Green Energy Efficient Homes | Best energy efficient windows
New efficient windows can save you a bundle on heating and cooling
The best energy efficient windows now available are significantly more energy efficient than even the current minimally certified ENERGY STAR windows. Before you look at house window replacements or a volume window purchase for a new house it helps to understand what makes high efficiency windows truly energy efficient and what features will maximize your comfort and minimize energy use.
In this article I will describe key features of the best energy efficient windows, the current ENERGY STAR standard and the new R-5 standard for high efficiency windows, ways you can make your existing windows more energy efficient without replacing them, and the Windows Volume Purchase program which helps US residents buy the most energy efficient windows at a volume discount of up to 20% from selected suppliers.
The best energy efficient windows cut your home energy costs in four ways, the first three of which are directly related to how heat is transferred from one object or space to another:
The following illustration should help you understand the four ways in which heat is transfered from one side to the other of a window:
Bear in mind that we’re talking about heat transfer both when you’re heating your home, and when you’re cooling it (or trying to keep it as cool as possible without air conditioning). The same principles apply to both situations: you want your high efficiency windows to be as strong a barrier as possible to heat transfer, from the hot side of the window to the cold side. The only difference is that in winter in cooler climates, you want the heat from the sun to radiate into your home, but you don’t want heat from inside to radiate out; while in summer or in hot climates, you don’t want heat from outside radiating in.
Which of these heat transfer mechanisms do you think is the most important when looking for house window replacements? They’re all important, but airflow is the most important, at least if your current windows are more than 10-20 years old. If you check the illustration, you’ll notice a squiggly air current moving from the indoors to the outdoors, between the main part of the window and the wall below. This represents a draft, which is the main way in which most homes lose energy through their windows.
Drafts can occur in cracked putty on old hand-glazed windows, cracked glass, cracked wood in wooden window frames, and improperly sealed moving sashes or casements in movable windows. Drafts can also occur in the window framing, and even in the area of the wall around the window frame.
In general, when looking at house window replacements, the biggest benefit of replacing your windows with high efficiency windows is that the best energy efficient windows cut down on many of these types of drafts. And while the ENERGY STAR windows specification does not have any requirements as far as air leakage, the newer, more stringent R-5 standard does require minimization of air leakage.
Convection affects window efficiency as well as your comfort indoors, particularly in winter. If the inside glazing is cold (because too much heat is being conducted outside through the glass and the gas filler between panes of glass), warm air from the room will cool as it strikes the glass, and that air will then tend to fall (since warm air rises and cool air falls). The falling cool air draws warmer air in its place, creating a convection loop that lets the window glass continually pull heat out of the room, all the while your ankles get colder.
I’ll point you to some resources a little later that can help you deal with the convection currents inside your room to turn your existing windows into high efficiency windows, or that can make the best energy efficient windows even more efficient. Hint: dress up your windows.
Most high efficiency windows have two or three layers of glazing, with space between them to act as a barrier against heat conduction. The space between the glazing layers is filled either with air or with an inert gas such as argon or krypton.
Ideally you would want a vacuum between the two panes – as little gas as possible to minimize heat conduction – but unfortunately this has the effect of pulling the two panes together, risking breakage, and since, as Faulkner taught us, nature abhors a vacuum, a vacuum-sealed double-glazed window has a tendency to develop leaks and fill up with air.
The reason a vacuum would be ideal in the best energy efficient windows, is that if there was no gas between the two panes, only empty space, there would be nothing for heat to be conducted through. (That’s why your thermos has a vacuum between the inside and outside walls of the container.)
Since a vacuum isn’t a viable option, the next best choice for the most energy efficient windows is a gas that has very low thermal conductivity, and is also non-reactive. Some common gases and their thermal conductivity (in W/mK) are:
You’ll notice that krypton has the lowest thermal conductivity of the bunch; unfortunately it is rather expensive, so is only used in a small percentage of the most energy efficient windows (particularly triple-glazed windows where the thinner space between panes allows a smaller amount of the gas to be used). Carbon dioxide has low conductivity as well, but can be reactive when it meets moisture, so it’s not suitable for a glazing gas. Argon is the next best gas, and is used extensively in the window industry to produce the best energy efficient windows. If you are buying ENERGY STAR windows or the new R-5 windows, you’ll almost certainly be looking at windows that are filled with either argon, or a combination of argon and krypton.
