One of the hardest decisions in the design of buildings is the choice of windows. There are so many variables and choices, and so much misinformation. For the LifeEdited project, where the windows are going into an existing building, both looking out and opening on to a fire escape, there are even more variables and limitations.
For the LifeEdited project, the goal is to provide the greenest alternative. But then the issue is, how do you define green? Is it just energy savings in operations, or is it a full life cycle cost? Which rules, efficiency or aesthetics? Security? Convenience? There are so many questions.
1. Should the windows be replaced at all?
watch a short video on windows Tampa Preservation. ""if you love the earth, you'll get vinyl windows"
As this little video notes, there is a lot to be said for keeping and restoring existing windows. I have noted in TreeHugger previously that old windows, properly serviced and restored, with an interior removable storm window, are pretty good;
- Properly repaired historic windows have an R factor nearly indistinguishable from new, so-called, "weatherized" windows.
- Regardless of the manufacturers' "lifetime warranties", thirty percent of the windows being replaced each year are less than 10 years old.
- One Indiana study showed that the payback period through energy savings by replacing historic wood windows is 400 years.
- These houses were built hundreds of years ago, meaning those windows were built from hardwood timber from old growth forests. Environmentalists go nuts about cutting trees in old growth forests, but what's the difference? Destroying those windows represents the destruction of the same scarce resource.
A Vermont study came to a similar conclusion:
Our study of old windows showed that the energy savings are similar for a variety of retrofit and replacement strategies. Rates of return on investment for energy improvements are quite low when starting with typical or tight windows with storms in place, but are significantly higher when renovating loose windows with no storm.
The difference in annual energy savings between renovating an old sash and replacing it with a new one was very small--retrofits saved only a few dollars.
Some go so far as to say that the old window with storm is even better than new windows; the old house guy writes:
An old window coupled with a storm window will give you a higher R-value than a double-glazed replacement. This is because there is more air space between the storm window and the inside window than between the two tightly squeezed panes (double pane) of glass in a replacement thermo pane window. Believe it or not, air is one of the best insulators. Since molecules are so far apart in air, heat cannot be transferred. Therefore, the three inches of air space you have between the two pieces of glass performs as a very good insulator.
And the numbers prove it; a properly restored window with an interior storm window can achieve U values between .32 and .44. That is good enough for Energy Star rating.
(What is U Value? The reciprocal of R value. Why do they use it? I think to hide the fact of how little value insulating windows are. More at If I See Another Full Page Pella Window Ad I Am Gonna Scream.
More on saving old windows instead of replacing:
Ask Pablo: Is Replacing Windows A Good Investment?
Building The Green Modern Home: Looking At Windows
National Trust for Historic Preservation
2. New Windows: Type of Window
The importance of this cross-ventilation cannot be overestimated. Before Air Conditioning (BAC) people knew how to tune their windows to get maximum ventilation. The air shafts, required under the building codes of the time, created a stack effect that drew air through the units, making even the crappiest tenement bearable in summer. Properly adjusted and tuned, one only suffered on the hottest days.
But as can be seen from the top image, without cross ventilation you get almost nothing. These sketches are for a single family home, and suggest that the most air flow would happen with a small upwind opening and a large downwind opening. But I think in a building with an air shaft, with the suction that comes from the stack effect, that a smaller window on the shaft and a big window on the street would do very well. The fact that ther is cross ventilation will significantly affect the choice of type of window.
Casement windows open outward and are usually operated by a crank, so that they can have an interior screen. This is very practical as the screens stay a lot cleaner. The windows, if properly placed, can funnel air into the unit. As can be seen in this sketch, there are other techniques one could use, like a panel between the windows, to create conditions that promote air flow. Casements have clamps that pull them in, sealing them tightly against the frame, creating probably what is the tightest seal.
Screen grab from Innotech Windows
Tilt and Turn windows magically offer more options; you can tilt them in at the top, giving good ventilation and some security, or you can open them completely like a door. The seals and mechanisms are a bit more complicated, but they are very popular in Europe. Screens must go on the outside.
