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Understanding Light and Solar Heat Gain

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    Temperature Gains and Losses

    There are multiple factors that affect the ability of a window to control heat gains and losses. In the winter you want a window that gains heat and keeps it indoors; in the summer you want a window that loses heat and prevents it from entering your home. So how can a window achieve both? A proper window will effectively “control” the four main processes in heat or cold transfer found in most windows: radiation, conduction, convection, and air leakage.

    How Windows Lose Energy

    Radiation – How much cold passes through the glazing (glass). About two thirds of heat lost through a window is due to radiation. Ordinary glass easily transfers heat to colder areas and is considered to have a high emissivity. In other words, glass in the summer months passes the outdoor heat to the cool interior of your home, and in the winter the glass passes the warmth of your home to the cold outdoors. Every Alpen Window's product is engineered with low emissivity (low-E) coatings that effectively control how heat passes through our glazing systems. These coatings help keep heat inside in the winter and block it in the summer. Conduction – How much cold passes through the edges and framing system of a window. The material of a window’s framing system is what determines how well a window controls conduction. Some materials are better conductors than others; for example, copper and aluminum pots and pans are the most coveted in a kitchen because they are extremely good conductors; however, you would never want that for your windows. Alpen Windows are constructed using fiberglass, the least conductive materials available for windows. This material prevents the transfer of unwanted heat or cold into your home. Convection – The warm air movement between the spaces in a multi-glazed window and the air that circulates within your home and touches the glass. The "drafty" feeling, often felt by low performing windows in the winter, results from warm interior air traveling along a window, cooling down, dropping toward the ground and then circulating out toward the interior of the home. This cycling of air is what causes that cold draft feeling in the winter. When warm air touches cold glass, heat is passed to the coldest side of the glass. In the summer that means the warmth is passed to the inside of your home, and in the winter the warmth is passed to the outside of your home. Too large of an air space increases convection and too small of an air space increases conduction and colder interior glass temperatures increase convection. Every Alpen Window's product has been engineered to have optimal spacing between panes of glass and suspended film, nearly eliminating temperature fluctuations due to convection or conduction. Also, the full frame R-Value of our windows create warm interior glass temperatures and nearly eliminate that cold drafty feeling. Air leakage – The air movement between all of the moveable parts of the window’s frame. Most air leakage occurs between the sash and frame, or the meeting rails of a sliding sash in operable windows. Air leakage performance is shown on a window’s NFRC label and is labeled as “air infiltration rate” - the lower the number, the better the performance. Posting air infiltration rates is considered voluntary for a window manufacturer and is not required on an NFRC label. We post all of our product’s air infiltration rates because we believe in being as open and honest as possible about our windows. Unlike some competitors, you will always and easily be able find performance data for all of our products.
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