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Choosing Windows: Looking Through the Options

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By Tristan Roberts and Alex Wilson

When we poke a hole in the wall and stick a window in it, we strike a high-stakes bargain. We want a visual connection to the outdoors that lets in daylight and that is itself pleasant to look at, both from the inside and the outside. We expect windows to provide fresh air and cooling breezes at times, but at other times we expect them to be completely airtight and provide good thermal insulation. Insects should be kept out; children and pets in. In heating climates, we want to get solar heat gain from windows, but not too much, and in all climates we don’t want glare.

By Tristan Roberts and Alex Wilson

Sorpetaler.jpg

Advanced triple-glazed, low-e Sorpetaler windows from Germany were used in this newly constructed Passive House in Palo Alto, California, designed by Arkin-Tilt Architects and built by Quantum Builders of Berkeley.

Photo: Bronwyn Barry, Assoc. AIA

When we poke a hole in the wall and stick a window in it, we strike a high-stakes bargain. We want a visual connection to the outdoors that lets in daylight and that is itself pleasant to look at, both from the inside and the outside. We expect windows to provide fresh air and cooling breezes at times, but at other times we expect them to be completely airtight and provide good thermal insulation. Insects should be kept out; children and pets in. In heating climates, we want to get solar heat gain from windows, but not too much, and in all climates we don’t want glare.

Along with these key functions, we need windows to be durable in every way: resistant to condensation, wind, driving rain and ice, as well as the occasional baseball from over the neighbor’s fence or hurricane-driven debris. Windows must operate easily and accommodate attachments like curtains, awnings, and other devices. We want windows that are quick to install, that integrate with the rest of the building envelope, and that won’t break the bank. Given that they are a big investment, they should last a long time—several decades at least. We want windows to not cause undue environmental harm during their life cycle, whether from material extraction, manufacture, disposal, or as a hazard to birds.

In this article we’ll try to untangle some of the many threads that go into choosing windows for our homes, offices, and other buildings. This article will be most relevant to light construction, but many of the concepts and technologies extend to commercial glazing and curtainwall systems. EBN will also cover window attachments and retrofit options later in 2011 as we learn from research we’re doing as part of a Lawrence Berkeley National Laboratory (LBNL) project.

The good news for anyone buying windows today and in the next ten years is the ongoing technical and market progress. Speaking about overall trends in the window market, John Carmody of the University of Minnesota told EBN, “Over the last 20 years or so, we’ve been through a generation of double-glazed low-e [low-emissivity] windows penetrating the market.” Now, said Carmody, the Energy Star program, codes, and other factors are moving us “toward the next generation of higher performance” with innovations in glazing, coatings, frames, and more.

The window frame material and appearance often get the most attention from occupants, so we’ll start there. Once that foundation is in place, we’ll look at glazing options.

Choosing a Frame Material

Window frame materials are chosen first for structural characteristics, but the material should ideally provide good thermal insulation. Durability, maintenance requirements, and cost also factor in to frame selection.

Vinyl: The market leader

Frame Materials in Residential Windows

FrameMat.gif

Source: Ducker Research for American Architectural Manufacturers Association and the Window

The share of the window frame and sash market owned by PVC, or vinyl, was only 5% in 1984 but now dominates with 66% (see Frame Materials table). In replacement windows, vinyl has an even bigger edge with 70% of the market. Most vinyl window frames are hollow extrusions but vinyl is also common as a cladding over wood windows.

Topping the list of vinyl’s advantages are its low cost and minimal maintenance requirements. Vinyl windows generally do not need painting, although some consumers complain about warping, cracking, fading, and yellowing that occur over time. Major downsides include their aesthetics, which many people consider inferior, and the environmental costs of PVC production. However, both in terms of structural qualities and energy performance, vinyl is comparable to wood. When the cavities in extruded vinyl are filled with polyurethane foam, energy performance exceeds that of wood.

Vinyl’s Achilles’ heel in practice is its very high coefficient of thermal expansion. Over time, expansion and contraction from temperature changes can loosen seals, and cause cracks at corners and on flanges.