Conduction through the solid parts of the window is the second way that conduction affects a window’s energy efficiency. A window frame made of a single piece of aluminum will conduct a lot of heat from the warm to the cold side, which is why aluminum window frames tend to gather a lot of frost around the edges in winter. Efficient aluminum window frames need some kind of thermal break between the indoor and outdoor portions of the aluminum; even so, these window frames are still less efficient than other types. Aluminum frames are definitely less popular than they were 10 or 20 years ago because of this, although some newer wood-cored frames are clad in aluminum. Here are the R-values of five different types of window frame material; higher is better. You’ll notice that all three versions with aluminum in them are less efficient than their vinyl cousins. The most energy efficient windows are almost always vinyl or fiberglass.
Notice the difference between the top and bottom types in the list? An insulated vinyl window frame has an insulation value between 6 and 12 times better than that of an aluminum frame with no thermal break, and between 1.3 and 3.3 times better than that of any aluminum-based frame. The message here is clear: aluminum is a poor choice for the best energy efficient windows.
Radiation through the glazing is the third type of heat transfer that affects a window’s energy efficiency. Radiation is really the same thing as light. Infrared radiation is long-wave radiation that you feel directly as heat. Visible light is shorter wave radiation; when intense visible light from the sun strikes an opaque surface, such as your face or a table or floor indoors, it turns to longer-wave infrared radiation.
How much visible radiation a window lets through is important for two reasons. In climates where keeping your home cool is important, you want to minimize visible light entering your home through windows, because it will turn to heat indoors. In cooler climates where you have the heat on for part of the year, you want to maximize light entering your home so you can take advantage of free heat from the sun. And you want to minimize infrared radiation passing from the warm to the cold side of the glass in either case.
Manufacturers of the most energy efficient windows address infrared heat transfer by applying specialized surfaces called Low-E coatings to the window glazing. These coatings reflect infrared light back to where it came from, rather than let it pass through, which significantly reduces the radiant heat gain you get from the best energy efficient windows.
Low-E coatings are applied to the best energy efficient windows to cut down on thermal radiation between the panes of glass in a double- or triple-gazed window. By coating one surface of one or more of the panes with a microscopic low-emissivity coating, window manufacturers can cut back on this radiant heat flow. The coating is most effective when it is applied to one of the surfaces between the panes – the inside surface of the outer pane, or the outside surface of the inner pane. These coatings block much of the radiant heat, and can have a significant impact on window efficiency. What’s more, the coatings can be more effective at blocking radiation in one direction than in the other.
You should select windows with a low-E coating that is suited to your climate. The three types of climates to consider are heating-dominated, cooling-dominated, and moderate climate with a mixture of heating and cooling. For heating-dominated climates you should look for a low-E coating that allows maximum solar gain (so you can benefit from solar energy in winter to help heat your home). Such coatings allow up to 71% of solar heat through the window glazing.
For cooling-dominated climates, you’ll need a low solar gain glazing, which reflects most of the solar radiation back to the out of doors – allowing as little as 27% of it into the indoors.
For climates between the two – those where you run heating and air conditioning about the same amount of time – you should look for low-E coatings that provide a moderate solar gain, somewhere between the 27% and 71% solar heat transmission.
Coatings can also be applied to tint the window to reduce the amount of visible light entering the room. A measurement called VT or visible transmittance gives you an indication of how close the window comes to allowing all light through it. A VT of 100% is equivalent to a hole in the wall – a window with no glass, such as they used a thousand years ago in castles in England! A VT of 60% or more will generally appear completely untinted to most observers. In hot climates, you may want a VT lower than 60% because reducing the amount of visible light entering your home will cut down on heat gain, as you’ll have less light indoors to turn into heat. But you don’t want to cut down on the light so much that you then have to use more indoor lighting, as you’ll pay twice for that – once for the electricity to power the light, and again for the extra energy it will take an air conditioner to move the heat from that light out of your home. (Of course if you use fluorescent or LED lighting the heat gain from the light will be minimal.)
