Image Credit: Michael Henry
More Musings of an Energy Nerd
Anyone who needs to choose windows for a new home has a lot of decisions to make. In this article, I’ll try to provide an overview of some of the factors to keep in mind when ordering windows.
Egress requirements
Most building codes require that every bedroom include at least one window that meets emergency egress requirements — in other words, a window that is large enough to allow a person to escape during a fire. These requirements can be found in 2012 IRC Section R310.1.
If any part of a basement has been turned into habitable space, then that area also needs at least one egress window.
To meet emergency egress requirements, a window:
- Needs to be operable (not fixed).
- When opened, needs to have an opening that begins no higher than 44 inches above the flooring.
- Needs to have an opening that is at least 20 inches wide.
- Needs to have an opening that is at least 24 inches high.
- Needs to have an opening that measures at least 5.7 square feet (unless the window is located on the ground floor, in which cast the minimum opening area is 5.0 square feet).
Note that a window that barely meets the minimum width and height requirements is not large enough to meet the minimum area requirement.
Fixed or operable?
Once you’ve specified at least one operable window for each bedroom and, if necessary, for the basement, you can think about how many of the remaining windows in your house should be operable.
For example, let’s say that your living room will have four windows. Should you buy four operable windows? Or should you instead consider buying two fixed windows and two operable windows?
In most cases, it makes sense to resist the temptation to make every window operable. Look around your house and…
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25 Comments
don't always avoid double hungs
Great article. Just thought I would add that in many towns and cities there are areas where double hung windows are required because of historic zoning. If you install anything other than double hung windows the historic zoning commission will make you remove them and they don't want to hear about air leakage rates or triple pane european style windows. It is important to know your local ordinances/zoning before you order anything.
It's past time for convenient exterior movable shading
The effect of natural light on human enjoyment of a space is very large. Changes in light through the course of a day, from season to season, and in different weather patterns make a fixed window and overhang design sub-optimum. The design profession has done much too little in addressing the needs for control over natural lighting, especially via movable exterior shading. While interior shades are also useful, only exterior shading can preserve most of the view and emotional connection with the outdoors, while significantly modifying the amount of light entering the room.
When I last visited high-efficiency houses in Europe, several years ago, I saw a variety of moveable exterior shade options, that the home owners said were satisfying and effective. Most were electric, and all could be operated conveniently from inside the house. Only a few were integrated with the thermostat/heating system. The owners said the prices were reasonable, and the reliability high. The USA market seems to have few of the options that I saw, and we tend to be erratic in our approach to what price we will call reasonable.
Given the huge role that windows play in the owners' satisfaction with a building, I think we are long overdue for the incorporation of convenient exterior movable window shading. It's a chicken and egg problem. People contemplating building a home aren't aware to the advantages. Designers see few products available. Manufacturers don't see a market. I hope this will change in the next decade.
Emulating double-hungs
Historic District committees will often accept casements with center bars to emulate the look of double-hungs. This is also useful when meeting egress codes.
I know that both Zola and
I know that both Zola and Klearwall have windows that have been approved by historic districts in NYC. They are typically a fixed sash on top and a TnT on the bottom.
There are providers of external shading systems in the US, although most of their products are actually sourced from Europe. Not a lot of folks in the US like the look of external shades so that may be part of the reason you don't see a lot of designers making provisions for them. New homes rarely have shades pre-installed and trying to retrofit with external shades can be a nightmare at that point.
Codes and Windows
Our code isn't as concerned with the size of the window used for egress, as the clear area available to escape through when the window is open. In practice this excludes awning windows and some others where the mechanisms impede egress.
Another consideration when designing, is that the area of glazing on exterior walls is often limited by the distance from the lot line. Under our code walls closer than four feet may be precluded from having any windows at all.
In buildings with more than one occupancy, like an apartment over a garage or shop, there are also limitations on where windows can be located so that a fire in the lower occupancy can not spread or impede exiting from above.
Response to Derek Roff (Comment #2)
Derek,
Like you, I've seen electrically operated exterior shading devices in Europe. By far the most common of these are roll-down metal shutters. These effectively shut out light (and solar heat gain), and provide improved security for homeowners worried about burglary. But they aren't very subtle, and I imagine that you are talking about more aesthetically pleasing options.
You noted that when you were visiting high-efficiency houses in Europe, "The owners said the prices [of their electrically operated exterior shading devices] were reasonable." I'd just like to point out that "reasonable" for the owner of a new home in Germany is different from "reasonable" for an American.
I imagine that the cost of an electrically operated shading device is at least as much as, and in some cases more than, the cost of the window.
Retractable awnings
Retractable patio awnings are popular in the U.S. Most people don't buy them for the purpose of shading their windows, but they do accomplish that.
Motorized Canvas Awnings
A Passive House in Maryland has:
"Motorized canvas awnings at all main west windows, capable of 100% shading when required"
I could imagine using something like this over south facing glass to provide adjustable shading here in Colorado.
See: http://www.phius.org/phius-certification-for-buildings-and-products/case-studies/the-new-american-foursquare
Motorized canvas awnings
Thanks for the link, Pat.
Anyone know what these cost? Or how much an electrician charges per window to install them?
.
Tempering
Do most windows generally come with tempered glass nowadays? Or does that incur additional cost as an option?
Also, I'd anyone offering windows with advanced features like electronic dimming or frosting (for privacy) or anti-intrusion glass?
