Each year, even with global warming, the residents of eastern Ontario and western Quebec survive a long tough winter. Canada is a cold country and in winter, we are forced to live indoors for a longer period than we would prefer. To help overcome these long winters, homeowners in Ottawa and Gatineau are increasingly turning to glass roof sunrooms. They want to bring the outdoors indoors; they want to brighten their home environment. Each year, on average, eastern Ontario and western Quebec receive over 1100 hours of sunshine between October 1 and May 31. With a glass roof addition from Four Seasons Sunrooms we have the opportunity to enjoy the entire 1100-right in the comfort of our own home.
Obviously, glass is the critical building component in sunrooms. The problem for homeowners is that few of us rarely pay much attention to this product and its properties. Given our experience with cold winters, we tend to focus almost exclusively on insulation or “R” value when selecting the type of glass for use in our sunrooms. We all understand that the higher the R value of glass, the greater its performance in terms of heat containment in winter. But when considering the purchase of a glass roof sunroom, this overriding concern with insulation value can be costly and even counter-productive.
“Can I live in this room in winter?” That is one of the most frequently asked questions we hear from clients living in Ottawa-Gatineau and considering the purchase of a Four Season sunroom. Our categorical answer is yes, absolutely. But in our opinion, these clients have often asked the wrong question. The more important, or at least an equally important question, should be “can I live in this room in summer?” Selecting the right glass so as to ensure optimum year round comfort for your sunroom, even in summer, requires some knowledge of the performance features of the major types of glass, especially in terms of solar heat reflection. This is often the key to understanding how to build a sunroom that will be comfortable throughout the entire year. In our view, control over the sun’s heat in summer is in fact the single most important factor in sunroom design-even in the Ottawa-Gatineau region!
Primary and Secondary Heat
In order to understand what makes one type of glass perform better in summer than another, it is necessary to differentiate between what is sometimes referred to as “primary” and “secondary” heat. Stated simply, primary heat comes from the sun and heats the earth. Heat returned from earth to the atmosphere is called secondary heat. More specifically, primary heat refers to short wave solar radiation from the sun. For the most part it is represented by the infrared and near infrared portion of the short wave spectrum. Other components of that spectrum include X-rays, UV, and visible light. Short wave solar radiation, or primary heat, has the ability to pass through the earth’s atmosphere without being absorbed or reflected back into space to any significant degree. Similarly, this short wave solar radiation will also pass through clear glass with very little being absorbed or reflected back into the atmosphere. In other words, both the earth’s atmosphere and standard glass are transparent and allow most short wave solar radiation to pass right through. However, once short wave solar radiation strikes an object, such a tree, a building, pavement, or the floor or furniture in a sunroom, it causes that object to heat up and to become a source of heat in the form of long wave thermal radiation.
Long wave thermal radiation, unlike short wave, does not pass as readily through glass. In fact, to a large degree, glass is opaque to long wave infrared. The result is that a large percentage of this long wave radiation coming from within the sunroom is in fact reflected by the glass back into the sunroom where it is absorbed by the inside air, often causing the room in summer to become unbearably hot and oven-like. This heat associated with long wave thermal radiation can be referred to as secondary heat and it is this secondary heat which is absorbed by and warms the earth’s atmosphere.
This is also exactly what happens to a car when it is left sitting in the sun, especially in spring, summer, and fall. Short wave solar radiation passes easily through the car’s glass, striking the interior materials, and causing them to heat up and emit long wave thermal radiation. A significant portion of this long wave radiation becomes trapped inside by the car and is then absorbed by and warms the car’s interior air. Due to the extremely poor insulation and low R value of the car’s metal surface and single glaze glass, this secondary heat is continuously dispersed to the outside atmosphere through conduction.
The above description has important implications for sunroom design. In order for your sunroom to be comfortable throughout the entire year, its glass must be able to carry out two separate functions. First, for all-year comfort, it must insulate against outside cold winter temperatures by reducing interior heat loss in winter. It must also help to insulate the home in summer from excessive outside secondary heat (i.e., ambient air in the 30 degree Celsius and above range). In other words, for your comfort, sunroom glass must have a good insulation or R value. Second, for summer comfort, it must have the ability to block a large percentage of the solar infrared short wave radiation (primary heat) from entering the sunroom and causing it to overheat. Although less important, it will also have to reflect the earth’s long wave infrared radiation from outside objects such as buildings, trees, rocks, etc. As a general rule, without solar protection, as much as 90% of excessive heat build-up in glass roof sunrooms is caused by short wave solar heat gain rather than the transfer of heat from the outside atmosphere to the inside of the sunroom.