The best energy efficient windows therefore combine features to address all of these heat transfer mechanisms: air leakage, conduction, convection, and radiation.
When you’re considering house window replacements it’s important to bear in mind that not all energy loss through windows is easily detected. If you have old single-glazed sash windows with aluminum framed (or even wood framed) storms outside, you will almost certainly see signs that you need house window replacements: gaps where the storm meets the window frame, cracked putty, cracks in the glass, the pulley openings for counterweights are all potential sources of air leakage, and you can often feel the cold air blowing in on a winter day.
Likewise if you have newer double- or triple-glazed windows with aluminum frames, you may see the condensation (or even frost) building up on the inside of the frames on a cold day, as heat from the room is wicked outside and the cold surface of the frame causes condensation. Or if the seal is broken in a newer double-paned window and the argon gas has been largely replaced by air, you’ll notice more condensation on that window on cold days than on others.
But some signs are less obvious, and sometimes even the experts don’t understand the factors that determine whether windows need replacing, and what kind of house window replacements to get.
My brother is a statistician and an energy efficiency nut like me. He once called a window installer to give him a quote on replacing his old aluminum-framed windows. He wanted the best energy efficient windows he could get – he heats with electricity, and every dollar he spends cutting the amount of heat escaping his home through cold Ontario winters has the potential to save him ten or twenty dollars in heating costs.
He told the installer he wanted low-E windows, and the installer told him it’s not really worth it – that he would be paying 20% more for the windows and only saving a few dollars a year on heating. So my brother pulled out his laptop, opened a spreadsheet he’d prepared, and plugged in the numbers: the amount of heat loss through his current, inefficient windows, the regular new windows the installer was pushing, and the best energy efficient windows available. While the most energy efficient windows had a much higher up front cost, over a 20-year life they would save him enough money to cover not only the increased up front cost, but the entire up front cost!
The installer was surprised. Even after years of selling and installing windows, he had never done such a calculation. “I guess that’s why you’re a statistician and I sell windows,” he said!
The point of this anecdote is twofold: first, that the savings from getting the best energy efficient windows can be substantial; and second that most people selling you windows – even people who claim to be looking out for your financial interests – have no idea what the full life cycle costs of different window efficiencies are. If you spend more than a few hundred dollars a year on either heating or cooling, you will definitely be better off if your house window replacements are as efficient as possible.
You should consider replacing your windows with new energy efficient windows if any of the following apply:
If you buy new ENERGY STAR windows you at least know that you are getting a guaranteed minimum efficiency level in your house replacement windows. But the ENERGY STAR standard is definitely not all that stringent by today’s standards.
Set in 2003, the ENERGY STAR specification for windows and doors requires different maximum U-factors for different climate regions of the USA. Remember that the lower the U-factor, the less heat can pass through the window – including through glass, frame and spacers. Factors ranged from 0.65 for the Southern climate region (Florida and the southern ends of Texas and states in between), to 0.40 in South/Central and North/Central regions, to a maximum efficiency of 0.35 in the Northern region (representing roughly the northern half of the Continental US).
Low U-factors are most important for increasing the efficiency of home heating. Low U-factors help reduce cooling costs in warm climates to a lesser extent than heating. U-factor is basically the inverse of R-factor, so a U-0.4 window is the equivalent of R-2.5. A typical 4″ stud wall will have R-12 to R-16 in it depending on the insulation used – that’s why windows, while a small part of a typical house wall surface, are a large part of the heat transfer.
If a window company or installer is trying to sell you ENERGY STAR rated windows, you’re at least getting something nominally efficient. But for the best energy efficient windows, make sure you ask for the new R-5 standard.
