Introduce yourself, who you represent, and some of your experience that qualifies you to present this material. State the three Learning Objectives for this presentation. See slide. Note that this educational program is designed to be an interactive program and as such we want to encourage audience participation. We will pose a series of questions throughout this presentation to encourage interaction. Suggestion: Use a flip chart to record architect responses to questions. Write one question on each sheet and record answers below. We also encourage you to use glass samples and a beam splitter as you discuss the third topic in this presentation, “Glass Thermal Performance”. Many sales branches already own a beam splitter. Pella has sold two different types in the past. The first was manufactured by Southwall Technologies and had a yellow-and orange housing with a light bulb on the side. The second was smaller and had a housing that was entirely black. Note that these two beam splitters may show different results. There are two versions of this presentation. The first is for those who choose not to use a beam splitter. All thermal performance values, including those for Visible Light Transmission and Solar Heat Gain Coefficient, are based on WINDOW 4.1 computer simulations. The second is for those who use a beam splitter. All thermal performance values, except those for Visible Light Transmission (VLT) and Solar Heat Gain Coefficient (SHGC), are based on WINDOW 4.1 computer simulations. VLT and SHGC are based on results from the black beam splitter using the specified glass samples. By the way, the building in this photo and all the buildings you will see in this presentation feature Pella Windows. P.S. We intentionally chose buildings of large size, interesting design, and by well-known architects to demonstrate Pella’s acceptance in the commercial market. FYI. Throughout these speaker notes you will see the letters FYI . These notes are background information for you. You can decide how much of this information you want to share with your audience. You don’t have to say everything in these speaker notes, but you do need to cover the information in the slides.
Here is a broad outline of the topics we will cover during this presentation. We will touch briefly on the first two topics, spend most of our time on topics three and four, and conclude with some brief recommendations on glass specifications. Question: Are there any other topics of interest that you would like us to address today? Suggestion: record their answers on a flip chart…one sheet per question.
When surveys are conducted about top interests in glass, these are the topics of greatest interest. Do these match the responses from your audience? We will touch on all of these today, except safety. Question: When you look back over the past 10 years, what have been the major architectural trends in glass? Let’s see if your perception matches what we have seen. Record answers on flip chart.
Here are some of the trends we have seen nationally. Higher % of glass in conjunction with energy conscious design and daylighting. High visible light transmission and clear appearance, there has been a move away from tinted and reflective glass. Increase in use of low-E glass has made all of these trends possible. When you look at the total energy use in and out of windows, low-E coatings can allow windows to perform better than R19 walls. In the winter, they prevent heat from escaping at night. In the summer, they minimize solar gain and thus reduce the need for air conditioning. In all seasons, they let in lots of daylight and reduce the need for artificial lighting. Quote from Steve Selkowitz, head of the building technologies department at Lawrence Berkeley National Laboratory, in the source noted below: “A properly glazed façade with these new coatings (I.e. low emissivity) can have a lower annual heating and cooling load than one with an R-19 insulated opaque wall. In cold climates, that’s true even when glass is placed on the north side. During the winter coated glass can gather more thermal energy than it loses over a 24-hour period.” Source: Architectural Record, “Improving Glass Performance”, 8/98
Because much of our discussion will involve basic thermal performance concepts, we want to briefly review some key definitions. You will probably remember some of this from your architectural technology course in college. However, there have been some major changes in definitions in recent years, so a quick review makes sense just to make sure we’re all thinking alike. All heat transfer occurs through conduction, convection, and radiation. Read the definitions on the slide. Source: 1997 ASHRAE Fundamentals Handbook, p. 29.2
All heat flow is measured in terms of U-value, SHGC or SC, and air infiltration. Because we’re talking about glass, we will spend all of our time on the first two and don’t plan to discuss air infiltration. Other topics we will discuss in some detail include visible light, occupant comfort, and fading. Let’s move on to some definitions. Source: 1997 ASHRAE Fundamentals Handbook, Chapter 29
We will start out with U-value and Emissivity. U-value is a measure of the rate of non-solar heat loss or gain through a material or assembly. A lower number is better (I.e. a perfect insulator would have a U-value of 0.00). Winter U-value is measured per the design conditions noted on the slide. The U-value is affected by a combination of conduction, convection, and radiation. The large yellow arrow represents conduction. The up-and-down blue arrows represent convection. And the small red arrows represent radiation. If you reduce any of these, you will reduce the U-value. When heat or light energy is absorbed by glass, it is either convected away by moving air or re-radiated by the glass surface. This ability of a material to radiate energy is called its emissivity. Again, a smaller number is better. Long-wave radiation is the heat energy given off by objects with relatively low temperatures when compared to the sun (e.g. humans, equipment, light bulbs). Reducing the window’s emission of heat can greatly improve its insulating properties. Clear glass, which has an emissivity of 0.84, absorbs 84% of the long-wave radiation striking the surface of the glass and reflects the remaining 16%. The thermal environment around us always seeks equilibrium. In the winter when it is cold outside, our bodies radiate heat toward colder surfaces like windows and we feel cold when the surface of the glass is cold. Glass with an emissivity of 0.04 reflects 96% of our long-wave radiation back into the room and thus reduces the need for supplementary heating. Glass with a low emissivity will also have a lower U-value. In the summer, we also see a benefit from low emissivity glass. Objects outside the building such as parking lots and neighboring buildings absorb heat from the sun and re-radiate it in the form of long-wave radiation. Glass with an emissivity of 0.04 will keep 96% of this long-wave radiation out of the building and thus reduce the need for supplementary cooling. Briefly mention the other two points on the slide. Source: Residential Windows: A Guide to New Technologies and Energy Performance, 1996