The Capability of Glass to Insulate Against Cold and Secondary Heat
Different materials resist the transfer of cold or secondary heat with different levels of efficiency. In house construction, R value refers to the ability of a material or substance to insulate, guard against, or reduce the transfer of heat through itself. (Energy always flows from a high temperature area to a lower temperature area). To help prevent interior heat escaping through sunroom glass in winter, or warm/hot outside atmospheric heat (e.g,. 30 degree Celsius and above) from entering your sunroom in summer, it is essential to install glass with an enhanced R value.
Obviously, different types of glass have different insulation values. Single glaze glass is exactly as described. It is one piece of glass usually with an R value of one. Thermal is two pieces of glass separated by a small air space and sealed at the edges. Depending on a number of factors such as the size of the air space and thickness of the glass, thermal glass generally has an R value of approximately 2. Triple glaze is three pieces of glass in a sealed unit with an R value of approximately 2.7. Low-E is a thermal unit to which a low emissivity coating has been added to the inside surface of the outside piece of glass, bumping the R value from 2 to 3.1. Low-E argon is a piece of low-E that has had the air removed from the cavity separating the two pieces of glass, and that air replaced with argon gas. Adding argon pushes the R value to approximately 3.7. Low-E2 with argon has a coating on both inside surfaces and an R value slightly above 4. Adding argon gas and two low-E coatings to triple glaze will push the R value as high as 6.5. R values for all types of glass can be boosted to as much as 10, or even higher, by replacing the argon with a heavier gas such as krypton, but at a considerable increase in cost.
At some point, the pursuit of higher glass R values, given the cost of boosting this insulation value, becomes no longer cost effective. Here it is critical to understand that the curve measuring glass insulation performance for each level of R value is exponential-not linear. In more simple terms, R4 is not twice as efficient R2, nor is R16 twice as efficient as R8. In fact the improvement to be gained by adding R value diminishes with each incremental addition. For example, where an R4 glass wall will actually cover off about 80% of potential heat loss, a conventionally framed 2 x 6 inch wall, with no windows or doors, and rated at R20, will only cover off approximately 95% of potential heat loss, or only 15% more of potential heat loss than an R4 all glass wall. And given that most conventionally framed house walls have glass windows and doors, this differential will drop to well below the 15% advantage.
At Four Seasons Sunrooms, we recommend the use of ConservaGlass Select® with an R4 rating. Not only will this enhanced R value keep you warm in winter, it will also function to keep those unpleasant hot summer temperatures under control.
The Capability of Glass to Control the Entry of Primary Heat
Dealing with primary heat is much more complex than dealing with secondary heat. To start with, it is important to understand that the R value of glass has essentially no role in protecting against short wave radiation or primary heat. This can be a difficult concept to grasp. Protection against short wave solar radiation is provided by what is called the “shading coefficient”. It is completely separate from R value. It refers to the ability of glass to selectively filter and prevent a portion of the sun’s solar heat, particularly infrared radiation, from passing through glass and entering your sunroom. It is measured in relative terms with a fully opaque surface such as a solid wall having a shading coefficient of 0. At the other extreme, a single pane of glass untreated will have a shading coefficient of 0.99. A second method of measurement of primary heat penetration is in quantitative terms, stated in Btu’s per hour per square foot (Btu/hr/sqft).
Without a strong shading coefficient, sunrooms run the risk of being unbearably hot and unusable in summer, regardless of the R value. This is a critical point. Even if it were available, and you were able to use glass with an R value of 50 or 100, but without a strong shading coefficient to reduce solar heat gain, your sunroom, although extremely energy efficient in winter, would still be oven-like in spring, summer, and early fall.
In summer, primary heat enters a sunroom mainly through the roof. In winter, due to the lower angle of the sun, it enters primarily through the front and side vertical glass. To be comfortable in summer, a south facing sunroom should have roof glass with a shading coefficient of 0.20 or below, and 0.40 or below for vertical glass. By accommodating the changing angle of the sun in winter and summer, this glass combination, by allowing for some passive solar heat gain in winter while providing significant solar protection in summer, will work to the home owner’s advantage. However, even with this level of solar protection, with shading coefficients of approximately 0.20 and 0.40, and in the absence of significant shading from trees or other buildings, adequate ventilation or air conditioning for hot summer days is still recommended. As a general rule, and certainly for south or southwest facing sunrooms, no shading coefficient can be too low.
