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Windows That Perform Better Than Walls

If you choose the right glazing, your windows can gather more energy than they lose

Posted on Dec 11 2009 by Martin Holladay

UPDATED on March 18, 2015

The common perception that windows are “energy holes” is a bad rap. Since today’s high-solar-gain triple-glazed windows gather more heat than they lose, good windows perform better than an insulated wall. After all, a wall can only lose energy, while windows can gain energy during the day to balance energy lost at night.

The Canadian window-rating system
In Canada, windows are rated according to the ER (Energy Rating) method. For those who live in cold climates, the Canadian ER system is arguably an easier-to-understand method of rating windows than any system used in the U.S., where NFRC window labels cause a fair amount of head-scratching.

According to a Natural Resources Canada document explaining the original ER rating system, “A window’s ER rating is a measure of its overall performance, based on three factors: 1) solar heat gains; 2) heat loss through frames, spacer and glass; and 3) air leakage heat loss. A number is established in watts per square meter, which is either positive or negative, depending on heat gain or loss during the heating season.”

Under the original ER rating system, windows with a negative ER were “energy holes,” while windows with a positive ER acted like heaters. Poorly designed windows had ER ratings as low as -25, while the best-performing triple-glazed windows had an ER of about +1 (for operable windows) or +8 (for fixed windows).

Window manufacturers weren’t happy to discover that many of their windows ended up with negative ratings. Canadian authorities decided to respond to manufacturers’ complaints by instituting a type of grade inflation: 40 points were simply added to every window’s old ER score. With a stroke of the pen, -10 became 30, and 1 became 41.

South-facing windows produce the most energy
Consider a 1-square-meter fixed window equipped with the best available triple glazing installed in southern Canada. Over the course of a heating season, this window will, on average, perform like an 8-watt heater. This level of performance is an average based on all four orientations; if the window faces south, it will contribute much more than 8 watts.

Good residential designers shouldn’t be choosing window sizes without performing energy calculations. To get an idea of the necessary arithmetic, consider this summary from Jesper Kruse, an energy consultant in Greenwood, Maine: “I’m working on a project for a future Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. in Maine. … I’m using triple-pane Thermotech windows with a low-e coatingVery thin metallic coating on glass or plastic window glazing that reduces heat loss through the window; the coating emits less radiant energy (heat radiation), which makes it, in effect, reflective to that heat; boosts a window’s R-value and reduces its U-factor. on surfaces #3 and #5, 90% argonInert (chemically stable) gas, which, because of its low thermal conductivity, is often used as gas fill between the panes of energy-efficient windows. , 10% air filled. On my south-facing wall I’m using glazing with a solar heat-gain coefficient (SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.) 0.61 and a [metric] U-value of 0.9 [equal to a U.S. U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. of 0.16]. For a total area of 12.7 square meters (around 110 sq. ft.), my total heat losses [attributable to south-facing windows] are 2,005 kWh per year, and gains are 3,880 kWh per year — a net gain of 1,775 kWh per year.”

North-facing windows need high-solar-gain glazing
Since American window manufacturers prefer to sell the same kind of glazing from Florida to Maine, it’s hard to buy high-solar-gain glazing in this country. That helps explain why few designers realize that high-solar-gain glazing is the best cold-climate choice — even for north windows.

According to Morgan Hanam, the manager for window services at Enermodal Engineering in Ontario, “In Canada, even if windows are evenly distributed between the north and south, it’s still worth putting in high-solar-gain windows on all sides – even considering air-conditioning loads.”

The reason is simple: even if sunlight never strikes a window directly, a north-facing window can gather a little bit of heat from reflected and ambient light. “We get a lower heating energy cost with a high-solar-gain low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. glazing on the north rather than the intuitive choice — the better insulating, lower SHGC low-e,” explained Stephen Thwaites, the technical director of Thermotech Windows. “Interestingly, the answer is the same when we use London, England, weather data instead of Ottawa, Canada weather data.”

Although high-solar gain glazing beats low-solar gain glazing for north windows, the amount of energy involved is small. Thwaites admits that it’s “a pretty minor difference.”

Can north-facing windows be net energy gainers?
In theory, if a window can be designed with a very low U-factor and a high enough SHGC, even a north-facing window can gain more energy than it loses.