The total window U-value is made up of three components (see black-and-white diagram): center of glass, edge of glass, and the window sash and frame. To get an accurate view of a window’s insulating ability, you must consider all three. For example, consider the thermal diagram for a Pella aluminum-clad wood casement with low-E glass (Pella’s Insulshield IG). When it is -15° F outside and +75° F inside, you can see dramatic differences in the colors in the glass, sash, and frame as you move from the center of the glass to the frame. Reds are warm. Purples and blues are cold. The temperature at the center of glass is 57.4° F. The temperature at the edge of the insulating glass is 24.4° F due to the cold aluminum spacer at its edge. The temperature in the sash and frame ranges from 51.5° F at the edge of the sash to 75° F at the frame. All of these components have an impact on the total window U-value. If the frame and sash were made out of aluminum, the frame would be much colder and would increase the total window U-value. Source: FRAME 4.0 computer program for analyzing window energy performance
Solar energy that reaches the earth is composed of 3% ultraviolet light (purple in the diagram), 44% visible light (white), and 53% infrared light (red). One of three things happens to all of these types of solar energy (see diagram at the right). It is either transmitted or reflected or absorbed and re-radiated. Today we will discuss how these three types of solar energy are affected by different kinds of glass. Source: Residential Windows: A Guide to New Technologies and Energy Performance, 1996
As solar energy passes through windows into buildings, it is the infrared component that causes buildings to heat up. The Solar Heat Gain Coefficient (SHGC) is a measure of that fraction of incident solar radiation that actually enters a building through the window as heat gain. For example, a window with a SHGC of 0.41 means that 41% of solar radiation enters the building as heat gain. For cooling, a smaller SHGC is better. For passive solar heating, larger is better. It can be expressed in terms of center-glass or total unit. SHGC = SC x 0.87 (approximate). FYI. Shading coefficient is the number that architects and mechanical engineers have used in the past to calculate heating and cooling loads for buildings. The shading coefficient (SC) is being replaced by the SHGC but it may still take a few years for design professionals to fully make the transition. The Shading Coefficient is the ratio of solar heat gain through the system relative to that through a single sheet of 1/8” clear glass. The SHGC is a more meaningful number because it relates directly to the amount of heat gain passing through the glass. The National Fenestration Rating Council (NFRC) uses the SHGC, not the Shading Coefficient. Currently the NFRC requires that the label on all certified windows show total window U-value. Total window SHGC and Visible Light Transmission will be required starting in January of 2001. Source: 1997 ASHRAE Fundamentals Handbook, pages 29.22 and 29.23
So why are we talking about glass today?…because selection of the correct glass can have a major impact on the total energy consumed by a building. For example, an energy consultant in Minneapolis, the Weidt group, has worked with Northern States Power (NSP) and many different architects on more than 75 projects since 1994. Because energy analysis was included in the early phases of design, starting in early schematic design, on average these buildings use 32% less energy than a code compliant building. What is significant for our discussion on glass is that daylighting and specification of the correct glass resulted in 10 to 15% of the savings (I.e. 15% of the 32% was the result of proper glass selection, the other 17% was due to other measures not related to the glass). FYI. The energy consultant fees were paid by NSP because NSP knows that energy efficient design lowers peak cooling loads and thus reduces the need for them to increase their energy capacity. Source: Northern States Power, 1999
In the section on heat transfer we discussed the importance of using total window values for U-value and SHGC. In this section on Glass Thermal Performance we’re going to digress for a few minutes and talk only about the performance of the glass (center-glass numbers) as it relates to the three components of the solar energy spectrum (UV, visible, and infrared). We will look at center-glass performance as it relates to two distinct glazing systems: sealed insulating glass and dual glazing. Sealed insulating glass relies on sealant to keep moisture out of the air space. Dual glazing relies on a gasket to keep indoor moisture out of the air space. It is also important that the air space of every dual glazing system be vented to the outside (e.g. like Pella’s breather system) to minimize potential condensation between the two pieces of glass during the winter when it is cold outside. Any indoor moisture that gets in between the panes of glass can then exchange with the dry outside winter air. FYI. AAMA /NWWDA 101/I.S.2-97 defines dual glazing in the following terms: 3.7.1 on page 37. “When a dual glazed window with a venetian blind between the glass is provided, the operating sash shall consist of the main sash and an access sash providing an air space in which the venetian blind is mounted.” 3.7.2 on page 37. “If a venetian blind is included in an enclosed air space, the air space should be vented.”
