Passive Solar 101: Glass And Glazing Scott Ely Picture yourself staring calmly out a large window at the snow-capped peaks or the roaring ocean. The light of day brightens the room. It's cold outside, but the sunlight shines in, warming your bones. Your dog is launched on the carpet. You don't need to explain passive solar gain to him! Harnessing the sun's light and heat is a clean, simple, and natural way to control the light and temperature in our homes. Passive solar design entails the arrangement of basic building materials to maximize the sun's energy. Glazing describes any material which allows sunlight to pass through it while retaining a certain amount of heat. Designing with glass and glazing not only provides our source of natural light and heat, but also a means for ventilation, moisture control, privacy, views, and access to the outdoors. Understanding the Solar Spectrum and Heat Transfer To make intelligent choices on glazings, you need to understand a bit about light and heat. The sunlight that strikes the Earth is comprised of a variety of wavelengths. Ultraviolet (UV) light is short wavelength light (5-400 nanometers) and is invisible to our eyes. UV light comprises about 2% of the solar spectrum. Visible light is the light you see. It is light of medium wavelength (400-800 nm) and accounts for around 49% of the spectrum. Finally, infrared radiation (light in the form of heat) has long wavelength (800-1000 nm) and makes up most of the remaining 49% of the spectrum. Different glazings will selectively transmit, absorb, and reflect the various components of the solar spectrum. For example, controlling ultraviolet light can save carpet, fabrics, and furnishings from fading. Likewise, reducing glare (via reflection or tinting) is helpful in the workplace. By allowing the transmission of visible light, or natural light, you can save many watts of artificial light. But perhaps the greatest effect on human comfort levels is determined by infrared heat transfer. By specifying the right type of glass, you may choose to trap the infrared heat for warmth, or reflect the infrared heat to prevent warming. There are three ways that heat moves through a glazing material. The first is conduction. Conductive heat is transferred through the glazing by direct contact. Heat can be felt by touching the glazing material. The second form of heat transfer is radiation; electromagnetic waves carry heat through a glazing. This produces the feeling of heat radiating from the surface of the glazing. The third method of heat transfer is convection. Convection transfers heat by motion, in this case, air flow. The natural flow of warm air toward colder air allows heat to be lost or gained. The R-value of a glazingÄits insulating capabilities or resistance to the flow of heatÄis determined by the degree of conduction, radiation, and convection through the glazing material. However, air infiltration will also determine the overall R-value of a glazing system. The amount of heat that travels around a glazing is as important as the heat transfer through a glazing. Air can leak in or out of a building around the glazing via the framing. The quality, workmanship, and the installation of the entire glazing system, including the framing, affects air infiltration. The concept of R-value is really "brought home" if you compare the insulation in your ceiling and walls with that of your glazings. If you were heat, and were plotting to get out, where would you go? Through the R- 11 wall or the R-1 window? Window manufacturers often boast about the high R-values of their products. These figures are usually based on measurements at the center of glass and do not include the type of framing and overall window design. Ask the salespeople about the workmanship of the unit, the weather stripping, and installation recommendations. Basic Glass Types Glazing materials include glass, acrylics, fiberglass, and other materials. Although different glazing materials have very specific applications, the use of glass has proven the most diverse. In all fairness, there should be a discussion on other glazing materials; however, I have found glass to be the best all-around passive solar glazing material. The various types of glass allow the passive solar designer to fine-tune a structure to meet client needs. (see chart). INSERT CHART SOMEWHERES AROUND HERE The single pane, or lite, is the simplest of glass types, and the building block for higher performance glass. Single lites have a high solar transmission, but have poor insulationÄthe R-value is about 1.0. Single pane glass can be effective when used as storm windows, in warm climate construction (unless air conditioning is being used), for certain solar collectors, and in seasonal greenhouses. Structures using single pane glass will typically experience large temperature swings, drafts, increased condensation, and provide a minimal buffer from the outdoors. Perhaps the most common glass product used today is the double pane unit. Also known as insulated glass (IG) or thermopane, double pane glass is just that: two lites manufactured into one unit. These IG units incorporate a spacer bar (filled with a moisture absorbing material called a dessicant) between the lites and are typically sealed with silicone. The spacer creates a dead air space between the lites. This air space increases the resistance to heat