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| Energy efficiency |
Energy Efficiency & Carbon Footprint Reduction
Reducing energy consumption and increasing the comfort of your home environment is a simple and achievable goal for any homeowner. Consideration must be given as to the window type, size and location, along with its potential problems or benefits. As an example, west facing windows will allow large amounts of radiant heat gain to enter during the summer months. This will tend to overheat the room and cause air conditioners to work excessively, attempting to cool off these areas. A south facing window however, will allow welcomed heat gain to warm up a cold room in December or January. It is important to remember that Summer Solstice occurs on June 21st (the longest day of the year) and Winter Solstice happens December 21st (the shortest day of the year). Generally speaking, the prevailing winds in the Southern Ontario are from the northwest. Try to follow the seasonal path of the sun’s rays and you will see that the following holds true:
I think it can be agreed that excessive heat gain is a good thing during the winter months and a problem in the summer. As you can see from above, west windows are the worst possible exposure since they allow unwanted summer heat gain as well as excessive winter heat loss, due to the prevailing winds. North windows offer no gain, only winter heat loss. Energy efficient homes are built so that they maximize the sun’s energy, hence lots of southeast glazing and fewer windows on the north and west. The sun also strikes the glass at different angles during summer versus winter months. Most winter heat gain enters the window at an angle closer to perpendicular. The most intensive summer heat gain hits the glass at a very sharp angle between the hours of 11:00 am to about 4:00 pm. Flat roof or low pitch skylights are summer’s heat gain worst-case scenario. Summer heat gain causes overheating of rooms and makes air conditioners run constantly. An energy efficient shading system can greatly reduce this heat transfer by reflecting radiant heat and creating pockets of trapped air to help insulate. When searching for solutions in reducing heat transfer through glass, it is important to understand how the heat is actually exchanged. Three separate processes move heat: radiation, convection and conduction. Reducing or omitting these factors will ultimately determine the effectiveness of the shading system. Radiant heat transfer occurs in the absence of matter. The sun’s rays carry energy in the form of a wave, which hits the earth and get absorbed. Our bodies also omit radiant heat at about 300 BTU (British Thermal Units) per hour. Light colours, shiny surfaces and metallic finishes reflect radiant heat. The difference in wearing a black T-shirt instead a white T-shirt on a hot sunny summer day clearly demonstrates the distinction in absorption versus reflection of radiant heat. Most shading systems are provided with a uniform “white” backing. This serves two purposes. Firstly, it creates a uniform appearance from the outside of the building. Perhaps the internal fabric colours are different from room to room. Houses look best with a consistent “visually clean” exterior façade. The second reason for the standard “white” flipside is to maximize the reflection of radiant heat. A silver or metallic backing is slightly superior in efficiency, however it looks esthetically unappealing in residential applications. Convection is the transfer of heat in a liquid or gas form. As we know, heat rises and cold sinks. A fan placed behind a wood stove will convect warm air to other parts of the room. A hot air balloon will rise once the air is heated. A lake is colder at the bottom than at its surface. Effective winter window insulation will attempt to keep the heat inside a home from warming up the glass. Since the glass panes can become extremely cold, they can help create wasteful convection currents which more warm air to colder spots within a room. Shading systems with “edge seals” or “side tracks” (or blinds tightly installed against the window frame) will reduce convective air currents. The final method of heat transfer is conduction. This is the transfer of heat through solids. Dense materials like glass are generally good conductors. They transfer heat from a warm spot to a cold one. Touching the bare handle of a hot cast iron frying pan demonstrates conductive heat transfer instantly! Trapped pockets of dead air will increase the thermal resistance of the window system. The amount of resistance is measured by the term “R factor”. The higher the R factor, the better thermal insulation it provides. Fiberglass inside a wall provides an effective barrier against conductive heat transfer. It is not the actually fiberglass which insulates, rather it is the air which is trapped inside the fiberglass. The same can be said about a thick sweater or down vest. The fabric only traps the air, which in turn creates the R factor to reduce the heat transfer.
The chart below (source: Window Energy Systems magazine, March 1981) clearly shows the relationship between R factor and percentage of winter heat loss. Please note the “diminishing return”.
