However, a wood window kept up well can last 20 or more years. the average price for these windows in between $400-$1,000.
Aluminum Framed Windows Aluminum framed windows last a very long time, but lose up to 1500 BTU/HR/IN so I am not going to spend much time on them. If you have a commercial building with large storefront windows, get these otherwise they are not a goo investment. There are some companies that make an aluminum window with foam in the frame or a foam frame separator as a thermal break so there are options. These windows average $1,000 to $2,000 for average size windows. Storefront windows are usually $5,000 plus.
Gas In The Glass
The most commonly used gas in between the panes of glass is Argon. Why Argon? Because it is a natural element easily accessible and it is denser than air making it a better insulator. This is a very smart option as it takes a typical double pane window from R-1.8 to R-2.3 of higher. The gas cannot fill more the 98% of the space because of expansion an contraction of the glass.
The gas that is used the least is Krypton. No superman cannot handle this gas (ha, ha). Krypton is three times denser than Argon so it is a very good insulator. It is also very hard to get and very expensive. It will boast R-values of R-5 and R-6, however. To some that may be worth the extra price tag.
This is the part of the window that is the most important feature in terms of efficiency. Most of the seals are made of felt that is dipped in silicon to maintane their structure when wet. The seals are generally called polypiles which look a lot like pipe cleaners when removed. Most window manufactures have replacement polypiles you can order so do not discard a window is the seals need to be replaced. you may be able to order new seals.
As I stated before, windows are complicated. Too many features that noone needs and many different variations. These easiest way to tell if a window is a "good buy" is when it has an EnergyStar logo on it. This means it has all of the features that make it an energy efficient window. Most of these windows also come with a rebate or tax deferral.
If you are unsure of what energy improvement to do, it is always a good idea to hire an energy consultant to walk through your property and show what energy efficiency improvements will be the most cost effective.