Aluminum: Losing market share

Due to its structural strength in window frames, aluminum (along with steel) has been used in great quantity but is gradually losing market share to vinyl. Aluminum is also used as a cladding for wood windows. Aluminum can have high recycled content and is readily recyclable, but manufacturing of the raw materials is very energy-intensive.

Metal window frames are very thermally conductive, and so they impose a big energy penalty on whole-window performance and can cause condensation. In its 2007 report comparing vinyl to other common building materials on a life-cycle basis, the U.S. Green Building Council (USGBC) found that aluminum windows performed much worse than either vinyl or wood windows, due to poor energy efficiency coupled with its energy-intensive production. To avoid energy penalties, metal window frames must incorporate thermal breaks.

Wood: A natural favorite

Wood was the first material used in window frame and sash construction, and it remains popular for homes, although it continues to lose market share to vinyl. Wood is attractive, it is a natural and renewable product, it has a warm feel, and it is relatively energy-efficient. With proper care it should be durable, although paints or stains will need to be reapplied over the lifetime of the windows.

The need for strength, dimensional stability, and durability means that only top-quality, knot-free wood is used for windows. Fortunately, the premium price brought by top-quality millwork has helped make wood from certified forests a more common option—although not nearly common enough. Marvin, for example, began in 2010 to offer Forest Stewardship Council (FSC) certification as an option for all its wood products, following Loewen, which made the same move in 2007. EBN doesn’t know of any wood window manufacturers using certified wood as a standard, however.

Fiberglass

Although it commands only a fraction of the market, fiberglass has emerged as the fourth-most-popular frame material, and is often found in the highest-performing windows. Fiberglass is durable and strong. Its coefficient of thermal expansion is far lower than that of vinyl or aluminum and much closer to that of glass—which makes it ideal in window frames. According to Robert Clarke, marketing director at Serious Materials, a maker of fiberglass and vinyl windows, the pultrusion process used to make fiberglass frames is relatively slow, and machining the fiberglass requires very hard tools. These factors are likely to keep it at a price premium compared to vinyl.

Fiberglass appears to have an open-ended lifespan, making it more durable than vinyl or wood. According to Clarke, the core material in fiberglass is unstable under UV radiation from sunlight, but “its finishing will protect it forever.” On the down side environmentally, fiberglass is difficult to recycle into anything other than aggregate. Use of recycled material in the windows may not be possible, and air pollution emissions from fiberglass can be significant. The curing of fiberglass resin during production emits pollutants including styrene and volatile organic compounds (VOCs), and fiberglass plants are regulated by the U.S. Environmental Protection Agency (EPA).

Composites and other frame materials

Despite the broad market-share statistics quoted above, window frames aren’t so easily categorized into single materials. It is common for combinations of materials to be featured prominently in window frames and interiors, providing benefits representing the best of each material.

Comparing the materials

According to Carmody, existing life-cycle assessment data for window frames is old. If anything, he said, it points to wood as the best material, but he hopes a project currently starting at the University of Minnesota will lead to more up-to-date data. In the USGBC study mentioned earlier, which relied on older data, neither wood nor vinyl emerged as a clear winner, although major concerns were raised about health impacts from vinyl manufacturing. Vinyl is often chosen based on cost, but if choosing on performance and environmental impact, then wood is a better choice, and possibly fiberglass. One of the most exciting reasons to update our life-cycle data will be to evaluate the performance of fiberglass, which is particularly common in super-energy-efficient windows.

As mentioned, aluminum was a clear loser. Aluminum or steel frames should be considered only in climates with little heating or cooling, or when their strength is essential. Even then, thermal breaks should be specified.

Glazing Options

In most cases, energy performance will determine the environmental impact of windows over their lifetime, and with most current windows on the market, that will be determined by glazing choices. To understand what goes into high-performance insulated glazing units (IGUs) here’s an overview of key window performance technologies.

Double- and triple-glazing

windows_house.jpg

The Edelglass residence in Vermont uses Loewen aluminum-clad double- and triple-glazed windows. Window design and selection was optimized for budget, durability, good insulation value, and passive-solar heat gain on the southern exposure.

