Window Replacement Q&A
Advice to homeowners planning to replace one or more windows
The choices of windows and shades on the market today are many and varied. It can be very difficult to know what window or shade to purchase, and to find adequate information about their performances. Sometimes the addition of an interior or exterior shade can solve a solar gain (overheating or glare) problem without the need for replacing the window.
Our first advice, for windows, is to look for the energy label from the National Fenestration Rating Council (NFRC). This organization is promulgating energy efficiency standards around the country and its label contains values for the U-factor, a measure of the ability of the window to conduct heat from the warm inside to a cold outside (or from a hot outside to a cool inside).
This number should be low. The label also contains values for the Visible Transmittance (VT), the fraction of incident sunlight and daylight that is permitted inside, and the Solar Heat Gain Coefficient (SHGC).
We see outdoor scenes from the light coming to us from them through the window. If the window blocks such light excessively, having too low a VT value, the window will appear overly tinted and dark.
Solar heat gain is reduced when the SHGC value is low, such as below 0.4. In general, we in Florida desire a low SHGC value and a VT value that is somewhat (or a lot) higher than the SHGC value. Such windows are said to be spectrally selective.
Here is more information about replacing or improving existing residential windows selection in hot climates like Florida, in the form of responses to common questions asked of us.
We are going to put in new windows but we want to have them as energy efficient as possible. We want double hung windows with insulated glass, but that is all we know. After having been in contact with a few installers we are confused because we get different information from all of them. We believe that vinyl frame windows are more efficient than aluminum. Is this right?
Thermally broken aluminum windows can be as effective as vinyl frame or even wood frame windows, at blocking conduction transfer of heat through the frame from warm interiors to cold exteriors.
The same is true for blocking transfer from the hot exterior to the cool interior, but the quantity of heat so blocked is modest. It is better first in hot climates to put any extra money you have into spectrally selective glazings, glass that reflects the infrared part of the solar spectrum while blocking the visible portion (light) only minimally. Such glass is a special form of “low-e” glass designed for hot climates.
More on this following the next question. If the window is well shaded, this coating is less necessary. It usually makes sense to provide more insulation for the window, such as adding a storm window in winter for cold climates and products are also available for adding a clear plastic film to the interior of windows in both hot and cold climates, to provide and extra trapped air space between the outside and the inside.
Regarding low ‘E’ glass. Some people say that it is very important here in Florida but others say that it does not make any difference. What is right?
Normal low-e glass was designed for cold northern climates. The best versions are used in a two-pane configuration, since you get the added insulation value of the trapped air space. The main function of such a glazing system is to admit copious quantities of solar radiation into the building as solar heat gain, while preventing the escape of that heat back outside. It produces and accentuates the greenhouse effect. This is not what we want for Florida, or for other hot climates.
There is a way to make the high solar gain type of low-e glass appropriate for hot climates. Instead of making the outer glass pane clear to all solar radiation, it is possible to tint the glass a special way so that it absorbs the unwanted and unused infrared part of the solar spectrum. This would be a mistake for single-pane glass, however, since the hot glass would radiate and conduct much of its heat to the interior.
If the absorbing pane is the outer one of a double pane glazing system, however, the northern, cold climate, low-e coating is effective at reducing the transfer of heat from the hot outer lite of tinted glass to the inner pane. Few residential window manufacturers may be using this technique for blocking solar gain currently.
The more common way to achieve the same effect is by reflecting rather than absorbing the unwanted solar infrared radiation. This is done with a special coating that is also a low-e coating, but a specially designed one for hot climates.
The new low solar gain low-e coating blocks some of the solar radiation incident on it, by reflection, and is quite effective in hot climates like Florida’s. There are various trade names for this hot climate insulated glass. The common feature is a solar heat gain coefficient (SHGC) lower than the visible transmittance (VT).
If this sounds complicated, there is a more complete description of the technology, with color graphic illustrations, in our publication on residential window options for hot climates.
Unfortunately, the best of the spectrally selective coatings intended for this use are soft, and will degrade when washed. So the coating must be protected by placing it between two panes of glass. The resulting double-pane glazing system just happens to be insulated, which is not a bad thing, but the main purpose, in our climate, for the second pane, is to protect the soft coating.
The second pane offers the additional advantages of enhancing thermal comfort in the room near the window, decreasing the peak heat load on the air cooling system, and protecting you from the added expense of peak demand charges, should your electric utility company ever decide to add such charges to the bills of its residential customers, like most utilities do currently for their commercial customers.
Manufacturers have developed coatings of this kind that are “hard” and can be cleaned without damaging them. These can be used with single-pane windows, a definite cost advantage for hot climate residents. At present the few of these currently on the market do not perform their solar-gain-rejecting magic as well as the soft coatings do, but they are an effective choice in some circumstances.