As is the case with R values, the capability of each of the more common types of glass to control heat gain will vary. Thermal glass has a shading coefficient of .87 which translates into a massive summer heat gain of 183 Btu/hr/sqft. Similarly, low-E and low-E argon, both with a relatively ineffective shading coefficient in the area of .80, will allow a relative heat gain of 170 Btu/sqft/hr. While these numbers may be considered a plus on sunny cold winter days, the sunroom will almost certainly be unbearably hot over the summer months, and even as early as April or May, unless it is equipped with heavy duty and fairly expensive to operate air conditioning. It should also be noted that low-E glass was developed for the window market-not the sunroom market. Its purpose is to boost winter R values by reflecting long wave infrared radiation back into the home. It was not designed to cut short wave radiation.
Four Seasons Sunrooms prides itself on having glass with strong shading coefficients. We lead the industry. Our special patented ConservaGlass Select® glazing system, with Stay Clean technology, is simply the best performing glazing available in the industry today. Available in two basic types-MC 16 and MC 7E-it has revolutionized the sunroom glass industry. MC 16 roof glass, with a shading coefficient of 0.18, is so effective it will actually reduce the relative heat gain from 183 BTU’s/sqft/hr for thermal glass down to approximately 38 BTU’s/sqft/hr. MC 7E, used for curved and vertical glazing reduces heat gain down to 79 BTU’s/sqft/hr. In fact, Four Seasons Sunrooms is so far ahead of the competition in this area that our competitors will usually prefer not to raise the issue of shading coefficient with their clients and instead focus their glass presentation almost exclusively on insulation values.
Accepting glass with a poor shading coefficient may be the correct approach if you are building a passive solar heater, or if you spend all summer away from your home and at your cottage. It certainly does not work for sunrooms that you intend to use all year long.
Low Maintenance Care
Four Seasons ConservaGlass Select® goes through an extra manufacturing process to permanently bond an exterior finish to the glass. This makes the surface extremely smooth and reduces the potential for dirt accumulation. It also has the ability to repel water and to resist staining. Put simply, ConservaGlass Select® stays cleaner longer.
Seal failure in glass is not a good thing. It is another topic often avoided by sunroom sales representatives. When seal failure occurs, condensation will collect inside between the two glass panes and impair visibility. Enhanced performance glass is expensive and in order to avoid facing high glass replacement costs, buyers should ensure they receive good warranty protection. Once your warranty protection has ended, you could be looking at $45 or more per sq. ft. for replacement glass. In addition, labour costs to replace one piece of glass could add between $100 to $150 to the overall cost.
It is our anecdotal experience that triple glaze is more likely to experience seal failure than double glaze. It is certainly far more prone to seal failure than ConservaGlass Select®. Increased wear and tear on the window hinges and casement cranks from the heavier weight of triple glaze due to the extra panel of glass is also of concern.
Four Seasons Sunrooms provides one of the strongest and most comprehensive warranties in the industry, including full 100% coverage against both seal failure and breakage for the first 20 years. (This warranty does not include replacement labour beyond the first year). And unlike other warranties, it is not prorated. After the first 20 years it simply drops from 100% to 50% of full coverage. Forever! And that warranty is transferable. That is security for the homeowner that is backed by craftsmanship, a quality product, and a company that has been in business for over 35 years.
There is one other warranty consideration that should not be ignored and that is the issue of long term availability. Glass manufacturers will often change the type of glass they produce and sell as new and improved technology becomes available. Sunroom owners with short warranties may not only be looking at unexpected replacement costs sooner than they would like, but may find that when they try to purchase the replacement glass they require to match their remaining glass, it is either no longer in production, or has become a relatively expensive custom order. Moreover, custom glass can take weeks for delivery from the factory.
Labour costs must also be considered. Not only do sunroom owners with short warranties face the probability of expensive glass replacement costs, they are also likely to encounter significant labour costs in replacing the failed glass. For new owners of Four Seasons sunrooms, even this replacement labour cost is not much of an issue since our failure rate for ConservaGlass Select® is extremely low.
Choosing the Best Glass for Your Sunroom
An efficient R value, an efficient shading coefficient, and a respectable warranty with real protection for the buyer are all critical to the economics and overall enjoyment of your sunroom. At Four Seasons Sunrooms, we believe ConservaGlass Select® provides our clients with excellent protection against solar heat gain, an enhanced R value for winter comfort, as well as the strongest warranty in the business.