Photo: Kirstin Edelglass

With storm windows dating back 200 years and sealed double-glazing units dating to the 1930s, adding a second layer of glazing has long been the first step for window manufacturers toward improving energy performance. A second layer of glazing—or a third in the case of triple-glazed windows—improves window insulation by trapping dead air. For example, going from one layer of glass to two with a ¼" (6 mm) air space increases the center-of-glass insulating value from U-1.11 to U-0.57 (R-0.9 to R-1.75).

Double-glazing has become ubiquitous over the last couple of decades, and triple-glazing is now becoming more common in both residential and commercial windows. Triple-glazing is provided through a third layer—either a third pane of glass between the two outside panes or, less commonly, a plastic film suspended between the two panes of glass (Heat Mirror is the best-known film product).

Increased thickness of air space

A thicker air space between the panes of glass reduces conductivity of heat through the gas in the space. For example, doing nothing other than increasing the air space from ¼" (6 mm) to ½" (13 mm) increases the center-of-glass insulating value from U-0.57 to U-0.49 (R-1.75 to R-2.04). If the air space gets too wide, however, convection loops form, increasing convective heat loss through the window. With air or argon gas fill (see next section), the optimal thickness is ½" (13 mm); the ideal space for krypton gas is 3/8" (10 mm) and ¼" (6 mm) for xenon.

Low-conductivity gas fill

Because heat conduction across the air space in a sealed IGU contributes to heat loss, we can improve performance by replacing the air with a lower-conductivity gas. The most commonly used gas fill is 90% argon, which is plentiful, inexpensive, and inert. With low-e glass in an IGU with ½" (13 mm) spacing, argon boosts the center-of-glass insulating value from U-0.29 to U-0.23 (R-3.45 to R-4.35). More expensive gases like krypton perform even better and can be found in the highest-performing windows or where a thinner profile is desired.

Although safe, these inert “noble” gases are very buoyant and difficult to contain, and they will leak from an IGU over time. As a technical bulletin from Cardinal Glass states, “No organic seal ultimately can prevent the internal atmosphere of an IGU from becoming the same as the ambient atmosphere over time.” Since the “ambient atmosphere” we breathe contains just 1% argon and much more nitrogen and other gases, that argon or krypton will eventually escape. Accelerated-weathering tests used by major manufacturers require 80% argon to remain after testing; this suggests that after years of service, most of the argon will remain. Citing German research, Clarke told EBN that a loss of 1% of the gas per year is expected. While the benefits of gas fills may not be permanent, they are substantial and long-lasting, and the incremental price premium is easily justified.

Low-emissivity coatings

Installing Windows

windowDetail.gif

All the engineering, energy-intensive production, and precision that go into making new windows can easily go to waste with improper installation. The two biggest problems with window installation are water infiltration from poor detailing, and air leakage around the frame. This schematic shows key features of a good window installation—and it’s not uncommon for even good builders to miss some of them.

Schematic: Steve Baczek, Architect

First introduced in 1979, low-e glazings have grown tremendously in popularity. A thin transparent coating of silver or tin oxide on the glass surface or on a suspended plastic film with such a coating allows short-wavelength sunlight to pass through but blocks long-wavelength heat radiation. Low-e coatings are not one-size-fits-all: both the type and the placement of these coatings contribute to wide variations in performance metrics.

The most common type of low-e coating is called soft, or sputtered, coat. Thin layers of silver and anti-reflective coatings are applied to the glass surface through a vacuum deposition process. Because the coating is delicate, it must be protected within the IGU.

Pyrolytic or hard-coat low-e glazings have a thin layer of tin oxide incorporated into the surface of the glass during manufacture when the glass is still hot. Hard-coat low-e glazings are durable and can be used in single-glazed windows or storm panels, but their emissivity is not as low as that of soft-coat low-e glazings. Hard-coat glazings generally offer weaker insulating value compared with soft-coat glazings but have higher solar-heat-gain values.

Low-e technology has changed tremendously since single low-e coatings first became common in the 1980s. In the 1990s, a double (layered) low-e coating came along, dubbed “low-e2” or “low-e squared.” According to Clarke, the evolution was due to a market demand for cooler glass, with lower solar-heat-gain. Particularly given that demand, the market also shifted to favor soft coats. Adding standard soft-coat low-e2 glazing to an IG unit with a ½" air space increases the center-of-glass insulating value from U-0.49 to U-0.29 (R-2.04 to R-3.45).