Also, be wary of claims of very high R-values (e.g. R-6 to R-9) or very low U-values (U-0.18 to U-0.1) on windows. There is a good chance this measurement reflects the insulation value of the glazing itself – in other words, the manufacturer is measuring only heat transfer at the center of the pane, not the heat transfer of the entire window including the frame.
While ENERGY STAR windows must have an efficiency between U-0.65 (least efficient) to U-0.35 (most efficient) depending on region, the new R-5 standard sets much more stringent requirements across the board. All R-5 windows must be – you guessed it – about R-5 or better. In fact the specification requires fixed windows to have a U-factor of U-0.2 (equivalent to R-5), but movable windows can have a slightly higher U-factor of U-0.22 or lower, which translates to R-4.54. So many movable “R-5” windows will actually be slightly below R-5. But that’s still a whole lot better than what you’ll get with a minimally qualifying ENERGY STAR window.
The other efficiency requirement for the R-5 standard has to do with air leakage. While the ENERGY STAR standard made no requirements on airflow, the R-5 specification requires manufacturers to measure airflow through the window and frame, and for that airflow to read less than 0.3 cubic feet of airflow per minute per cubic foot of window (including both the glazing and the frame).
One additional requirement of the R-5 standard that will benefit consumers is the product warranty. Qualifying windows must have at least a 20 year warranty on the glass (visibility or seal failure), and a 10 year warranty on other components (free from manufacturing defects).
R-5 windows are typically triple-pane. Note that triple pane windows from the 1970’s and 1980’s were not particularly efficient – you are probably better off replacing these with, at the very least, new double-glazed ENERGY STAR windows. These new R-5 triple-glazed windows are built using a new process that improves the quality of the seal, and features the low-E coatings that minimize unwanted heat transfer. There is at least one R-5 manufacturer who makes double-glazed R-5 windows.
A typical rule of thumb for ENERGY STAR windows is that you’ll pay about $240-250 per window. For as little as an extra $30 to $50 you can get an R-5 window instead.
There are many federal, state/provincial, local, and utility incentives and rebates available for people who install ENERGY STAR or R-5 windows. For example many municipalities and utilities in California and some in Florida offer rebates of $1 to $2.50/square foot on ENERGY STAR windows. There are often loans available to help finance purchase and installation of energy efficient windows, although the programs I reviewed had fairly high interest rates, so if you have good credit you are better off financing an upgrade with a second mortgage or line of credit loan.
If you don’t want to spend a fortune on the best energy efficient windows but you still want to cut your energy losses through your windows, there are a number of less expensive things you can do to at least gain some energy efficiency.
I cover many of these concepts in my article on energy saving window coverings, where you can learn about ways to cut the convection currents around glass, how to reduce air leakage, and how some types of window coverings can add an additional layer of insulation that substantially reduces your heat loss in winter. And of course window coverings can reduce solar heat gain in hot weather by preventing sunlight from entering a room in the first place.
Other articles to consult are:
Investing in house window replacements can be a major financial burden in the short term, although in the long term the best energy efficient windows will pay for themselves several times over. If you’re considering a major window retrofit, you may benefit from the US government’s Windows Volume Purchase program.
The Windows Volume Purchase program was established to encourage window manufacturers to produce R-5 qualified windows and offer them for sale at volume discounts. Participating manufacturers offer discounts- typically, 10 to 20% – on purchases of 15 or more windows for a retrofit, where you replace existing windows; and for purchases of 20 or more windows for a new installation.
You can take advantage of the Windows Volume Purchase program either through your own window installer (or by installing windows yourself), or by using an installer provided or recommended through one of the manufacturers participating in the program. The program provides a website that lets you select window configuration and size, and provides a list of manufacturer names, websites and phone numbers that can provide windows of that type. The program started in May 2010 and as of January 2011, about 1,000 windows have been sold with a total sales volume of about $260,000 (which should give you an idea of the typical cost of a discounted R-5 window).
This content was originally published here.