Ultra-violet light is invisible to the human eye, but its effects are readily seen in terms of fabric fading, plastic deterioration, and skin cancer. FYI. Please note that ultra-violet light is not the only component of solar energy that causes fading. Visible and near infrared light also contribute to fading. Blocking all or most of the ultra-violet light will not stop fading. Visible light is the only portion of the solar spectrum visible to the human eye. Without it, we can’t see. Capturing visible light is one of the main reasons we put windows in buildings. Infrared light is invisible to the human eye and has a penetrating heat effect. Near infrared (I.e. short-wave radiation) converts to heat when it is absorbed by an object. Objects and people re-radiate this energy to the environment in the form of Far Infrared (I.e. long-wave radiation). As we discussed earlier, Near Infrared transmission is measured in terms of the SHGC. Far Infrared transmission is measured in terms of emissivity. Windows with lower emissivities have lower U-values. Question: If you could specify/describe the ideal/perfect glass, how would it affect the transmission of these four types of energy (UV, visible, Near Infrared, and Far Infrared)? Source: Residential Windows: A Guide to New Technologies and Energy Performance, 1996
Let’s assume that the ideal glass would act like a wall, but we could see through it. We would block out all of the Near IR to prevent solar heat gains, block all Far IR transmission to keep heat in, and we would block UV to block additional fading rays. But we would let in all of the visible light because that’s why we put windows in buildings. Of course, for days when the sun is bright, we need to control how the visible light enters the building so we can minimize glare. We can do this through the use of daylighting principles. Did your group of architects come up with a similar scenario? You can see that by constructing a chart like this, we can clearly see what performance characteristics we should look for as we search for the ideal glass. FYI. Please note that there is a nationally-known dayligting consultant, Steve Ternoey with LightForms LLC, who is promoting the use of dark gray glass with very low visible light transmission. He argues that the sun provides so much visible light that we don’t need very much of it. Another goal of his strategy is to minimize eye discomfort as you look outside. He promotes the use of Viracon’s gray low-E glass: VE 3-85 (38% visible light transmission) VE 3-55 (23% visible light transmission) VE 3-40 (18% visible light transmission) The Weidt Group, mentioned previously, disagrees with this strategy and promotes the use of glass with visible light transmission of 50% to 70%. FYI. If we want passive solar heating, we would let in some Near IR, depending on how much heating would be beneficial. However, for the typical commercial application, the use of passive solar heating is not very common so we’ll assume that we want to minimize near infrared transmission as much as possible. Now we’re going to look at a series of glass samples and see how each of these samples affects these four types of solar radiation. If you are using the beam splitter, explain how it can be used to measure two of the four types (visible light and infrared light). Values for ultra-violet and far infrared are based on computer simulations using WINDOW 4.1. If you are not using a beam splitter, all four values are based on WINDOW 4.1. Encourage the architects to go to the chart in their handout and record the values for the ideal glass and then the corresponding values for the subsequent glass samples. This will allow them to compare as we go.
We will start with clear insulating glass. Refer to the product section in the slide. Show sample. Both pieces of glass are clear and the space in between is filled with air. FYI. Both lights are 3 mm as is the case in most Pella windows with clear glass or low-E glass. For the tinted and reflective glass options we will also use the glass thickness that is typical for Pella windows. Note that these are different from those for clear and low-E glass. Clear glass is still used in some historic buildings like this one, the Reliance Building in downtown Chicago. Often, clear glass is used for one of two reasons: reduce initial cost maintain historic authenticity How does it affect the four types of energy we defined previously (see text on slide)? All types are primarily transmitted. Some reflectance and absorption occur, but for the most part clear glass transmits these types of energy. Clear glass has an emissivity of 0.84, which means that only 16% of far infrared radiation is reflected. So how does clear glass compare to the ideal glass? See next slide.
Not very favorably…good in visible light but poor in the others. Note that glass performance numbers are based on the WINDOW 4.1 computer program for analyzing energy performance, just like the numbers in the Pella Architectural Design Manual. Ultra-violet is the % of UV light passing through the glass ( in this case, 56%) Visible is the % of visible light passing through the glass (in this case, 81%) Near Infrared is based on the SHGC of the glass (in this case, 0.76) Far Infrared is based on the Emissivity of the glass (in this case, 0.84) FYI. Note that SHGC and Emissivity are typically expressed as decimals rather than percentages. But for the purposes of this comparison we will show them as percentages.