The 2000s have seen “low-e3” (or “low-e cubed”) glazing take hold, with yet another layered low-e coating. Clarke told EBN that again a demand for reduced heat gain, particularly for cooling-dominated office buildings, has driven this shift, along with technical advances allowing coatings that cut out the low and high infrared light while leaving more of the visible light to pass through. Today, low-e, low-e2, and low-e3 coatings are all available, with single low-e coatings making a comeback for heating-dominated climates, and improved hard coatings also available for applications favoring solar heat gain.

Warm-edge glazing spacers

IGUs are sealed around the perimeter by spacers that maintain the distance between the panes of glass and help seal in any gas fills being used. Aluminum has been the most common material for glazing spacers, but it is very thermally conductive. Warm-edge spacers using rubber, foam, silicone, thermally broken steel, and other materials have become common in high-performance windows, and drastically reduce heat loss or gain at the edges of IGUs. Warm-edge spacers with integrated desiccant beads also reduce the risk of fogging within the IGU.

Airtightness

Fixed windows that don’t open are less expensive and, due to simpler construction, more durable than operable units. They are also more airtight, offering an important energy benefit. While operable windows provide ventilation and a better connection to the outdoors, it’s worth considering for each window location whether it should be fixed or operable.

Operable units use weather-stripping to ensure airtightness. As a general rule, hinged casement or awning windows that open out use a compression-type, synthetic weather-stripping gasket that offers a tighter, more durable seal than a seal used on a sliding-sash window. However, there are significant differences among manufacturers and products, so it always makes sense to examine product labels carefully.

Glazing dimensions and lites

Because high-performance glazings generally lose more heat at the edges, the larger the glazing-area-to-perimeter ratio the better the overall window energy performance. Divided lites, in which the sash is divided into multiple individual panes separated by muntins, offer a more traditional look, but they result in reduced glazing area and thus lower overall (unit) insulation value. (These windows also tend to cost more due to a more complicated assembly.)

Windows with larger glazing areas and no subdivided lites, or with simulated lites (through a combination of applied grilles, grilles inside the IGU, or both), offer better energy performance.

Adding it Up: Performance Metrics

NFRC.jpg

An NFRC label for a double-glazed south-facing window in a northern climate shows good SHGC for solar gain.

Photo: Kirstin Edelglass

Heat loss through windows, solar heat gain during heating periods, and avoidance of solar gain during cooling periods should all factor into window selection. To enable apples-to-apples comparisons among windows, industry leaders formed the National Fenestration Rating Council (NFRC) in 1989 to create standard metrics for comparing performance of windows.

U-factor

The NFRC procedure for U-factor accounts for the U-factors of the window frame, muntins, and glazing, each of which has different characteristics. It is useful to consider lower center-of-glass U-factors along with whole-window U-factors, but be careful not to confuse the two. U-factors are calculated using a prescribed computer program, and then those values are verified for a small number of products within a product line through actual testing by an accredited testing laboratory. See Choosing Windows table for guidance on what U-factors to aim for.

Solar heat gain coefficient

The solar heat gain coefficient (SHGC) is the ratio (between 0 and 1) of the solar heat gain coming through the window to the incident solar energy striking the window. The value includes both radiant solar gain and solar heat conducted through the window. SHGC replaces an older, confusing, “shading coefficient” metric that sometimes still appears in product literature. Recommended SHGC values for heating and cooling climates are listed in the table.

Visible transmittance

We install windows to let in light and enjoy the outdoors, so it’s important that we don’t lose sight of this in our search for highly insulating windows. Visible transmittance (VT) is a value from 0 to 1 that indicates the amount of visible light transmitted, taking into consideration light blocked by the frame and muntins. Most double- and triple-glazed windows have values between 0.30 and 0.70. Higher VT is desirable, but lower VT values are often delivered along with lower U-factors. According to Clarke, people start to notice a gray appearance to windows with a VT less than 0.40.