Guidelines. Look for a visible transmittance around 70-80% if the window is well shaded from the sun or from 40 to 60% if it is not. Then get the lowest solar heat gain coefficient (SHGC) you can find and afford. The ratio of VT to SHGC should be as high as possible but definitely over 1.0.
Argon gas between the glass is offered by some manufactories but again different people have different opinions about whether it helps with the insulation. What is right?
Since our needs for insulated glass in hot climates are minimal, paying more for an insulating gas between the panes may not be that good an idea. It is more important to put any extra dollars you might spend on insulating gases such as argon or krypton into lowering the solar gain of the window without drastically reducing the visible transmittance.
On the other hand, energy prices are expected to rise as developing countries with huge populations demand more energy at the same time that the world is reaching and passing its peak of oil production. If you want to protect your pocketbook against future energy price shocks, it would be good to have insulated glass in addition to good solar gain prevention.
If the electric utilities every decide to charge us each month an extra surcharge on our electric bills for our peak usage over the course of a month, called “demand charges,” having double pane windows with insulated frames will reduce these peak loads, providing protection against excessive demand charges.
Since windows last decades, and the oil crisis is expected to come upon us in the first decade of the 21st century, now is not too late to replace single pane windows with better insulated ones as the need arises.
In addition to the above we would be grateful for any other important information you may have for us before we buy our windows.
There are additional reasons favoring the use of insulated glass and insulated frames on residential windows.
There are less tangible human-factors benefits that can be realized with insulated glass:
a. Insulated windows offer more of an acoustic barrier, blocking outside noises from reaching the interior as strongly. Of course if you like to hear the birds chirping and are not bothered by other sounds from the outside, this would not be an advantage.
b. Insulated windows are more comfortable to be near when it is very hot or very cold outside, since the interior glass is closer to room temperature and does not produce the radiant heating or radiant cooling of single pane glass in such weather conditions.
c. In new construction, or if you are planning to replace your air conditioner the same time you replace your windows, insulated windows can lower your peak need for air conditioning and your peak need for heating — on those especially hot or especially cold days.
Because of this, you may be able to install a smaller, lower capacity, air conditioner, and save money on the smaller unit. Sometimes the money saved pays for the extra cost of the insulated windows.
Two final factors can affect your choice of new or replacement windows for your home.
1. The shading of the window has a big impact on the properties needed for the window. If the window is well shaded, as with a big full oak tree, or well-shading awning, then there is less need for solar heat gain prevention and the SHGC value can be permitted to be relatively high–over 0.6.
If the window is not well shaded, and if it receives direct beam solar radiation, and if you cannot add shade to the window, then you need a window with a low solar heat gain coefficient, below 0.4. If this results in too low a VT.
However, you could find yourself having to turn electric lights on in the room in the daytime, costing energy and at least partially defeating the purpose of the low solar gain glazing. Keep VT values above about 0.2 (or 20%) in the worst case and 0.4 (40%) if the problem is not severe.
2. A new Florida building code recently went into effect that requires impact resistant windows be installed during new construction in a large portion of Florida, mostly the coasts around the state and for some distance inland. If you are replacing some of your windows and you live in this zone, you might like to consider providing some impact resistance when you make the change.
There are various ways of achieving the desired protection. Exterior shutters, which can be closed tightly during extreme weather events, is one way. If these are closed when the sun is incident on the window, they can be excellent solar heat gain blockers too, reducing your need for low-SHGC glass. Find shutters that are very easy to open and close if you wish to operate them for this purpose.
Another way to provide impact resistance is to incorporate laminated glass in the glazing system. Laminated glass consists of two sheets of glass sandwiched together without an air space, and held together with a clear, transparent plastic material. This laminate, most often polyvinyl butyrate (PVB), adds strength to the window.
If a heavy object is projected onto the window with force, the glass may crack, but the PVB holds it together, keeping the wind and rain out (and also providing some protection from intruders). If you decide to spend the extra money on installing an impact-resistant window pane, you should shop around and find a vendor which can also offer this kind of glazing with a solar infrared rejection coating also sandwiched in between the panes.
As usual, in the final analysis, the resulting window will be a good one when exposed to the sun if the visible transmittance (VT) is reasonably high while the solar heat gain coefficient (SHGC) is as low as possible without an excessive loss in VT. If you are going to pay more for an impact-resistant window, it may not cost very much more to make it energy efficient too.
A problem frequently encountered in Florida is that the window vendors either do not understand the various trade-offs mentioned above, or do not stock and are therefore reluctant to sell, the best of the high performance windows, or both.
You may have to be quite persistent to get the kind of window you want at a price you can afford to pay. The U.S. Department of Energy and the Alliance to Save Energy are working to improve the knowledge of energy-efficient residential windows amongst window vendors and to encourage them to stock and sell and promote these windows, as well as the awnings, shades, and shutters that also can be effective.