Next sample is tinted gray insulating glass. Refer to product section. Show sample. The outside piece of glass is tinted gray. The inside piece is clear. And the space in between is filled with air. FYI. Exterior glass is 5 mm. Interior glass is 4 mm. Tinted glass was invented for the purpose of reducing solar gain in commercial buildings like this one, the Rio Casino and Hotel in Las Vegas, NV. How does it affect the four types of energy we defined previously (see text on slide)? All types are primarily absorbed. Some reflectance and transmission occur, but for the most part tinted glass absorbs these types of energy. Tinted glass can become hot to the touch when it is hot outside. Like clear glass, tinted gray glass has an emissivity of 0.84, which means that only 16% of far infrared radiation is reflected. So how does it compare to the ideal glass? See next slide.
Not very favorably…better in ultra-violet and near infrared, but at the expense of visible light. Note that glass performance numbers are based on the WINDOW 4.1 computer program for analyzing energy performance, just like the numbers in the Pella Architectural Design Manual. Ultra-violet is the % of UV light passing through the glass Visible is the % of visible light passing through the glass Near Infrared is the SHGC of the glass (in this case, 0.50) Far Infrared is the Emissivity of the glass (in this case, 0.84)
Next sample is reflective gray insulating glass. Refer to product section. Show sample. The outside piece of glass is clear with a mirror-like gray reflective coating on the #2 surface (inside surface of outside piece of glass). The inside piece is clear. And the space in between is filled with air. FYI. Exterior glass is 5 mm. Interior glass is 4 mm. Reflective glass was invented for the purpose of reducing solar gain even more for buildings where the exterior skin was composed almost entirely of glass similar to this building, Purvine Hall at the Oregon Institute of Technology. How does reflective glass affect the four types of energy we defined previously (see text on slide)? All types, except far infrared, are primarily reflected. Some absorption and transmission occur, but for the most part reflective glass reflects these types of energy. Reflective glass, like tinted glass, can become hot to the touch when it is hot outside. Like clear glass, reflective gray glass has an emissivity of 0.84, which means that only 16% of far infrared radiation is reflected. So how does it compare to the ideal glass? See next slide.
Good in ultra-violet and near infrared, but at the expense of visible light. Note that glass performance numbers are based on the WINDOW 4.1 computer program for analyzing energy performance, just like the numbers in the Pella Architectural Design Manual. Ultra-violet is the % of UV light passing through the glass Visible is the % of visible light passing through the glass Near Infrared is the SHGC of the glass (in this case, 0.34) Far Infrared is the Emissivity of the glass (in this case, 0.84)
Next sample is clear low emissivity insulating glass. Refer to product section. Show sample. The outside piece of glass is clear with a spectrally-selective low-E coating sputtered on the #2 surface (inside surface of outside piece of glass, shown in yellow in the product section). The inside piece is clear. And the space in between is filled with argon gas. FYI. Both pieces of glass are 3 mm. The sprayed on coating must be protected in a sealed piece of insulating glass to protect it from: moisture, the low-E coating is made of microscopically thin layers of silver, and will corrode if not protected from moisture. scratches Argon gas is more dense than air, and thus reduces the conduction of heat through glass. The result is a lower U-value. Clear Low Emissivity glass (Pella’s Clear Insulshield) was used on this project, the Greensboro Public Library in Greensboro, NC. How does low-E glass affect the four types of energy we defined previously (see text on slide)? The low-E coating makes the glass spectrally selective. It transmits visible light while reflecting the other types of energy. Due to the low-E coating, low emissivity glass has an emissivity of 0.04, which means that 96% of far infrared radiation is reflected. So how does it compare to the ideal glass? See next slide.
Good in ultra-violet, visible, and far infrared, and almost as good as reflective gray glass (0.41 vs. 0.34) in rejecting near infrared. And it has a clear appearance. This option comes closer to ideal glass than anything else available today. Note that glass performance numbers are based on the WINDOW 4.1 computer program for analyzing energy performance, just like the numbers in the Pella Architectural Design Manual. Ultra-violet is the % of UV light passing through the glass Visible is the % of visible light passing through the glass Near Infrared is the SHGC of the glass (in this case, 0.41) Far Infrared is the Emissivity of the glass (in this case, 0.04) FYI. What happens if you tint the outside piece of glass gray (5 mm) and move the low-E coating to surface #3 (similar to Pella’s Gray InsulShield)? Ultra-violet drops to 7%. Visible drops to 40%. Near Infrared (SHGC) drops to 0.33. Far Infrared remains the same. (emissivity = 0.04) There is a dramatic reduction in visible light while getting just a small improvement in reduction of near infrared.