 

Special considerations in heating climates

Lower U-factors are desirable in both heating and cooling climates, but SHGC introduces complications in heating climates. In climates with significant heating seasons, passive solar heat gain from south-facing windows is desirable. (Not much solar heat gain is available from the north, and overheating is a risk with western windows in winter or summer. Some experts say that solar heat gain is also desirable from the east on winter mornings, but that needs to be balanced against risks of summer overheating.)

The need for passive-solar heat gain is met with higher SHGC values. Unfortunately, due to the different low-e coatings used, higher SHGC values generally also come with somewhat higher U-factors, meaning reduced insulation value. Experts EBN spoke with agree that due to this tradeoff, those higher SHGC/higher U-factor windows should only be sought for the south and possibly east, and other orientations should get the lowest U-factor possible. For locations where summer overheating is a danger, overhangs or other types of shading for high-angle sun should be installed on these passive-solar windows.

For buyers in the northern U.S., however, getting high-performance windows with high SHGC and low U-factor can be tricky. Standard window options from U.S. manufacturers are typically tuned to southern needs. Although Pella, Marvin, and other major U.S. companies are increasingly offering products sensitive to northern buyers, Canadian companies are generally ahead of the game.

Air leakage

Minute cracks in the window assembly can add up to significant heat loss and gain, so it’s worth looking for air leakage numbers (typically lower in fixed or casement windows—see above). NFRC labels sometimes provide an air leakage rating (AL), although it is not required, and manufacturers often omit it. AL is expressed as cubic feet of air passing through a square foot of window area (CFM/ft2). The lower the AL, the less air will pass through cracks in the window assembly. Select windows with an AL of 0.30 or less.

“Superwindows”—At a Cost

Choosing Windows: Decoding Window Labels

ChoosingWin.gif

Source: Compiled by EBN, with inspiration from Martin Holladay and input from Andy Shapiro.

An increasing number of window manufacturers are combining multiple layers of glazing, multiple low-e coatings, and very-low-conductivity gases such as krypton to create super-high-performance windows, or “superwindows,” a term coined by Dariush Arasteh, a staff scientist at Lawrence Berkeley National Laboratory. In the early 1990s, Arasteh predicted that advances in technology could make all windows, even north-facing windows in northern climates, net-energy-gainers. Whether or not that day has arrived is a matter of debate, but there’s no doubt that the advances since the 1990s, when window buyers were dreaming of U-0.05 (R-20) windows (and window makers were making tantalizing demonstrations), have been astounding.

Today, that race has cooled off to some extent in favor of climate-specific solutions. As Stephen Thwaites of Thermotech in Canada told EBN, “A window doesn’t have to be R-20 to be as energy-efficient as the wall around it,” due to the ability of a window to gain solar heat and provide ventilation. “A home with no windows will use more energy than a properly designed home with R-5 windows,” he said. That puts the emphasis on proper design by orientation, shading, and window-to-wall ratios, and on buying the best windows for each application according to the budget.

It’s still exciting to dream of what we’ll see in the next 30 years. For example, vacuum glazing, in which most of the air is evacuated from the space between panes, reduces thermal conduction and convection to nearly zero (leaving radiation as the primary means of heat transfer), and can currently offer U-0.08 (R-12) with double-glazing and one advanced low-e coating. Steve Selkowitz of LBNL cautions that vacuum glazing is still largely in research and development, however, telling EBN, “The key issue is the seal, which is a much tougher problem than a normal IGU.” Because the vacuum between the glazing tends to pull the panes of glass together, tiny glass or ceramic spacers are also needed in a grid pattern to hold the glass apart—a technical and aesthetic complication.

Other technical wonders are electrochromic glazing, a technology available now in some commercial and residential windows in which the glazing darkens at the push of a button (see “Tinting on Demand with SageGlass,” EBN June 2006) and photochromic glazing (darkening triggered by light) or thermochromic glazing (darkening triggered by temperature), both of which are under research. For windows where light but not views are wanted, glazing filled with the translucent material aerogel can offer good thermal performance—up to about R-8 per inch.

The biggest limit on energy performance is and may continue to be the wallet of the buyer. Windows imported from Germany meeting the Passivhaus standard, for example, offer U-factors under 0.14—at a cost of over $90/ft2 of window area. Triple-glazed Canadian windows typically cost $40–$50/ft2, in contrast with a price range for more conventional double-glazed windows of $30–$35.