transfer; the R-value for double pane is about 1.8-2.1. Huge air spaces will not drastically increase R-value. In fact, a large air space can actually encourage convective heat transfer within the unit and produce a heat loss. A rule of thumb for air space is between 1/2 inch and 3/4 inch. You could go as large as 4-5 inches without creating convective flow, but at that point you are dealing with a very large and awkward unit. The demand for greater energy efficiency in building and retrofitting homes has made insulated glass units the standard. With good solar transmission and fair insulation, the IG unit is a large improvement over the single lite. Windows, doors, skylights, sunrooms, and many other areas utilize double pane glass. High Performance Glass High performance or enhanced glass offers even better R-value and solar energy control. By further improving the insulating capability of glass, you can dramatically increase your design options. What were once insulated walls may now become sunrooms. Solid roofs and ceilings become windows to the sky. Dark rooms can "wake up" to natural light, solar heat gain, and wonderful views! For a relatively small increase in cost you improve efficiency, provide better moisture and UV protection, and gain design flexibility. A variety of high performance glass is now available. What makes it so great? Low emissivity (Low-E) glass is quickly becoming the successor to double pane glass in energy efficient buildings. Emissivity is the measure of infrared (heat) transfer through a material. The higher the emissivity, the more heat is radiated through the material. Conversely, the lower the emissivity, the more heat is reflected by the material. Low- Eúcoatings will reflect, or re-radiate, the infrared heat back into a room, making the space warmer. This translates into R-values from 2.6 to 3.2. In warmer climates you can reverse the unit and re-radiate infrared heat back to the outside, keeping the space cooler. Low-Eúglass improves the R-value, UV protection, and moisture control. Heat Mirror Low-E glass has recently taken a back seat to Heat Mirrortm glass as the leader in energy efficient glazing and window technology. Heat Mirror glass incorporates a double pane unit with a suspended film stretched between the panes of glass (see diagram). This polyester film or substrate has a wavelength-selective coating of metallic particles which controls emissivity and thus the re-radiated infrared heat, and controls UV transmission and visible light through varying degrees of reflectivity. The result is a glass unit with superior insulating capacity (R-values from 3.8 to 7+), outstanding UV protection (>99%), excellent condensation control (due to the warm interior pane), an improved sound barrier, and incredible design flexibility. INSERT DIAGRAM Heat Mirror offers a variety of film types with different reflective properties. Southwall Technologies, the manufacturer of Heat Mirror film, has assigned numbers to the film types corresponding to the reflective properties. For example, Heat Mirror 88 has the lowest reflectivity and highest transmission which translates into more heat gain. In the Northern Hemisphere, south-facing rooms provide the best orientation for light and heat. Typical locations for Heat Mirror 88 include vertical glass on the north, east, and south. Conversely, the Heat Mirror 66 film is more reflective. While visible transmission is slightly less, the higher reflectivity is very effective at blocking out unwanted heat. This can be useful in skylight applications and west-facing glazings, where overheating is a potential problem. Low Iron Glass Another enhanced, or specialized glass is called low iron. Low iron glass has a reduced iron content which allows for maximum solar transmissionÄeven better than single pane! Almost every solar hot water collector uses low iron glass for that reason: high transmission for maximum heat gain along with the strength of tempered glass. Low iron lites have a smooth side and a patterned, or textured side. The textured side allows the incoming light to be diffused into the space. The diffuse light is great for many plants and the diffused translucence provides privacy; this makes low iron a nice option for greenhouses and private spaces. Low iron lites are often fabricated into IG or even Heat Mirror units to increase their R-value. Gas-filled Glass You may have heard about gas-filled units that increase R-value. Properly done, gas-filling will increase the overall R-value of a glass unit by about 1.0. The air within an IG unit is displaced with an inert, harmless gas with better insulation properties. Typical gases used are Krypton and Argon. While the consumer certainly can benefit from greater glazing insulation, there have been some frequently asked questions regarding the gas-filled units: How do I know it's in there? How can I tell if it's leaking out? What kind of guarantee do I get for the extra money? The benefit lies in lower utility bills and increased comfort levels, but with so many variables involved, the consumer should be wary of the sales pitch and get some straight answers. Glass Specification When specifying glass, you will need to know certain characteristics in order to get the right glass for the job: 1) DimensionsÄglass units are ordered from "block sizes", that is, the closest rectangular shape. Keep in mind that the spacer bar and sealant will invade the viewing area. When ordering high performance glass in an angular shape (trapezoids, triangles, etc.) you will need to note dimensions for proper orientation in the opening. The glass people should help clear up any confusion. If in doubt, draw a sketch or have them visit the site. 2) Glass thicknessÄthe thickness of the single pane(s). This is determined by the overall square footage of the unit (1/8 inch minimum for 15 square feet., 3/16 inch minimum for 30 square feet.). Larger units require thicker glass. 