Now let’s look at an example of dual glazing that closely resembles the sample of sealed insulating glass that we just looked at. Refer to product section. Show samples. The outside piece of glass is clear (3 mm). The inside piece (3 mm) is pyrolytic low-E. A typical pyrolytic coating is a metallic oxide, most commonly tin oxide with some additives, which is deposited directly onto a glass surface while it is still hot. The result is a baked-on surface layer that is quite hard and thus very durable. The space in between is filled with air. It can’t be filled with argon gas since dual glazed systems are designed to breathe to the exterior. Since the interior piece of glass in a dual glazed system is often subject to handling, the low-E coating must be pyrolytic so it isn’t damaged or scratched during handling. Dual glazing works well in a project like the one shown here, Lincoln Elementary school in Green Bay, WI, where blinds are placed between the glass to protect them from dust and damage. How does this glass affect the four types of energy we defined previously (see text on slide)? The low-E coating makes the glass spectrally selective. It reflects far infrared light while transmitting the other types of energy. Due to the low-E component, pyrolytic low-E glass has an emissivity of 0.15, which means that 85% of far infrared radiation is reflected. So how does it compare to the ideal glass? See next slide.
Good in visible and far infrared, but not as good as the sputtered low-E in ultra-violet and near infrared. It has a clear appearance. This type of glazing works well in a passive solar building because it transmits the near infrared component that heats the building and then keeps it inside at night by reflecting the far infrared component back into the building. Note that glass performance numbers are based on the WINDOW 4.1 computer program for analyzing energy performance, just like the numbers in the Pella Architectural Design Manual. Ultra-violet is the % of UV light passing through the glass Visible is the % of visible light passing through the glass Near Infrared is the SHGC of the glass (in this case, 0.71) Far Infrared is the Emissivity of the glass (in this case, 0.15)
Now let’s consider triple glazing. What happens when we combine a piece of clear low emissivity insulating glass with a pyrolytic low-E triple glazing panel? Refer to product section. Show samples. In the outer piece of insulting glass, the outside piece of glass is clear with a spectrally-selective low-E coating sputtered on the #2 surface (inside surface of outside piece of glass, shown in yellow in the product section). The inside piece is clear. And the space in between is filled with argon gas. The third piece of glass is pyrolytic low-E. And the space in between the insulating glass and triple glazing panel is filled with air. FYI. All three pieces of glass are 3 mm. We often see this type of glazing in hospital projects like this one, Columbus Children’s Hospital in Columbus, OH. Triple glazing makes it possible to minimize room-side condensation in buildings like this where high inside relative humidity levels are needed to maintain patient comfort. Another building type where we often see triple glazing for similar reasons is in Assisted Living facilities. How does low-E glass affect the four types of energy we defined previously (see text on slide)? The low-E components make the glass spectrally selective. It transmits visible light while reflecting the other types of energy. Due to the low-E coating, this glazing system has an emissivity of 0.04, which means that 96% of far infrared radiation is reflected. So how does it compare to the ideal glass? See next slide.
Good in ultra-violet, visible, and far infrared, and nearly as good as reflective gray glass (0.36 vs. 0.34) in rejecting near infrared. And it has a clear appearance. This option also matches up nicely with the ideal glass. Note that glass performance numbers are based on the WINDOW 4.1 computer program for analyzing energy performance, just like the numbers in the Pella Architectural Design Manual. Ultra-violet is the % of UV light passing through the glass Visible is the % of visible light passing through the glass Near Infrared is the SHGC of the glass (in this case, 0.36) Far Infrared is the Emissivity of the glass (in this case, 0.04)
The chart in the architect’s handout should now match this chart. Conclusion: Clear LoE 2 IG (similar to Pella’s InsulShield) most closely resembles the ideal glass. In a few minutes we will look at the impact of between-glass blinds on the thermal performance of the dual glazed system (Clear/LoE DG).
Now let’s take a look at center-glass U-values for each of these glass options. Of the double-glazed options, Clear LoE2 IG also has the best U-value due to the sputtered low-E coating and the argon gas fill. The low-E coating contributes the most to reducing the U-value. The argon gas doesn’t contribute nearly as much…it reduces the U-value by approximately 0.04. U-values are based on the WINDOW 4.1 computer program for analyzing energy performance, just like the numbers in the Pella Architectural Design Manual.
How does cost compare? The best performing glass, Clear LoE2 IG, is also one of the least expensive. Costs are based on the actual price Pella Corporation pays to Cardinal IG for glass. However, we have adjusted the cost of Clear IG to $1.00 per square foot so it is easier to see the % add for the other types of glass. Based on this assumption, Reflective Gray IG costs 2.77 times more than Clear IG. Clear LoE2 IG costs 1.35 times more than Clear IG. FYI. These are glass prices , not window prices. We are trying to show that gray tint and reflective gray are more expensive than low-E. Why would an architect specify tinted or reflective glass when it costs more and does not perform as well? If you use PDQ to compare window prices , (e.g. an Architect Series Classic casement with Insulshield IG to the same window with clear IG), you will see about a 9% difference in the window price. FYI. We suspect that this is similar to what an architect might discover if he asked a commercial glass supplier like Viracon what the relative difference in cost is between these types of glass.