Choosing your windows

Window Recommendations

WindowRec.gif

There are a lot of window options; making a choice should include environmental and energy considerations (see Choosing Windows table), but in the end can come down to subjective factors such as aesthetics or how well the sales rep sells the durability of the showroom model.

Whatever you choose, don’t be too cheap, said Selkowitz. “People make most of the decisions about a home based on preferences and amenities.” The same homeowner who doesn’t think twice about a granite countertop might balk at the cost of a higher-performing window, but Selkowitz said it’s an important investment that offers more “payback” than the countertop ever will.

To get the best performance for your money, you may also need to go your own way. In southern Vermont, Kirstin Edelglass invested countless hours in coming to a decision on windows and doors for her family’s home, looking at seven manufacturers before settling on a combination of three of them. In the process, she up-ended several applecarts, as she related to EBN.

Edelglass cut her costs by over 30%, largely by substituting fixed windows for operable (keeping enough operable glazing for natural ventilation). She used the company recommended by her homebuilder for only the basement windows after deciding at the showroom that those less-expensive models weren’t built as well. She rejected one company because it was sacrificing performance for her climate by putting the low-e coating on the wrong glazing surface, and with the company she used for the bulk of her windows, she insisted on a glazing different than usual for the company for higher SHGC values. She also asked her builder to read the installation guide provided by the manufacturer, which led to a fruitful conversation about best practices in window installation.

Continuing Education

Receive continuing education credit for reading this article. The American Institute of Architects (AIA) has approved this course for 1 HSW Learning Unit. The Green Building Certification Institute (GBCI) has approved the technical and instructional quality of this course for 1 GBCI CE hour towards the LEED Credential Maintenance Program.

Learning Objectives

Upon completing this course, participants will be able to:

    1. Choose a frame material based on its structural characteristics, thermal insulation, durability, maintenance requirements, cost, and effects on the environment.
    1. Describe at least three glazing options.
    1. Differentiate between R-value and U-factor and recognize how the U-factor relates to SHGC values.
    1. List the factors involved in "tuning by orientation."

To earn continuing education credit, make sure you are logged into your personal BuildingGreen account, then read this article and pass this quiz.

Discussion Questions

Use the following questions to inform class discussions or homework assignments.

    1. Which window frame materials are most environmentally sound, least environmentally sound, and why?
    1. Many factors influence window performance and durability, including specific performance metrics it is designed to meet, specific components and pieces of its deisgn, and how it is installed. Pick one specific factor of window performance and discuss why it is important.
    1. How should your climate influence your choice of glazing? Would you choose different windows for predominantly heating climates versus predominantly cooling climates? Why?
    1. You are working for a custom homebuilder, and are assigned to work with the homeowners on window design and selection. Their primary concerns are both cost-effectiveness and environmental friendliness. What would be your key recommendations to them?

 

 

 

Comments (8)

1 other ways to cut costs posted by Tristan Roberts on 01/28/2011 at 07:49 am

I heard some feedback from Kirstin Edelglass since I completed this article. She wanted to note other ways in which she saved money on the home discussed in the article:

"There were other significant ways I cut costs (besides substituting fixed panes for operable): 1.) eliminated three windows from the plans, 2.) reduced the size of several windows on the E,W, & N sides of the house, and 3.) changed the design so that four triple-glazed windows that originally required tempered glass would comply with the code w/o tempered glass 4.) bought doors and basement windows from companies with significantly cheaper prices but the same performance values.

"The tempered glass change made a huge difference on the triple glazed windows. (Over $1,000 reduction for each on the larger windows.)... folks can read the building code for tempered glass on this web page: http://www.thewindowstore.com/Installation.aspx?PageIndex=3.3.0.0.0.0.0.)"

2 Window add-ons may be most im posted by David Guenette on 02/01/2011 at 06:46 am

Thank you for an excellent article on window buying and designing/installing factors, but I was very glad to see, "EBN will also cover window attachments and retrofit options later in 2011 as we learn from research we’re doing as part of a Lawrence Berkeley National Laboratory (LBNL) project."