Almost two decades ago, Dariush Arasteh, a staff scientist at Lawrence Berkeley National Laboratory, started calling high-performance triple-glazed windows “superwindows.” Along with co-authors Brent Griffith and Paul LaBerge, Arasteh produced a 1994 paper, “Integrated Window Systems,” that predicted, “Highly insulating windows with moderate solar heat gain coefficients will transmit more useful solar radiation during the day than they will lose, even during cloudy days, on north orientations, and with significant sky obstructions.”

A year earlier, Arasteh wrote “Monitored Thermal Performance Results of Second Generation Superwindows in Three Montana Residences,” in which he stated, “Simulation studies have shown that highly insulating windows with moderate solar transmittances (R-values greater than 6 [in other words, U-0.166 or less] and shading coefficients greater than 0.5) can outperform insulated walls on any orientation, even in a northern U.S. climate. Such superwindows achieve this feat by admitting more useful solar heat gains during the heating season than energy lost through conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow., convection and infrared radiation.”

Unfortunately, Arasteh was unable to verify his energy calculations with field measurements because he couldn’t locate or build any windows meeting his rigid specifications. Arasteh wrote, “Testing of first generation superwindows in three new homes in northern Montana during the winter of 1989-1990, reported in an earlier study, indicated that the glazed areas of superwindows” — that is, the windows not including the frames — “can in fact outperform insulated walls on obstructed off-south orientations. However, this same study also showed that further improvements in the thermal performance of window edges and frames are necessary if the entire window is to outperform an insulated wall.”

To sum up, good triple-glazed windows facing south, east or west gain more heat than they lose during the heating season in a cold climate. Windows facing north may lose just a little more heat than they gain — at least until window manufacturers develop a window meeting Arasteh’s specs. As it turns out, Passivhaus-certified windows from Europe may already be able to meet Arasteh’s threshold.

Perhaps a little gain, perhaps a little loss
Although the possibility that north-facing windows could be energy-positive is intriguing, the potential energy gains, if any, would be small. Whether a north-facing window with excellent glazing is slightly energy-positive or slightly energy-negative will depend on several factors, including the glazing specifications, the building’s location, and the time frame under investigation. A north-facing window will perform better over a seven-month time frame (say, from October to April) than over a four-month time frame (say, November to February). Of course, the number of months that a building requires space heat varies with climate and with building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. quality.

According to calculations performed by energy consultant Marc Rosenbaum, a north-facing window with excellent glazing in Boston will be slightly energy-negative from November 1 to February 28. If the window has what Rosenbaum “calls super-duper German glazing” — U-0.11, SHGC 0.51 — each square foot of glazing (not considering frame performance) will gain 9,894 BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. and lose 10,608 BTU during the four months under consideration. Rosenbaum notes that these calculations do not include an adjustment of the SHGC for off-angle radiation.

A house without walls
If it’s possible to buy windows that perform better than an insulated wall, why not design all-glass houses?

In addition to the obvious drawbacks — triple glazing is expensive, and most people feel that a glass house doesn’t provide enough privacy — there are two basic problems with the all-glass house idea: overheating on sunny days, and excessive heat loss on cold nights.

Although good windows gather more heat than they lose, the gains and losses happen at different times. In a well designed house, this isn’t a problem, because the house includes enough thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. to absorb heat gained during the day, and because the amount of glazing is appropriate — not too much, not too little.

Although interior thermal mass can help dampen the see-saw effect between daytime heat gain and nighttime heat loss, there is an upper limit to the amount of effective thermal mass that can be included in a passive solar house. Generally, thermal mass thicker than 4 inches won’t absorb useful amounts of heat during the time frames under discussion.

The Canadian ER window-rating system assumes that no heat gains are wasted. Although this is a reasonable assumption in a well designed house, it won’t be true in a house with excessive glazing.

Anyone who has ever lived in one of the experimental passive solar houses built during the 1970s knows that houses with too much south-facing glazing overheat on sunny afternoons in February and March, forcing the residents to open the windows to stay comfortable. Of course, once the windows are opened, some of the heat gathered by the windows is wasted.

Fortunately, the principles of passive solar design are now better understood than they were in earlier decades, so newer passive solar homes are much less likely to overheat.

The bottom line
Although windows were “energy holes” forty years ago, the best modern windows are now energy assets. They not only provide a view and a method of ventilation — they also help heat your house.

Last week’s blog: “Roofing and Siding Jobs Are Energy-Retrofit Opportunities.”