Now let’s return to our discussion about Total Window Performance as it relates to the two glazing systems mentioned previously: sealed insulating glass and dual glazing. For these two glazing systems, we are going to compare the items listed on this slide. Mention each. Remember our definitions from earlier in the presentation? Sealed insulating glass relies on sealant to keep moisture out of the air space. Dual glazing relies on a gasket to keep indoor moisture out of the air space. Question: Has anyone had previous experience with dual glazing? If so, on what kind of projects? Why was dual glazing specified? These are two distinct glazing systems and each has its place in commercial buildings as illustrated by the next two slides.
Here is a speculative office building in the Chicago area with sealed insulating glass. Note the aluminum curtain wall in the center with punched openings (Pella Clad Frames) on the sides.
Here is a corporate office building in Ohio with dual glazing at the punched openings and sealed insulating glass at the aluminum curtain wall…two buildings with very similar uses and design solutions…and yet different glazing systems. Why? We hope to answer this question as we compare the two glazing systems. Any initial thoughts from the audience about why?
Both aluminum and wood windows offer the glazing systems shown in the slide. Dual glazing is common among most aluminum window manufacturers. It is not as common among wood window manufacturers. FYI. As far as I know, Pella is the only wood window manufacturer that offers dual glazing.
There is a lot of discussion in the window industry today about warm edge technologies and what technology offers the warmest edge of glass. With the advent of low-E glass, center glass temperatures have become so warm that cooler edge conditions now have a much more dramatic impact on the total window U-value. Thus, the search for the warmest IG edge continues. Question: which of these do you think has the warmest edge and why? We will look at the five edge conditions shown on this slide. The three on the left represent the most common edge technologies in use today in the manufacture of sealed insulating glass (aluminum, stainless steel, and silicone foam). In the two on the right, the glass is separated by a piece of wood.
The two sets of bars on the left are for clear glass. The three sets of bars in the middle are for low-E argon glass with different edge conditions. The first has an aluminum spacer. The second has a stainless steel spacer. The third has a foam spacer similar to the type of foam spacer used in between the muntins on Architect Series products. Note that the foam provides a slightly better edge temperature but won’t last nearly as long as a metal spacer. Cardinal and Pella will not offer a 20 year warranty against seal failure on insulating glass with a foam edge spacer. SSII is for a clear prime with low-E DGP. SSIII is for Insulshield IG and a low-E DGP. If you’re customer is looking for warm-edge technology, there is nothing better than SSII and SSIII. That big piece of wood between the panes of glass makes a big difference on the temperature at the edge of the glass (compare 50°F to 39 °F). In terms of Condensation Resistance Factor (CRF), the ultimate window is one where: the inside glass temperature is warm when it’s cold outside and the edge of glass and center of glass have the same temperature. Condensation will always occur at the point of the lowest temperature. The higher the glass temperature, the more resistance to condensation. Smart Sash II and III offer the best solution for resistance to condensation. Source: FRAME 4.0 computer program.
Source: Cardinal IG, Glass Products for Windows and Doors, 1998, p.11. Question: How long should a building last? How long should a glazing system last? The chart on this slide shows the results of subjecting samples of sealed insulating glass to the rigors of an ASTM test called the P-1 Hot Box. It is an accelerated aging test. The goal of the test is to cause failure of the seal between the two lights of glass. In the test, samples of sealed insulating glass are subjected to 100% relative humidity and temperatures of 140° F until the seal fails. Look at the drawing in the lower left-hand corner. The red is the primary seal (typically polyisobutylene which is highly impervious to moisture but lacks the structural characteristics needed to hold the unit together). The blue is the secondary seal (typically polysulfide or silicone which provides the structural characteristics needed to hold the unit together). Manufacturers who supply single seal insulating glass with a seal similar to the 4 yellow bars on the left only survive 2 to 8 weeks in this accelerated aging environment. Single seal insulating glass typically has a 5 year warranty against seal failure. Unfortunately, there are some manufacturers who still use this technology today. Manufacturers who supply double seal insulating glass with a dual seal similar to the 4 yellow bars on the right survive 15 to 48 weeks in this accelerated aging environment. Dual seal insulating glass typically has a 10 year warranty against seal failure, especially among aluminum window suppliers. Pella and Andersen offer 20 years. Even the best sealed insulating glass will fail some day. If your owner is looking for a glazing system that will last as long as the building and/or window, then you should consider dual glazing. Replacing a gasket will be a lot less expensive than replacing an entire piece of sealed insulating glass. The largest life cycle cost on any window system will occur when the sealed insulating glass starts to fail. Eventually every piece of glass will need to be replaced because it cannot be repaired. The cost of replacement will probably exceed the initial cost of the windows.
