Image Credit: Stephen Thwaites, Thermotech Fiberglass Fenestration
More Musings of an Energy Nerd
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…
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32 Comments
Cold Climatist
Martin,
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.
Tunnel vision?
John,
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.
Climate zones
John,
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.
smiley face
Martin,
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.
Consider me goaded
John,
Thanks for the goading. I'll try to maintain a better balance between Vermont stories and Texas stories.
Thanks from a glass lover!
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 IntegratedFraming.com
Clarification
Martin,
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?
Windows That Perform Better Than Walls
Martin,
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.
Heating versus cooling costs
James,
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."
Arasteh's U-factors
Ross,
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.
Super windows
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.
Comparing consumption in KWH to BTU
Martin,
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?
Site energy versus source energy
John,
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:
http://www.eia.doe.gov/emeu/mecs/mecs94/ei/elec.html
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.
Rules of thumb
Doug,
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."
_____________________________________________________
http://www.lrc.state.ky.us/kar/115/002/020.htm
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:
http://www.newenergy.org/sesci/publications/pamphlets/passive.html
"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."
Energy Rating little used in Canada
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.
Cheers,
Andrew
Windows that aren't energy holes
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.
Payback for retrofit windows?
Ann,
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.
Efficient windows
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.
Privacy issues and all-glass houses
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.
SHGC of South Facing Windows
For a quick qualitative evaluation of SHGC effects on passive solar heating, see
http://greenbuildingindenver.blogspot.com/2009/12/solar-cats.html
Walls as net energy gainers
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?
Jon
Site-Source Factors (for Martin H.)
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": http://www.nrel.gov/docs/fy07osti/38617.pdf It has site-source numbers broken down on a per state mix, if you are inclined to use them.
Thanks, Kohta
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.
Thanks.
Walls that are net energy gainers
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.
Better than walls
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
Calculating window heat loss and gain
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:
http://www.susdesign.com/windowheatgain/index.php
BUILDING AND RENOVATING
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.
John
http://www.cavalok.com/
Haters!
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?
Victorian window solution
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....
Heavy curtains don't stop solar heat gain
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.
beg to differ
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.
Response to Steven
Steven,
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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