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 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 “most efficient” designation that is given to those ENERGY STAR windows that are a cut above their peers.

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Key features of energy efficient windows

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:

  • Convection: The transfer of heat through moving air currents. Think of the warmth you feel with your hand just above the toaster.
  • Radiation: Direct heat radiating from a heat source through space or a transparent surface such as glass. Think of the warmth of the sun shining through a south facing window on a winter day.
  • Conduction: The transfer of heat carried through matter. Think of what happens when you touch that toaster while it’s hot!
  • Airflow: When heated matter moves, the heat moves with it. Think of the cold you feel inside if you leave your windows open in winter!

The following illustration should help you understand the four ways in which heat is transfered from one side to the other of a window:

Heat transfer mechanisms through windows

Bear in mind that we’re talking about heat transfer both when you’re heating your hohme, 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.

Cross section of a double-glazed window

Double-glazed window cross section

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 earlier ENERGY STAR windows specifications did not have any requirements as far as air leakage, the newer, more stringent ENERGY STAR requirements specify air leakage of under 0.3 cubic feet of air per square foot of window area per hour. If that sounds like a lot, it’s not. A tiny gap between a window and its frame can easily achieve this airflow, and that’s exactly what you don’t want. Imagine you have a large bicycle pump and you slowly press the handle down over a period of two and a half minutes. That’s about 0.3 cubic feet of air per hour.

Convection heat transfer

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. ENERGY STAR Most Efficient windows have three layers of glazing, which not only provides a stronger barrier to convection, but also results i a quieter home, as less noise travels through the glass. 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 near-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:

  • Krypton: 0.0094
  • Carbon dioxide: 0.015
  • Argon: 0.016
  • Ammonia: 0.022
  • Air: 0.024
  • Nitrogen: 0.026
  • Methane: 0.030
  • Helium: 0.142

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 ENERGY STAR Most Efficient 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

Conduction through solids 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.

  • Insulated fiberglass or vinyl: R-3.2 to R-5
  • Wood or vinyl: R-2 to R-3.2
  • Aluminum clad wood or reinforced vinyl: R-1.5 to R-2.5
  • Aluminum frame with thermal break: R-1
  • Aluminum with no thermal break: R-0.4 to R-0.5

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

The light, in particular the infrared radiation, passing 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, or either surface of the middle pane in a triple-glazed window. 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.

Getting windows that match your climate

If you buy your windows from a trusted local window manufacturer, they will most likely design their windows for the local climate, and therefore you’ll get the right compromise between solar heat gain and solar isolation for your climate. This isn’t just because they’re trying to do the right thing. It’s because the ENERGY STAR standard has different specifications for solar heat gain depending on the region. The “Solar Heat Gain Coefficient” is required to be as follows based on the region:

  • Northern climate zone: ≥ 0.17
  • North-Central: ≤ 0.40
  • South-Central: ≤ 0.23
  • Southern: ≤ 0.23

You’ll notice that the solar heat gain must be higher than 0.17 in the Northern climate zone, because the predominant climate control energy use in northern homes is heating, so we want additional heat to enter the house through solar heat gain. For all other regions, we want to keep solar heat gain below a threshold, and that threshold grows as we go further south, since the hotter the climate is, the bigger the heating impact of sun on the windows.

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 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.

How to determine if you need to replace windows

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. Any condensation between the window panes of a window indicates that the window needs to be replaced. It’s no longer providing the thermal barrier you want in an energy efficient window.

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.

Window salespeople aren’t always energy efficiency experts

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.

When to replace your windows

You should consider replacing your windows with new energy efficient windows if any of the following apply:

  • Age: If your windows are 20 years old or more, you will almost certainly save money by replacing them with the best energy efficient windows.
  • Style: If your windows are single-glazed with separate storms, you should definitely consider replacing them as these do not have argon filling and almost certainly do not have low-E coatings.
  • Drafts: Hold a candle or lighter around the edges of the window frame and see if the flame flickers.
  • Filling: Argon degrades by about 1% a year, so a 20 year old argon filled window could have 20% less argon in it than a new one
  • Comfort: Do you feel a lot of cold coming through the glass?
  • Noise: Is too much noise coming in from outdoors? This isn’t typically a sign of an inefficient window, but can be an additional rationale for upgrading an old window.
  • Ease of opening: Are your windows hard to open or close? A warped window or frame may indicate the need for replacement, and may also signal that the seal around the opening is not good.
  • Frame damage: Cracked or warped vinyl or metal framing, or rotten or chipped wood framing increases the chance of air leakage, and may also indicate that the framing is not doing as good an insulating job as it should
  • Cold framing: If the edge of the glass, or the frame itself, is cold to the touch on a cold day, it is not effectively insulating.
  • Metal framing: aluminum and other metal framing has fallen out of favor as it is less efficient than vinyl or vinyl-clad wood.
  • Moisture: Condensation on windows on cold days may indicate too much heat loss through the glass – often a sign of a broken seal between the panes. Black mold around the edges of the glass may indicate condensation has occurred, even if it’s not visible now. (For example, look for signs of mold in summer that may have formed during condensation in winter.)
  • Difficulty cleaning: Dirty windows that won’t come clean don’t really indicate that your windows are inefficient, except that dirty windows won’t allow as much solar heat gain in winter. But if the glass becomes scratched or scuffed due to weathering, this strengthens the case for window replacement.
  • Financial incentives: If there are financial incentives to replace your windows, that may mean you need fewer of the other factors to apply to motivate you. For example, Canada’s Greener Homes program provides incentives of up to $250 per window  that you replace with an ENERGY STAR Most Efficient window, and local gas utilities may top that up – Enbridge in Ontario adds another $75. In the USA, the Inflation Reduction Act provides tax credits of up to $1200 for home envelope improvements (but strangely, this is limited to $1100 for

ENERGY STAR standard for windows

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. Bear in mind that the standard is continually evolving, with new specifications published every so often, so if you bought ENERGY STAR windows fifteen or twenty years ago, they are probably well below the requirements for ENERGY STAR windows today.

The ENERGY STAR specification for windows and doors requires different maximum U-factors (more on U-factor below) 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 0.32 for the Southern climate region (Florida and the southern ends of Texas and states in between), to 0.28 in South/Central, 0.25 in North/Central, and 0.22 in the Northern region (representing roughly the northern half of the Continental US). For comparison, the same standards 20 years ago ranged from 0.65 to 0.35, so efficiency requirements have approximately doubled on the U-factor front.

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 ENERGY STAR Most Efficient windows.

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. For ENERGY STAR windows, the proof is in the label that is printed on the window when it is installed – the U factor stated on the window is the overall U factor of the window including window pane and window frame.

Most efficient designation for windows

While ENERGY STAR windows must have an efficiency between U-0.32 (Southern region) to U-0.22 (Northern region) , the ENERGY STAR Most Efficient designation is reserved for windows at the high end of the scale. For example, the U factor for ENERGY STAR Most Efficient windows ranges from a high of 0.20 to a low of 0.12 (and remember, lower is better for U-factor). Solar heat gain ranges from 0.05 to 0.64 for the Northern region for Most Efficient (higher solar heat gain is good in the north where it helps warm a home in cold but sunny weather), compared to a range of 0.17 to 0.40 for standard ENERGY STAR. Air leakage is not separated out as a distinguishing factor for Most Efficient designation.

ENERGY STAR Most Efficient 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 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. I installed ENERGY STAR Most Efficient windows in my home in 2023 and you wouldn’t even know they are triple glazed unless you looked carefully at the window frame from an oblique angle. The view out the window is just as clear as out of any double-glazed window.

There are many federal, state/provincial, local, and utility incentives and rebates available for people who install ENERGY STAR Most Efficient windows. This includes tax credits (e.g. from the Inflation Reduction Act, as mentioned above), reimbursements for improvements made after a home energy audit (Canada’s Greener Homes program), and interest free loans to help finance whatever part of purchase and installation is not already covered by grants and rebates.

Making your current windows more efficient

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:

  • Energy efficient window film: You can install this film on existing windows to reduce the radiation heat transfer, particularly from sunlight.
  • Plastic window insulation: For old or leaky windows, a good stop-gap measure while you wait for your new windows to arrive (or while you save up for them) is this shrinkable plastic, which you apply on tape, and then blow dry to shrink to a tight fit. For $2-5 per window each winter, you can apply this insulation and stop a lot of heat from escaping through older windows.
  • Insulating window blinds help cut heat transfer by adding a well-sealed layer of insulation between the window glass and your room.
  • Energy saving curtains work on the same principle as insulating window blinds. Factors such as curtain length, color, and openings around the edges have a major impact on curtain efficiency.
2 replies
  1. Marc
    Marc says:

    Argon or other gases leak even from perfectly constructed IGU (insulated glass units). They leak at the rate of 1% per year. After 16 years, there should be no more gas in the IGU. I don’t know if they are the best things to do?

    Reply
    • Robin
      Robin says:

      I wasn’t aware of this, but if they leak at a rate of 1% per year, presumably compounded, then after 16 years there would still be 85% of the original gas between the panes.

      Reply

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