The incidence of glass breakage in most buildings is extremely low (less than 1%), but in buildings where it is an issue like the student dormitories at the University of Wisconsin, you may want to consider dual glazing to reduce life cycle costs. The University of Wisconsin has two full-time employees dedicated to replacing broken glass. That’s all they do. They started using dual glazed windows in 1981 as they began replacing windows in the dormitories. Since then, they have replaced more than 3200 windows in 12 different dormitories, all with Pella double-hungs. They chose dual glazing for several reasons: In-house maintenance staff can replace broken glass. Since the system consists of two separate pieces of glass, if one piece gets broken, the owner’s maintenance crew can cut a new piece in their shop and install it as soon as they can. They don’t have to wait for someone else to manufacture a new piece of sealed insulating glass for them. Immediate protection from the elements. If both pieces of glass in the bottom sash get broken, the maintenance crew can grab the double-glazing panel from the top sash and cover the hole until they have time to replace the broken glass…no need for plastic or cardboard to cover up the hole. As we saw in the previous slide, it is a glazing system that lasts as long as the window. It gave them the ability to put the muntins between the panes of glass where the students couldn’t damage them.
Dual glazing allows for between-glass options like tilt-only blinds, muntins, pleated shades, and raise-and-lower blinds. These are located between the glass where they are safe from dust and damage which means lower life-cycle costs: less cleaning less replacement since they aren’t easily damaged They also provide a lower SHGC than room-side blinds because they stop the infrared energy before it gets through the inner pane of glass. Per the AAMA Window Selection Guide (page 40), “the enclosed blind is much more efficient than the room-side blind since only about 40% of the heat absorbed by the enclosed blind is transmitted into the room”. FYI. It’s always best to stop infrared energy before it ever gets to the outer piece of glass, by trees or some type of exterior shading device. Second best is between the panes of glass for the reason mentioned above. Third and worst is room-side because the energy gets inside the building before it has any chance of being blocked. Between-glass blinds also improve the overall U-value of the window because they reduce convective loops within the air space. Source: 1997 ASHRAE Fundamentals Handbook, Chapter 29
Here is a quick recap of the thermal performance numbers that we looked at earlier. Now let’s compare the glass types highlighted in blue (sealed insulating glass similar to Pella’s Insulshield) and red (dual glazing similar to Pella’s SmartSash II with low-E DGP). They compare somewhat favorably in terms of visible light transmission, far infrared transmission, and center-glass U-value. But the sputtered low-E coating in sealed insulating glass performs much better in ultra-violet and near infrared transmission.
But when you look at Total Window U-value (0.37 vs. 0.40), the numbers are nearly equal. You can see the effect of the edge condition on total window performance. That big piece of wood between the two pieces of glass makes quite a difference. Source: WINDOW 4.1 computer program
Now let’s consider the effect of placing blinds between the glass in the dual glazed option. When the blinds are closed, they provide the ability to reduce ultra-violet light, visible light, near infrared transmission and far infrared transmission to almost zero. The total window U-value is improved by 25% when the blinds are closed. Source: Window 4.1 computer program and Laboratory Testing using NFRC methods
How does this compare to Clear LoE2 IG with room-side blinds? There isn’t much data available to help us make that comparison. The 1997 ASHRAE Fundamentals Handbook (page 29.39) does provide the following data on Shading Coefficients. The shading coefficient for clear insulating glass is 0.87. If you add room-side venetian blinds (light color, closed), it drops to 0.58. The shading coefficient for clear/clear dual glazing is 0.87. If you add between-glass venetian blinds (light color, closed), it drops to 0.33, which is a significant improvement over room-side blinds. Based on this, one might speculate that Clear LoE2 IG with room-side blinds might offer performance that is comparable to Clear/LoE DG with between-glass blinds. Source: 1997 ASHRAE Fundamentals Handbook
In summary, dual glazing has much to offer when compared to sealed insulating glass…warmer edge, longer life, easier to re-glaze, between-glass options, and better thermal performance when between-glass blinds are included. If blinds are not included, then sealed insulating glass offers better thermal performance. Aesthetics is more subjective, and we’ll leave it up to you to make that judgment. For Pella windows, the cost is exactly the same when the windows have clear or low-E glass. There are minor differences in cost when tinted glass is introduced.
References to glass in Section 08550 - Wood Windows are limited to those noted on the slide. References to NFRC 100 and NFRC 200 are good because they are based on total window values (glass and frame). Architects need to seriously consider revising IG warranty to at least 10 years, and preferably 20 years to protect the long-term interests of their owners. FYI. This is an advantage for Pella since it limits competition. The Masterspec requires them to choose insulating glass or dual glazing. We encourage them to consider the recommendations on the next slide. FYI. Many architects use AIA Masterspec as their office master specification. It is the starting point for most project specifications. Section 08800 - Glass and Glazing offers broader coverage, but is more oriented to aluminum windows and field glazing with 1” insulating glass and often conflicts with specifications in 08550. Source: 1999 AIA Masterspec
First, architects need to choose insulating glass or dual glazing. Second, they need to determine whether they will specify center-glass or total unit values. Encourage them to be consistent in all spec sections which include glazing. Encourage them to use total unit values, which are more accurate because they include the entire window and not just the glass. Note that ultra-violet light transmission and LBL Fading Damage Function are only measured in terms of center-glass values. The others have both center-glass and total unit values. LBL Fading Damage Function is a better way of expressing fade protection than UV transmission. It is a better predictor of potential fading damage than UV transmission because it takes into account the fading effects of the entire spectrum of solar radiation, not just UV. Not all low-E glass is created equal so we should encourage architects to specify specific values for each of the performance criteria listed. Only then can they be sure they will get the glass they want. All of these values are referenced in the Pella Architectural Design Manual. Since wood windows are factory-glazed, they may want to consider specifying these values in 08550 instead of 08800 to avoid potential conflicts between sections. FYI. Aluminum companies will resist the use of total unit numbers because their U-values are not very good due to the high conductivity of thermally-broken frames. Many of them do not even publish total unit numbers in their manufacturer’s technical literature. They like to focus on center-glass U-values which give architects the impression that their products perform better than they really do. NFRC has recently given its approval for a procedure that will allow them to rate and certify U-values of site-built products like storefront and curtain wall. It will be interesting to see if aluminum companies embrace the procedure.
