Post by ogeezer on Mar 14, 2007 6:42:19 GMT -6
I got these questions the other day from a couple seeking info about windows for a new home I'm designing for them on my CAD:
DOES WINDOW SIZE REALLY MATTER?
This is one decision where smaller is better, energy wise that is. Windows are a major source of heat loss due to their low R-value, even the double glazed ones. The best way to counter this problem is to minimize the overall amount of glass put into the home. As a rule of thumb, the glass area should run no more than 15% of the floor area. In a conventional home, glazing runs about 13% to 18%. These numbers may seem low compared to houses of the past. But remember houses today are being built better so it will require less heat and can afford to capture less sunlight. Whether the house will be dark depends more on the placement of the windows than on the amount of window area.
WHAT'S THE BEST WAY TO PLAN FOR WINDOW ORIENTATION?
Due to the high heat sink capacity of windows (to outside in winter; from the outside in summer), the goal should be reduce the number of windows used.
It's also important to place the windows so they will help heat the house in winter or to catch a breeze when not using air conditioning. The best way to do this is to locate the majority of windows on the south side, while minimizing windows on the north, east, and west. This will not only allow the sun to provide additional winter heat, but will help to prevent overheating as well. When trying to cool a house with a comfortable breeze, it will be necessary to open a window on the opposite side of the house or room to create a draft (flow of air from one place across to another). You should take care to also position the windows so the rooms will have enough light and will meet code requirements. (The Uniform Building Code requires 10% glazing in habitable rooms.)
WHAT DO THEY MEAN BY WINDOW GLAZING?
Glazing describes the transparent material used in the window (typically glass or plastic). You need to decide both the number of glazing layers and the type. The minimum choice would be a plain double-glazed window, with a U-value of .65 for a gas heated home, and .40 for an electric heated home. The next step up is a triple-glazed window or a double-glazed Low-e window, both around U-.33. Low-e refers to a manufacturing process that enables the window to retain more heat inside the house.
The window's thermal performance depends not only on the number of glazing layers, but also on the air space between the glazing layers. A spacing of 1/2" to 3-1/2" is recommended. One advantage of using a higher-efficiency window is that you can add more window area for views and daylighting without increasing the heat loss.
WHAT'S THE BEST FRAME MATERIAL FOR WINDOWS?
Common frame materials are wood, vinyl, metal, and fiberglass. Like other parts of the window, the R-value of the frame varies among materials. Wood, vinyl, and fiberglass are about the same, and metal is much lower.
Insulated metal frames are usually referred to as thermally improved frames and have a better R-value than uninsulated metal frames. They also are less likely to produce condensation. But even with a thermal break, insulated metal frames are not as efficient as wood, vinyl, and fiberglass frame options.
Regardless of the framing material, you should look at the weatherstripping and check to see if the window has been tested for air leakage.
WHY DO WINDOW MAKERS USE U-FACTORS INSTEAD OF R-FACTORS TO MEASURE THERMAL PERFORMANCE?
Some building components, such as walls, floors, or ceilings, are given R-factors to indicate how well they perform thermally. An R-factor value measures the thermal resistance of a material. In other words, it measures how slowly heat flows through a material. The higher the R-factor, the better an insulator the material is.
Although R-factors have been applied to windows as well, current practice is to use U-factors to describe their thermal performance. U-factors are just the opposite of R-factors. It measures how quickly heat flows through a material. Thus the lower the U-factor, the better an insulator the material is.
The terms are interchangeable by the formulas R=1/U and U=1/R.
DOES WINDOW SIZE REALLY MATTER?
This is one decision where smaller is better, energy wise that is. Windows are a major source of heat loss due to their low R-value, even the double glazed ones. The best way to counter this problem is to minimize the overall amount of glass put into the home. As a rule of thumb, the glass area should run no more than 15% of the floor area. In a conventional home, glazing runs about 13% to 18%. These numbers may seem low compared to houses of the past. But remember houses today are being built better so it will require less heat and can afford to capture less sunlight. Whether the house will be dark depends more on the placement of the windows than on the amount of window area.
WHAT'S THE BEST WAY TO PLAN FOR WINDOW ORIENTATION?
Due to the high heat sink capacity of windows (to outside in winter; from the outside in summer), the goal should be reduce the number of windows used.
It's also important to place the windows so they will help heat the house in winter or to catch a breeze when not using air conditioning. The best way to do this is to locate the majority of windows on the south side, while minimizing windows on the north, east, and west. This will not only allow the sun to provide additional winter heat, but will help to prevent overheating as well. When trying to cool a house with a comfortable breeze, it will be necessary to open a window on the opposite side of the house or room to create a draft (flow of air from one place across to another). You should take care to also position the windows so the rooms will have enough light and will meet code requirements. (The Uniform Building Code requires 10% glazing in habitable rooms.)
WHAT DO THEY MEAN BY WINDOW GLAZING?
Glazing describes the transparent material used in the window (typically glass or plastic). You need to decide both the number of glazing layers and the type. The minimum choice would be a plain double-glazed window, with a U-value of .65 for a gas heated home, and .40 for an electric heated home. The next step up is a triple-glazed window or a double-glazed Low-e window, both around U-.33. Low-e refers to a manufacturing process that enables the window to retain more heat inside the house.
The window's thermal performance depends not only on the number of glazing layers, but also on the air space between the glazing layers. A spacing of 1/2" to 3-1/2" is recommended. One advantage of using a higher-efficiency window is that you can add more window area for views and daylighting without increasing the heat loss.
WHAT'S THE BEST FRAME MATERIAL FOR WINDOWS?
Common frame materials are wood, vinyl, metal, and fiberglass. Like other parts of the window, the R-value of the frame varies among materials. Wood, vinyl, and fiberglass are about the same, and metal is much lower.
Insulated metal frames are usually referred to as thermally improved frames and have a better R-value than uninsulated metal frames. They also are less likely to produce condensation. But even with a thermal break, insulated metal frames are not as efficient as wood, vinyl, and fiberglass frame options.
Regardless of the framing material, you should look at the weatherstripping and check to see if the window has been tested for air leakage.
WHY DO WINDOW MAKERS USE U-FACTORS INSTEAD OF R-FACTORS TO MEASURE THERMAL PERFORMANCE?
Some building components, such as walls, floors, or ceilings, are given R-factors to indicate how well they perform thermally. An R-factor value measures the thermal resistance of a material. In other words, it measures how slowly heat flows through a material. The higher the R-factor, the better an insulator the material is.
Although R-factors have been applied to windows as well, current practice is to use U-factors to describe their thermal performance. U-factors are just the opposite of R-factors. It measures how quickly heat flows through a material. Thus the lower the U-factor, the better an insulator the material is.
The terms are interchangeable by the formulas R=1/U and U=1/R.