3) Overall unit thicknessÄyou want to maximize R-value while staying within the limits of the framing detail. Overall thickness is the sum of one lite plus air space plus one lite. 4) Annealed (standard) or tempered (safety) glassÄdepends on the application. Annealed glass is standard (or float) glass, direct from the manufacturer. It is used for a majority of household applications. Annealed glass can break fairly easily from surface tension. And when it does break, the resulting shards of glass can be pretty dangerous. Building and safety codes therefore require the use of a stronger safety glass in many areas of a structure. Tempered glass is four times stronger than annealed glass. Tempered glass is annealed glass that has been heated to about 1200ø F and then rapidly cooled. Tempered glass has a much higher resistance to surface tension. Unfortunately, this does not make tempered glass unbreakable. Pressure on the edges of tempered glass units may cause breakage. However, when a tempered unit shatters, the pieces remain small and relatively harmless. Other types of safety glass include laminated glass (two lites bonded together), glass treated with a shatter proof film, or glass with a wire mesh screen. 5) Glass typeÄsingle pane, double pane, low-E, Heat Mirror, or low iron. 6) Other specificsÄit may be necessary to specify the type of seal desired. Most units with small overall dimensions use a "hot melt" seal. This is a single silicone seal primarily to prevent moisture from getting inside the unit. The other option is the double seal silicone construction. This consists of polyisobutylene caulk as an inner moisture barrier and pure silicone for an outer structural seal. If units are to be transported through varying elevations, a breather tube should be included. The breather tube allows the unit to equalize pressure as it travels through changing elevations. Standard Size Glass Units Glass distributors, architects, and knowledgeable contractors will refer to"standards" when selling, designing, or installing fixed glass. Their dimensions have been determined by the size of patio door replacement glass. Low iron standards come in somewhat different sizes. Their dimensions have been determined by solar hot water collector glass replacement sizes. All standards use tempered glass because of the need for safety glass in patio doors. Standards can be single lites, IG, Low-E, Heat Mirror, or any other glass type. The glass thickness varies from 1/8 inch to 3/16 inch and they generally have a 1 inch overall thickness (except Heat Mirror which typically runs 1 3/8 inch) Glass standards include these sizes (in inches): 28 x 76, 34 x 76, 46 x 76, 34 x 90, 46 x 90. Low iron standards include: 34 x 76, 34 x 96, 46 x 76, 46 x 96, 46 x 120 Standards are usually manufactured in quantity and consequently carry a lower price tag than custom size units. They are typically a stock item and therefore readily available. Square One Understanding glass and glazing systems is only one piece of the total passive solar design puzzle. Ventilation and air quality, thermal mass, moisture control, material selection, framing detail, aesthetics and integration, usage patterns, budget and future needs, and a host of other design issues must all be considered. By learning about the basics of glass and glazing, you are better suited to speak the language of the sales people and designers out there to get your business. Further Info For more background on glass and glazing systems: 1) Rocky Mountain Solar Glass in Boulder, Colorado has a wealth of information on glass, glazing and passive solar design information. Write or call: Rocky Mountain Solar Glass, 7123 Arapahoe, Boulder, Colorado 80303 ù 303- 442-4772. 2) Your local energy office should be able to steer you toward glass companies that carry high performance glass. 3) Contact your local glass company. If they are up-to-date with the technology, they should be able to supply literature on high performance glass as well as assist you in understanding local building codes. Ask about passive solar projects in the area. 4) Many local builders and related associations sometimes offer owner/builder seminars where the issues of passive solar design can be explored. If you are looking for in-depth study of passive solar design principles, contact the Solar Technology Institute in Carbondale, Colorado (see ad this issue). 5) The local chapter of the National Association of Homebuilders should also be able to steer you toward information regarding glass and glazing options. Access Author: Scott Ely, POB 301, Carbondale, CO 81623 ù 303-963-1420. Scott is the owner/operator of Sunsense, a solar design, consulting, and construction business located in Carbondale, Colorado. Heat Mirror is a registered trademark of the Southwall Technologies, Inc.