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Image Credits:

  1. Stephen Thwaites, Thermotech Fiberglass Fenestration

Dec 11, 2009 7:47 AM ET

Cold Climatist
by John Brooks

I suspect that there is only a very narrow climate zone (or neighborhood) where your "bottom line" would make sense over 4 seasons.
Carl is right... I believe you may have Northern and non-urban tunnel vision.

Dec 11, 2009 9:07 AM ET

Tunnel vision?
by Martin Holladay

Several times in my blog, I made it clear that I was addressing window specifications for cold climates. I don't think that reporting about cold climate issues represents tunnel vision.

My opinions on hot-climate design and hot-climate window specification can be found in an earlier blog, "Hot Climate Design."

A reminder to hot-climate readers: Americans consume about six times more energy for residential space heating than they do for residential air conditioning.

Dec 11, 2009 9:16 AM ET

Climate zones
by Martin Holladay

To evaluate window performance during the heating season, the energy losses are subtracted from the gains. I have provided some data in this article from Maine, Ontario, Montana, and London, England.

For any climate warmer or sunnier than these climates, the heating season energy performance of windows will be better, not worse, than the energy performance reported for Maine, Ontario, and Montana.

Dec 11, 2009 10:20 AM ET

smiley face
by John Brooks

I should have put a smiley face next to my comment.
Many climates have more than one season ;-)
Just goading you into more blogs for more climates with more seasons.

Dec 11, 2009 10:24 AM ET

Consider me goaded
by Martin Holladay

Thanks for the goading. I'll try to maintain a better balance between Vermont stories and Texas stories.

Dec 11, 2009 6:20 PM ET

Thanks from a glass lover!
by Anne Elliott Merica, RA

Thank you for the reminder that glass is a material that can have a wide range of characteristics, and that design ought to be regional. It is a shame many homes are built with the worst of modular and site-specific design: copying styles and features that are not appropriate, and yet modifying plans so that any house can be placed in any orientation, rather than proper siting for solar and other considerations. Anne Elliott Merica, RA

Dec 11, 2009 6:41 PM ET

by Brett Moyer

In this post you state "Americans consume about six times more energy for residential space heating than they do for residential air conditioning" and in the Hot Climate design post you state, "Americans spend about twice as much for residential heating as they do for cooling."
Could you clarify this?

Dec 12, 2009 3:27 AM ET

Windows That Perform Better Than Walls
by Ross DePaola

You quoted Dariush Arasteh's comments from a few decades ago. At that time, we didn't have some of the LowE products that are on the market today. If I gave you this choice what would you choose? A product with a a center of glass R-value of 8.8 with a very limiited winter North solar gain or a product with an R-value of 11.6 with about 1/3 the value of limited North solar gain? These are the current choices we face with today's LowE technology (LOF Energy Advantage compared to Cardinal 366 glass). I think we have to go back to the simulation programs and re-evaluate. I don't dispute that we could get higher gains on the South for high solar gain glass but for all nights and many winter days, the game is retaring heat loss and having windows with a high R-value can outweigh the gains on certain orientations.

Dec 12, 2009 7:16 AM ET

Heating versus cooling costs
by Martin Holladay

Both statements are true, although the statement in this blog is a little more accurate than the statement in the "Hot Climate Design" article.

The two statements appear different because one refers to energy use (kWh or BTU) while the other refers to cost (dollars).

The statement in this blog is based on the 2001 Residential Energy Consumption Survey from the US Energy Information Administration. That survey reports that the average US household used 2,263 kWh of electricity per year for air conditioning; that amounts to 7,723,393 BTU. The average energy use for residential space heating was reported 44.9 million BTU. That means that air conditioning site energy use is only 17.2% of heating energy use. This accounts for my 6-to-1 statement.

The ratio will narrow, of course, when one converts BTU to dollars, since electricity costs more per BTU than natural gas or oil.

When I wrote a November 2006 article for Enegry Design Update, I converted these energy figures to dollars. Of course, the cost of energy changes all the time. Using 12 cents per kWh for electricity and $1.36 per ccf for natural gas, I calculated that the average family that burns gas would spend $610 for heating, while the average air conditioning cost was $271 per year. These two numbers account for the statement that "Americans spend about twice as much for residential heating as they do for cooling."