For example, a triple cell honeycomb shade with an “additive” R factor of 3.5 will reduce heat loss through a single pane window from 99% down to 22%. This represents a reduction in total heat loss of approximately 78%. The same shade however installed over a low E window with argon gas will reduce the heat loss from 30% down to 15%, cutting the total reduction in half. The same shade will reduce total heat loss from 50% to 78%, depending on the type of window it is covering. The weaker the existing window, the better the moveable window insulation will perform. As a comparison, the Ontario Building Code stipulates the following levels of insulation in new home construction:
As you can see, windows have a substantially lower insulation value than other areas of the building. It stands to reason that the amount of summer heat gain and winter heat loss is directly connected to the quantity and location of glass in the home. Energy efficient window coverings can greatly assist you reducing your heating and cooling demand. The thermal resistance of the shading system is very important but more important is the seal or how tight the shade will fit. As an example, if you had a 48” x 48” window and inserted a 2” thick piece of Styrofoam which was cut to 46” x 46”, just how well would this work? Certainly the insulation value of the material is high (R10), but all the heat would simply escape around the perimeter. Convective air currents would easily transfer the temperature differential through this “gap”. Shading systems which have a side tacking system or which are installed to fit “tight” against the window jamb are ideal. Alternatively, a shade, which overlaps the window, should be installed so that it snugs up to the window casing. The better the shade fits the window, the better it will be at reducing the convective air currents. There is an extremely cost effective product sold through large retailers such as Canadian Tire, Home Depot and other hardware stores. It is called the “3M window insulator kit”. It’s basically a clear plastic sheet, which is attached on to a “two sided sticky tape”, mounted around the perimeter of the window. Once installed, a hair dryer is used to blow hot air, which shrinks the plastic to a drum tight, see through condition. This small plastic layer with virtually no R factor to speak of provides fantastic results. I would estimate its payback in terms of weeks, rather than months or years during the cold winter season. There are however some draw backs with this product. First of all it is a temporary, seasonal item of use, generally installed in the fall and removed in the early spring. It leaves a sticky tape residue on the window frame, which is very difficult to remove especially when used year after year. This product is easily damaged, should something sharp puncture its plastic membrane. Naturally, one cannot use the window when this material is installed. Disposable plastic products cannot yet be recycled so that also creates its own set of problems. Even considering the negative aspects, the concept of this product is excellent. It creates a pocket of trapped air and seals it to the window frame so that convective air currents cannot allow the cold air to enter the room. In theory, it should also greatly reduce condensation. Reducing winter heat loss and stopping summer heat gain require slightly different strategies. The reason is simple: temperature differential. For example, let’s take a typical home in Southern Ontario. As a general rule, homeowner’s like to keep the internal temperature at a constant 22 degrees Celsius, summer and winter. In the summer time during a heat wave, daytime external temperatures can exceed 35C. In the middle of the night in January however, the cold can hit minus 25C or even worse. 22C inside vs. 35C outside = 13 degree maximum summer time temperature differential 22C inside vs. -25C outside = 47 degree maximum winter time temperature differential The greater the temperature differential, the quicker the heat will be exchanged. Summer heat gain can be reduced by maximizing the reflection of radiant heat, adding minimal R factor and making sure the shade fits reasonably snug to the window frame. Since the interior versus exterior temperature is only a maximum of 13 degrees, most of the heat transfer is of a radiant and convective nature. The conductive aspect of heat transfer plays a far greater role during the cold season. Winter heat loss is much more difficult to control since a high R factor shade is required to create “loft”. Not only is superior thermal efficiency required, but also the seal is very important. Since temperature differential is far greater than in the summer time, the winter heat exchange will occur much faster. Energy efficient windows shade should be sealed inside a track to reduce or omit the convective air currents. This unfortunately limits the number of choices, since only a handful of window shades are truly designed to deal with winter heat loss. The key to successful reduction of winter heat loss through glass is having high thermal resistance in conjunction with a superior edge seal and a uniform white backing. Winter heat loss requires each and every section of glass to be covered with high performance shades to ensure complete coverage. If 90% of the windows in a house are covered, the internal heat will simply escape through the remaining 10% of uncovered glass. Summer heat gain enters the house primarily through skylights and west facing windows. Covering these selected areas with energy efficient shades will provide the homeowner with proper protection to ensure a comfortable environment. It will also ensure optimum efficiency for air conditioners so that no unnecessary energy is wasted. Besides the “thermal” aspects, moveable window insulation will also offer the following benefits:
When one considers the life span of a building between 60 to 100 years, the long-term energy savings can be staggering. Various studies show that windows and doors account for up to 40% of heat exchange in a typical Canadian home. Installing energy efficient shades will drastically reduce this source of waste. High performance window shades will greatly increase room comfort. Homeowners will be cooler in the summer and warmer in the winter. Moveable window insulation will add tremendous value to the building. Improve the quality of your home environment with energy efficient shades. Some type of window treatment will inevitably be required to cover each section of glass in your home. Why not incorporate an energy efficient product to greatly assist in the reduction your carbon footprint? |