Although new window efficiency issues are crucial in new light construction or major house energy retrofits, there are millions of homes that have reasonably good windows or replacement windows already that can't justify the costs of replacements for the incremental benefits the very latest, most efficient windows could provide. Unfortunately, there is little solid information available on such things as insulating curtains--to name just one of the solutions to improving low-e windows in cold climates--and especially on the practical results and installation and use challenges present in realizing good results.

I'm looking forward to hearing more.

3 International Residential Cod posted by Doug Horgan on 02/01/2011 at 10:36 am

Great idea to minimize tempered glazing to save costs. The provided link is helpful (it appears to be written in English rather than "code"!) but not in complete alignment with recent code updates at least to residential codes. For direct access to the International Residential Code, try this link: http://publicecodes.citation.com/icod/irc/index.htm In the 2009 IRC, which is in use or in process of being adopted in much of the country, the requirements are in section 308.4: http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_3_sec008_pa... In older versions of the IRC, still in use in many jurisdictions, the requirements are different.

4 Double-pane glazing posted by William Edmisten on 02/02/2011 at 06:11 am

Pella has a glazing approach called "double-pane glazing". It consists of a single exterior glass lite, silicone glazed and a hinged, gasketed interior glass panel. The air space between the lites is 1-9/16 inch. They claim it has lifecycle cost advantages because of the expense of anticipated failure and replacement of insulating glass panels. Does anyone have experience with this glazing approach and is there any information about how it fits into the glazing options from a performance standpoint?

5 Pella glazing posted by Tristan Roberts on 02/02/2011 at 08:15 am

William, have you seen performance numbers on these double-paned Pella windows? If so, how do they compare with the targets discussed above in the article? I looked on their website and couldn't find any specific information. I have seen older versions of these windows in the field, with no low-e coating. I wonder if they are providing low-e coating today as an option—it would have to be a hard coat, but it could improve the performance. My educated guess is that these windows would be hard-pressed to perform better than R-2, which is a long way from the R-5 or better windows out there. With such a diminished insulation capacity, I would wonder about the validity of their life-cycle cost numbers. If anyone has more information, please post it!

6 Choosing on Cost alone posted by Charles Brown on 02/15/2011 at 05:28 am

It has been my experience that with windows in most cases you get what you pay for, almost. Vinyl is cheapest, but the thermal expansion problem should not be understated. I have personally seen south facing vinyl sashes with a visible gap between the sash frame and glazing. Its my understanding that the NFRC label is testing in lab conditions, so does not reflect the impact of exposure to sun, etc. See air leakage...

To gain owner buy-in over the perceived best solution for operable commercial windows in our region (thermally broken aluminum), I have often bid clad wood, and fiberglass as alternates. In every case the clad wood and fiberglass alternates came in at equal or lower initial costs.

7 Tilt Turns etc. posted by Jamie Wolf on 03/22/2012 at 05:12 am

There are a couple things missed in the article that I would like to encourage BG to continue to follow and report on. They stem from the impact that Passive House has had on the growing interest and market for the inswing windows that predominate in Europe. Here are the things I find myself required to learn to think differently about as I design as a result of this wholly different approach to glazing:

Where do we place the window in the wall? What is it like to live with an inswing window both functionally and in terms of ventilation? These frames and hardware support much large expanses of glazing and consequently their performance, cost, and architectural integration require fundamentally different design thinking. Their "vinyl" is not PVC but u-PVC, something I rely on BG to place in context.

As a designer I like the challenge of this wholly different concept of a window and am looking forward to the many ways designers will integrate these products into their projects with sensitivity to our existing vernacular. There are some good examples already, with more sure to come.

What I think we all really want is for the domestic market to produce a North American "superwindow" that transcends the legacy of what are becoming outdated building practices and integrates more readily with the demands of installations in high performance walls (that's a whole other topic!).

8 uPVC and PVC are the same thi posted by Nadav Malin on 10/14/2013 at 03:33 pm

Responding to one part of Jamie's comment, below, there is no difference between PVC and uPVC in window frames. The "u" just stands for "unplasticized," and no window frames are made with PVC that contains plascizers. Those are just for flexible PVC applications (vinyl flooring, wallcoverings, shower curtains, etc.).

I whole-heartedly agree with all the rest of that message!

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January 27, 2011