Dec 12, 2009 7:41 AM ET

Arasteh's U-factors
by Martin Holladay

In his 1993 paper, Arasteh discussed "highly insulating windows with ... R-values greater than 6." Assuming that he wasn't confusing windows with glazing, he appears to be discussing windows with a whole-window U-factor of 0.166 or less. That U-factor is quite decent. Few builders are buying windows with U-factors that are lower. The lowest available U-factor for a Thermotech casement window with triple glazing is 0.17.

Of course, you can go to Heat Mirror glazing with multiple plastic films and obtain a lower U-factor. But these windows have very low VT ratings and appear gray or tinted to most homeowners.

Dec 12, 2009 12:14 PM ET

Super windows
by Doug McEvers

Great article, Martin

If only we would have had such windows when the superinsulation revolution started, the one weak thermal area in those homes. With net solar gain possible using the right windows, placement and sizing, is there a percentage of glazing area per total floor sf for cold climates that makes sense?

With a very tight and well insulated building, overheating due to too much south facing glass is a design challenge. Most new homes, even if superinsulated are of lightweight construction with drywall, lumber and furnishings being the main heat storage medium. My research has shown the thermal storage for lightweight homes to be about 6 Btu's per square foot per degree F. The charge and discharge period is about 8 hours for each. With this in mind and a solar F-chart one should be able for a given location to properly size windows for a new or existing home. For existing homes getting an extreme energy makeover, careful attention must be paid to window sizes, orientation and thermal mass for proper comfort.

Dec 12, 2009 1:33 PM ET

Comparing consumption in KWH to BTU
by John Brooks

Is it really fair to convert KWH to BTU without factoring transmision losses?
Aren't the Americans really consuming the BTUs that are consumed at the power plant?
The reason that Electricity costs more is because it consumes more....right?

Dec 12, 2009 2:26 PM ET

Site energy versus source energy
by Martin Holladay

I usually try to specify whether I am talking about site energy or source energy; for example, in my recent blog "Houses Versus Cars," I wrote, "Consumption of energy for space heating, hot water, and electrical appliances averaged 97,734,040 Btu (site energy) per household." However, in my Dec. 11 response to your comment, it's true that I referred to the 2001 Residential Energy Consumption Survey data without specifying that the energy was site rather than source energy. Sometimes, when dashing off a response to a blog, I don't include all the asterisks and parenthetical explanations that might be required in more formal writing.

As most people know, it takes more than 1 kWh of fossil fuel to generate 1 kWh of electricity in a fuel-burning generation plant. Of course, some electricity is produced in hydroelectric facilities, and some is generated by wind turbines; with these sources, the site-versus-source issue is really irrelevant.

Your concern over transmission losses is somewhat misplaced. Transmission losses represent about 2% of the energy input into electrical generation. Most experts accept that fossil-fuel generating plants deliver about 1/3 of the energy value of the fossil fuel as electricity, while about 2/3 becomes waste heat.

For more information, see:
Converting kWh of electricity to Btu is not a trivial issue, because the amount of input energy needed to create a kWh of electricity is far greater than the amount of useful energy in the kWh at its point of use. Therefore, meaningful conversions of electricity use from kWh to Btu can be given in terms of:
* Site (point-of-use) electricity, at the universal value of 3,412 Btu/kWh. This value is useful to engineers, energy managers, and others trying to evaluate energy efficiency.
* Primary electricity, at a value that reflects the content of the energy inputs used to produce the electricity. This rate is most useful to policymakers and analysts who are considering global resources and environmental issues.

For convenience and consistency, a factor is traditionally used to convert point-of-use electricity use to primary electricity: 10,447 Btu/kWh for 1985, 10,324 Btu/kWh for 1988, 10,352 Btu/kWh for 1991and 10,280 Btu/kWh for 1994. These factors represent the average energy input to the generation process for fossil-fuel utility plants in the United States for their respective year, as given in EIA's Monthly Energy Review (DOE/EIA-0035(95/03)). However, the reader should understand that the true conversion values for the range of electricity estimates are unknown. Applying the single value to the range of electricity estimates in this report provides only a rough approximation of primary electricity because:
* For some type of utility energy inputs--hydroelectric, wood/waste, wind, and solar (thermal or photovoltaic), there is no generally accepted conversion rate.
* The fossil-fueled, nuclear, and geothermal generation processes have known but different conversion rates, so the overall conversion rate for these energy sources is a function of their mix.

Dec 12, 2009 2:36 PM ET

Rules of thumb
by Martin Holladay

You ask, "Is there a percentage of glazing area per total floor sq. ft. for cold climates that makes sense?"