David
The companies I buy from will supply any type of glazing I request, but their basic glazing only differs in thickness depending on how large the window is. Tempered glass is expensive and generally reserved for where it is mandated by building codes, such as on stairs, windows extending close to the floor and some locations in bathrooms. Similarly security glass or fire rated glazing can be installed but at a considerable premium. I would imagine that none of these specialized glazings are included as part of any company's basic packages because they are unnecessary.
What is the cost of tempered glass?
Malcolm says, "Tempered glass is expensive". I've been trying to find out how much more it costs than regular glass. The only site that offered an overview said regular glass costs $12-$64 per sq ft. and tempered glass costs $14-$70 per sq ft. A minor difference. Other sites that would give me online quotes for a sample window in either tempered or regular glass showed a price difference from 10%-20%. My favorite glass supplier will only make double-pane windows in tempered glass, and their price was about 10% above the cheapest standard glass quote that I found, and 20% below the highest standard glass quote that I found.
There are so many variables, but my previous experience and my current online search both indicate that tempered glass is not very expensive, averaging something like 10% more. Does anyone else have specific cost comparisons?
Double hungs vs Casements
I've never read anything about what type of windows to use in high wind areas. Some years ago I replaced old wooden double hungs on a few upstairs windows. The new vinyl casements had heavy duty Truth hardware, multi-point locks and triple weatherstripping. All well and good, except that we like the windows cracked while sleeping. There is enough play in the hardware when the wind is howling that the open sash flops back and forth. I'm sure we'll have to replace the hardware at some time. I could have used good quality double hungs (Marvin maybe) and achieved the same amount of air leakage. Also, my daughter left a casement wide open all summer and the sash frame sagged such that we couldn't close and lock the window. Double hungs don't suffer from these issues.
I don't recall ever reading an article on GBA about variations in DH weatherstripping. Which brands and models work the best?
Derek
I'll call my glass supplier and get an exact amount. It's a holiday here tomorrow so it won't be until Tuesday.
Response to Jim Baerg
Jim,
Q. "I don't recall ever reading an article on GBA about variations in double-hung weatherstripping. Which brands and models work the best?"
One of the possible ratings that appear on an NFRC window label is the AL (air leakage) rating. For manufacturers, AL testing is optional -- but it is required for any window seeking an Energy Star designation.
If you are shopping for double-hung windows, you might want to compare AL ratings.
For more information on the topic, see this Q&A thread: "What's the range of window air leakage values for good-quality US windows?"
Reply to Derek
For a 2 ft x 3 ft double glazed unit with 1/2" spacer, I was quoted $90 for regular glass and $145 for tempered. That is quite a difference, but depending on what portion of the window price is glazing and what part frame and hardware, it may well be closer to the 10% to 20% you cite.
Operable panels instead of windows?
I've been trying to figure out why it is necessary for the location of light penetration (windows) to be the same as the location for ventilation and/or egress. Is it out of convenience? Habit? Why not install very well insulated and thermally broken fixed windows and then create fully functional ventilation/egress panels (doors?) which, could be cheaper, and much easier to insulate, while actually providing a safe means of egress? Maybe the combination window/egress is cheaper in the long run? But by asking the window to perform so many functions, we are sacrificing design, and ultimately, function.
Doors?
I guess another way to ask my question is... how does the best door compare to the best window when it comes to cost, thermal performance, etc? Perhaps egress doors, where applicable, would be more cost effective than egress windows?
Doors for egress
Ethan, unless you needed a door anyway, then you are proposing an incremental penetration. The fixed window would be a little better than the egress window, but the door would be much worse than uninterrupted wall. Comparing the best door to the best window isn't the right comparison.
Response to Ethan T
Ethan,
In addition to Reid Baldwin's point, here are a few more:
1. Most people don't want an exterior door in their bedroom. They'd rather use the wall space for book shelves or a dresser. Moreover, rightly or wrongly, an exterior door in a bedroom makes people feel insecure and more vulnerable to burglars.
2. A second-floor door is dangerous.
William Shurcliff
Back in the 1970's William Shurcliff, one of the pioneers of both thermal solar and super-insulation, experimented with separating the glazing and ventilation functions by using fixed windows and installing small insulated openings in the walls nearby. I don't know how he dealt with egress. Perhaps it wasn't in the code at that time?
Thermal Shutters and Shades By William Shurcliff
Thanks everyone for helping me think through the insulated opening/window question. It has led me to Thermal Shutters and Shades by William Shurcliff, which I'll add to the reading list (http://www.builditsolar.com/Projects/Conservation/ThermalShades/intro.htm).
Response to Malcolm and Ethan
Malcolm and Ethan,
I have a soft spot in my heart for William Shurcliff, the author of several pioneering books on superinsulation. That said, his ruminations on movable insulation for windows sound dated.
He's right, of course, that covering windows with movable insulation every night, and removing the insulation every morning, saves energy. The problem is that no-one (except Ven Sonata and his Buddhist community members in British Columbia) wants to take on the twice-daily chores to ensure that the insulation is in the right position.
So his advice makes me smile with nostalgia rather than alter my plans for my next passive solar house.
William Shurcliff wasn't the only voice advocating small doors to admit ventilation air, and other doors to allow the ventilation air to leave the house. The idea was popularized by Rex Roberts in the early 1960s, in his widely read book, Your Engineered House. I read the book and built the house. The ventilation doors were fussy to build and hard to weatherstrip, and they leaked boatloads of air. I never opened them. But the doors, even when closed, ensured that the thermal envelope was leaky, winter as well as summer.
Martin
His enthusiasm was infectious. I secretly still want to load my attic with hundreds of carboys filled with water. Luckily I have a sensible wife.
Good operable windows simply don't represent enough of an efficiency drop from fixed units to warrant worrying about them.
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