Other options include awnings and hoppers, which have fallen out of favour and don't provide enough ventilation.
3. New Windows: Materials
Image credit Green Building Advisor
This is truly a thing of beauty. FSC wood frame. Maintenance free aluminum cladding on the exterior. High tech triple glazing with natural cork spacers. Built to Passivhaus standards.
It also is German and incredibly expensive, probably $ 150 per square foot of window area. This is where compromises get made. But there are options for frames:
Credit Loewen Windows
a) Wood. Cheaper wood windows are made of finger jointed pine and won't last that long; high quality ones like Loewen are made from FSC certified western douglas fir.
credit innotech windows
b) Vinyl The only knockout material in the Cradle to Cradle certification system, there is so much wrong with the stuff from manufacture (chlorine) to the plasticizers (phthalates) to disposal. It is hugely controversial on TreeHugger, where we have been debating its use in windows for years.
Some are better than others; Innotech claims their windows are lead and plasticizer free, and strongly reinforced with steel. But most vinyl windows are not, and most will sag in the sun if they are any colour but white and will fail eventually, as the stuff has a high rate of expansion and contraction. Donovan Rypkema says "that's why they are calle replacement windows, every twenty years you have to replace them!"
c) Pultruded Fiberglass has a lot of benefits. It is incredibly strong, has low conductivity, has a rate of expansion that is very similar to glass so the windows and the frames stay together, and lasts almost forever in window frames. The most energy efficient windows in Canada (Inline Fiberglass) and the States (Serious Windows) are made from this.
Downsides are cost (it is more expensive than other materials) and composition, as it is made with styrene, which was recently declared a possible carcinogen. However this is not an issue for the end user, only for the manufacturer.
More: Serious Materials Makes Serious Windows
4. New Windows: Glazing
Here it gets so complicated. One has to consider the U factor (insulation value) and the solar heat gain coefficient. (SHGC). Convention in a climate like New York is to put windows with a high SHGC on the east and west windows of a house, so that you pick up the heat gain through the windows and reduce heating costs. Some people are even suggesting that one picks up heat just from skylight and reflections as well as direct sunlight and are putting them on north walls of houses. But we are in downtown Manhattan, and the lower winter sun probably never will reach these windows.
But in summer, the high summer sun might reach the street windows, and that is a problem. So in this particular case, we want a low SHGC window.
Noise is also an issue in New York, and the conventional wisdom is that triple-glazing is the best way to deal with this issue. However a study by the National Research Council of Canada determined that it doesn't make much difference; the thickness of the glass is far more important, and having different thicknesses helps.
In cases where substantial noise reduction is required, double glazing is the most sensible choice. The airspace should be sufficiently large to provide the desired TL.
Using different thicknesses of glass for double or triple glazing gives greater noise reduction. The highest STC values shown in Figure 2 are for double 6 mm glass; windows with 3 mm glass substituted for one of the layers of 6 mm glass would have equal or higher STC ratings. Using different glass thicknesses is effective because resonances for each layer fall at different frequencies. A ratio of about 2 in thickness of the glass layers is most effective.
Read more on glazing:
Green Building Advisor: Martin Holladay on glazing options
Alex Wilson thinks otherwise about glazing: Making the Case for Triple-Glazed Windows
1. Save and fix the existing windows
My recommendations should be taken with the note that I am past President of the Architectural Conservancy of Ontario, and believe that the greenest window is the one already in the wall. I believe that if you fix the existing windows and put on a high quality interior storm window that you will get a good insulating value, and that on the basis of a life cycle analysis is the greenest window going. Furthermore it will last the longest; all sealed glazing units leak eventually, losing their rare noble gases.
2. If you are going to replace the windows, go all the way.
The best window made in North America is the Serious 1125 series with an R value better than most walls. However it is not available in tilt and turn, only in casement or awning.
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