In a word, no. Sometimes there is no substitute for calculations.

Two houses can both measure 2,000 sq. ft. but be very different -- different orientations, different envelope surface areas, and different orientation of fenestration.

Some rules of thumb have been proposed, however. Most refer to the area of south glazing as a percentage of floor area. Many caveats surround such rules of thumb, however; glazing with a SHGC of 0.33 will obviously transmit less heat than glazing with a SHGC of 0.65.

Even back in the 1970s and 1980s, when glazing choices were fewer, rules of thumb were all over the map. I made of hobby of collecting such rules of thumb at one point. Some are pretty good rules of thumb, and some are absurd.

For example:

Green Building: Project Planning and Cost Estimating [R.S. Means]: "The required window area [for direct gain spaces] varies from 10-20% of floor area for a temperate climate, to 20-30% for a cold climate."

Green Building Guidelines from SBIC: "If a house has no additional internal mass (other than the typical amount provided by furnishings, drywall, construction, etc.), the maximum allowable area of south-facing glass is about 7% of the finished floor area."

The Sustainable Buildings Industry Council (SBIC) suggests the following amounts of non-south glass: north 4%, east 4%, and west 2% of the total floor area. Based on standard guidelines for sun-tempered homes, the net amount of glass on the south wall should be no more than 7% of the home’s total floor area. If more window area is added, additional thermal mass will be needed to avoid overheating on clear winter days.

Lenny Gibson, homeowner: “I calibrated my glazing area to 6% to 7% of floor area. The early guidelines were 10%, but it turned out that was too high.”

Daniel Chiras, The Solar House: "In direct-gain passive solar homes, solar glazing should range between 7 and 12 percent of the total floor space."
Kentucky regulation: "Income tax credit for active solar, passive solar, wind, and geothermal energy systems": "Where the ratio of the passive solar glazing area to the floor area of the direct gain space does not exceed sixteen (16) percent, additional storage mass beyond normal home furnishings and wall finishes is not required."

"The Sun-Inspired House" by Debra Rucker Coleman:
"Place a minimum of 5% and a maximum of 12% (of the conditioned area of the house) of glass on the south wall of the house ... If south glass exceeds 7% of the floor area, install heat-absorbing materials ...(thermal mass)."

For Canada:
"To let the sun in, a ratio of roughly eight per cent window to floor area is recommended for south walls."

The Green Studio Handbook by Alison Kwok and Walter Grondzik:
[This sounds too high] “For a cold to temperate climate, use a solar glazing ratio of between 0.2 and 0.4 square feet of south-facing, appropriately-glazed aperture for each square foot of heated floor area. In mild to temperate climates, use between 0.10 and 0.20 square feet of similar aperture for each square foot of heated floor area.”

Green Building Guidelines from SBIC: "The rule of thumb is that the thermal mass should be about six times the area of the direct-gain, south-facing glass. ... For most thermal mass materials, their energy effectiveness increases up to a thickness of about 4 inches. Mass thicker than 4 inches typically does not absorb and release heat quickly enough to be effective and worth the additional investment."

The Green Studio Handbook by Alison Kwok and Walter Grondzik: "A general rule is to provide a concrete mass of 4-6 inches thickness that is about 3 times the area of the solar glazing. This assumes the mass is directly irradiated by solar radiation. A ratio of 6:1 is generally recommended for mass that receives only reflected radiation."

Dec 14, 2009 4:32 PM ET

Energy Rating little used in Canada
by Andrew Henry

Hi Martin,

NRCan's ER method for rating windows is a sensible and straightforward way to rate a window. That's what I thought about ER when I first came across it when I was choosing windows for a renovation in 2004.

I live in Canada's National Capital region and so have a little more awareness, simply because of proximity, of the various progressive programs that the likes of NRCan and CMHC have developed over the years. Along with R2000 and it's commercial cousin C2000, the ER rating method was ahead of it's time; for the wrong reasons. Mostly because we in Canada, like in the States weren't mandated to build to a higher energy efficiency standard, instead the market would decide. Actually it was really the builder's who decided. So ER and R2000 became nice but little used innovations.

Unfortunately, the Energy Rating method as a useful consumer tool is pretty much meaningless, even here where Natural Resources Canada is located. Almost every window retailer in Ottawa has windows that show nothing but the NFRC label. Why? Because it would probably be pretty embarrassing to display their window's ER rating. It certainly wouldn't be a positive number.