We’re back to where we started so let’s recap our discussion. Clear low-E glass comes closer to the ideal glass than anything else on the market today. It offers lots of visible light while reducing heat gain and ultra-violet light transmission. Tinted and reflective glass offer more aesthetic options but at the expense of thermal performance, especially visible light. Dual glazing offers many advantages that you may want to consider for current or future projects. Ask if they have any additional questions and whether or not they agree with our assessment. Ask them to complete the evaluation form. We are always looking for ways to improve the content of this continuing education program and appreciate receiving their thoughts on what they found most valuable, what they found least valuable, and what other window topics they would find interesting for future programs.
Glazing Systems:Balancing Aesthetics,Performance & Costq Compare today’s glass options to the ideal glassq Discuss trade-off between aesthetics & thermal performanceq Advantages/Disadvantages of sealed insulating glass vs. dual glazing systemsMount St. Helen’s Visitors CenterCastle Rock, WASRG Partnership
Glazing Systems -Overviewq Architectural Trendsq Review of Heat Transfer Mechanisms & Definitionsq Glass Thermal Performanceq Comparison of Insulating glass and Dual glazingq SpecificationsWorthington High SchoolWorthington, OHNBBJ
Top Interests in Glassq Aestheticsq Initial Cost/Life Cycle Costq Durabilityq Fading Blockageq Warrantyq Energy Performanceq Optionsq Safetyq Resistance to CondensationLong-term Care FacilityWashington, DCSmith, Hinchman & Grylls Associates, Inc.
Architectural Trendsq Higher % of glass use in building skin in conjunction w/ energy conscious designq High visible light transmission, Low infrared transmissionq Low reflectance, clear appearanceq Low-E coatings gain share – standard on all buildings in 5 years – lower heating and cooling loads than an R19 wallThe Lansburg CondominiumsWashington, DCGraham Gund.
Heat Transferq Heat flows through a window assembly in three ways: – Conduction (heat traveling thru a solid material) – Convection (heat transfer by air movement) – Radiation (movement of heat energy thru space)
Heat Transferq Heat flow measured in terms of: – Insulating value (U-value) – Heat Gain from solar radiation (Solar Heat Gain Coefficient or Shading Coefficient) – Air Infiltration (cfm/sq ft)q Other Considerations – Visible Light – Comfort – Fading
Heat Transferq Insulating value (U-value & Emissivity) – Design conditions 15 mph, 70 F inside, 0 F outside – combination of conduction, convection, & radiation – Measured in terms of Btu/hr-sq. ft.-degree F – R-value = 1/U-value
Heat Transferq Insulating value 57.4° F @ center of glass (U-value) -15° F +75° F – center of 24.4° F @ edge of glass glass 51.5° F @ edge of – edge of sash 75.0° F @ edge of frame glass – window sash and frame
Heat Transferq Heat Gain from solar radiation Double -glazed window Solar transmittance Reflected radiation Absorbed radiation OUTDOORS INDOORS Inward flowing component of absorbed radiation
Heat Transferq Heat Gain from solar radiation – Solar Heat Gain Coefficient (SHGC) q that fraction of incident solar radiation that actually enters a building thru the window as heat gain q center-glass or total unit q SHGC = SC x 0.87 (approximate) – Shading Coefficient (SC) q the ratio of solar heat gain thru the system relative to that thru 1/8” clear glass at normal incidence q center-glass only q not recognized by NFRC
Impact of Glass Performanceq Minneapolis Case Study - NSP Energy Assets Program – Energy analysis as part of design process – more than 75 buildings since 1994 – Energy consultant is The Weidt Group – Buildings use 30-35% less energy than a code- compliant building (average of 32% savings) – Daylighting & specification of correct glass result in 10-15% of the savings
Re-glazingResidence Halls 12 dormsUniversity of Wisconsin at Madison 6428 sash
Between-Glass Optionsq Tilt-only blindsq Muntinsq Pleated shadesq Raise-and-lower blindsq Provide lower SHGC than room- side blindsq Extra insulating value by reducing convective loops w/in air space