Fast forward to 2006-2007 when my wife and I planned an addition to our house and I again started looking for windows and doing my research. I heard about the Passive House concept, and my Google search took me to the article about Katrin Klingenberg's Passive House by an 'energy nerd' who wrote for this obscure journal called Energy Design Update. Figuring that windows good enough for a Passive House are probably pretty good I did a search in Google for the windows she used and it turned out Thermotech windows were and are manufactured pretty much in my backyard.

I don't think I would have come across Thermotech because of the ER rating method. It took a lot of searching around the periphery to stumble upon them. If only EDU had been online back in 2004 when I did my renovation. I might not be stuck with double glazed double hung windows in the original part of my house.

The absurd thing, and I suppose this kind of absurdity is all too familiar to you Martin, is that the Canadian federal government, NRCan itself, hands out rebates for replacing windows that are Energy Star qualified when it could be using it's more effective and meaningful ER rating method, that it developed, to create a better awareness of window energy efficiency.

Hopefully as people demand better windows, manufacturers and governments will start using the ER rating as their standard way of conveying to the consumer a windows energy efficiency.

Finally, thanks for writing that long ago Passive House article.



Dec 15, 2009 2:37 PM ET

Windows that aren't energy holes
by Ann Edminster

Some analysis I did for my new book on net-zero homes (Energy Free - Green Building Press) also suggests that replacing windows can be an attractive payback item when you're replacing with R-5 or better, e.g., Canadian triple-panes or Serious Windows (for which I serve as an advisor, in the interest of full disclosure). This wasn't typically true in the past when the best you could do was replace with ~R-2 to R-3. These high-performance windows can be game-changers for home retrofit economics.

Dec 15, 2009 2:54 PM ET

Payback for retrofit windows?
by Martin Holladay

I'd be interested in seeing your payback calculations. As someone who paid $3,002 for two Thermotech windows in 2008 — that's the price of just the windows, not including other materials or installation labor — it's hard to juggle the figures in such a way as to yield an economic payback. (Admittedly, each window was a ganged unit that included two operable casements. But these windows aren't cheap.)

I know that the windows are saving energy, but I've only managed to afford these wonderful windows for one room in my house. Even if I were heating my house with propane (I actually cut my own firewood) it would be decades before I'd see an economic return on my investment in two new windows.

Dec 15, 2009 2:56 PM ET

Efficient windows
by jlbaerg

I might add the following factors to the discussion. First, in our area (Montana) glare and privacy issues have frequently trumped solar gain resulting in window shades that are drawn most of the time. Secondly, an all glass house would still have some mean radiant comfort issues (an R 6-8 window is still colder than an R25 wall) during winter nights. We need to design for both year round performance and worst case scenarios. Finally, lets not forget that houses need to be useful and loved. Window design also needs to incorporate privacy, use of wall space, lighting levels, exterior and interior views, aesthetics and other, more subjective experiential values. That's the hard part, in my experience.

Dec 15, 2009 3:01 PM ET

Privacy issues and all-glass houses
by Martin Holladay

J.L. Baerg,
I agree with all of your points. The all-glass house was proposed to make a technical point; it was never proposed as a desirable house design.

As you probably know, triple-glazed windows are much less likely to provoke comfort complaints due to radiant effects than are double-glazed windows.

Dec 15, 2009 4:36 PM ET

SHGC of South Facing Windows
by Kevin Dickson

For a quick qualitative evaluation of SHGC effects on passive solar heating, see

Dec 16, 2009 4:36 PM ET

Walls as net energy gainers
by Jonathan Beers

Martin H. wrote: "If it’s possible to buy windows that perform better than an insulated wall..."

I recall some research done in Alberta that showed south-facing walls were net energy gainers even on very cold winter days (which tended to be sunny). I don't recall how east and west orientations performed. Has anyone compared the net energy performance of well-insulated wall assemblies vs. very good windows?


Dec 17, 2009 1:54 AM ET

Site-Source Factors (for Martin H.)
by Kohta Ueno

However, the reader should understand that the true conversion values for the range of electricity estimates are unknown. Applying the single value to the range of electricity estimates in this report provides only a rough approximation of primary electricity because...

Hi Martin--I don't know if you already have seen this, but the most thorough analysis I have seen of site and source energy conversion factors for various fuels was the NREL paper by Deru and Torcellini "Source Energy and Emission Factors for Energy Use in Buildings": It has site-source numbers broken down on a per state mix, if you are inclined to use them.

Dec 17, 2009 5:05 AM ET

Thanks, Kohta
by Martin Holladay

The NREL report you link to has more than most of us will ever need to know on the issue -- including quite a few state-by-state tables at the end.

Dec 17, 2009 5:54 AM ET

Walls that are net energy gainers
by Martin Holladay

Jonathan Beers,
Interesting question about south-facing walls; I don't know the answer. As usual, though, residents of Colorado or the Canadian prairie provinces will make out better than those of us in cloudy Vermont, where sun is very rare in November, December, and January.

Jan 20, 2010 7:29 PM ET

Better than walls
by Dr Duck

please provide quantitative energy data based for a pair of 30 x 48 super windows for toledo ohio, 24/365 net btu heat gain/loss, direct northern exposure vs. southern exposure; data for both exposures. Thanks dd

Jan 21, 2010 5:15 AM ET

Calculating window heat loss and gain
by Martin Holladay

Dr. Duck,
I'm afraid you haven't provided enough information. The term "superwindow" is not a technical term. To know the performance of a window, we need to know the window's U-factor and SHGC.

Heat loss in BTUs per hour per square foot is calculated by multiplying the U-factor by the temperature difference between outdoors and indoors. To calculate window heat gain, you may want to use a free online tool:

Feb 25, 2010 10:39 PM ET

by John

Hello Martin, your blog has certainly generated a lot of comment. The issues I have is that there are standards and ratings set for consumer to select wisely when building or renovating their homes.
These standards and ratings do not appear to be mandatory, nor as one commenter claimed does the builder, architect and or consumer have to comply to any standard when placing orientation of the proposed home, and, or best use of materials for that particular climate. Where are the local authorities who audit the building process? Why is that the local authorities who regulate and monitor building "code of ethics and standards not encouraging mandatory standards. This is not unique to Canada, but surely all homes built or renovated must comply to a minimum standard or rating, that includes orientation, type of materials, window glazing and other building materials. Do we need to increase the minimum standard? I am amazed that with all the attention on "global warming" and "spiraling" costs of energy use that we still continue to leave the decisions to consumer and or retailers who often are considering other factors rather than what is best for the society on a whole to live comfortably in a home that is affordable and considers the weather conditions of the area.


Apr 1, 2010 7:39 PM ET

by Arron

Man oh man, some of these comments are getting a tad nit picky. John you make some great points initially but why is JLBaerg thinking Martin was making a point for an all glass house as a desirable design choice? Anyhow, Martin, your points are all strong and you have great rebuttals.

It's really awesome to see 27 comments on an issue like energy efficient glass. To all the commenters great job! Is commenter a word?

May 25, 2010 1:19 PM ET

Victorian window solution
by Miss M

Could it possibly make sense to simply VARY both the SHGC/R-value WITH THE SUN
by opening and closing the beautiful
heavy, double/triple-layered (dead air space) CURTAINS
over inexpensive high-SHGC glazing?

Of course, heavy curtains require more than drywall for mounting, but....

May 25, 2010 1:45 PM ET

Heavy curtains don't stop solar heat gain
by Martin Holladay

Miss M,
If sun is shining through high-solar-gain windows, as you suggest, and the sun is hitting your heavy curtains, the sun will raise the temperature of the curtains. Once the curtains are nice and hot, the heat is already in your room. It is transferred to other objects in the room by radiation and convection.

In other words, once the sun has passed through the high-solar-gain glass and entered the room, it's too late to stop it. The curtains won't help.

Jun 15, 2010 2:30 AM ET

beg to differ
by Steven Estergreen

A wall can do more than lose energy. Depending on the absorptivity of the outer surface, it could easily be above the indoor temperature when the sun is shining on it. However, if the wall is also well-insulated, the outside surface will just get hotter and hotter without letting much of that energy into the interior of the building. Even though it won't be admitting as much energy as a window designed for solar gain, it won't only be losing energy, either.

Jun 15, 2010 4:36 AM ET

Response to Steven
by Martin Holladay

Of course there are many hours during the year when walls are not losing energy. These include good chunks of time, sometimes referred to by their technical names: spring, summer, and fall.

Energy modeling programs take this fact into account when they determine the performance of a wall assembly, however.

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