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Bob Crager
Bob Crager of Lewis Street Glass is a 26 year veteran in the glass business. Lewis Street Glass is a leading Wichita Glass company, serving the entire Wichita/Sedgwick County area since 1919. They do anything and everything having to do with glass, both residential and commercial. They also do Auto glass. They are located at 743 South Market, facing Kellogg on the South, and you can reach them by phone at (316) 263-8259. You can email Bob Crager at bcrager@lewisstreetglass.com
Glass
2009-09-01 14:47:00
Summer heat flooding my home
Question: Last month we talked about answers to the question: “My house is an older house and I’m starting to think my windows are my biggest problem! The AC is running full blast, and it’s HOT in here...what should I do?” Let’s continue!
Answer: We told you last month about the first 3 of the 4 major ways that the windows in your home gain and lose heat…in other words, how you gain or lose heat through the glass of your home to the inside or to the outside. They are conduction, convection, and radiant heat transfer. This time we’re going to cover the 4th way this transfer of energy happens, air leakage. Air leakage siphons about half of an average home’s energy, used for heating and cooling, to the outside, mostly through the windows. Well designed windows have durable weather stripping and high quality closing devices that will normally block air leakage. Hinged windows, such as casements and awnings, clamp tighter against the weather stripping than double hung windows. However, the difference is slight. Well designed and constructed double hung windows are more than acceptable. Air leakage is also affected by how well the pieces of the window unit are joined together. Glass to frame, frame to frame, and sash to frame connections must be tight. When you’re replacing your windows, technical specs should list values for air leakage as “cubic feet per minute” per square foot of window glass. You should look for windows with certified air leakage rates of less than 0.30 cubic feet per minute/ft2. Lower values are best. In the colder part of the year, we welcome the sun’s heat and light most of the time, and once we are able to capture that heat, we don’t want to give it up. In a warm climate, we might not want the heat, but we do want the light. Recent advances in window technology have let us have our cake and eat it too! Only a fraction, less than half of the sun’s energy is visible to us. Longer wavelengths, those beyond the red part of the visible spectrum, are known as infrared, which is felt as heat. Shorter wavelengths, those beyond purple, are known as ultraviolet, or UV. When the sun’s energy shines on a window, visible light, heat, and ultraviolet light rays, are either reflected, absorbed, or transmitted into the interior of the building. There are windows that “selectively” block fabric or upholstery fading ultraviolet light, which as we said is felt as heat. Windows that block most of the UV and infrared rays while admitting visible light work well in the cooler climates. In hot climates, we have low-e glass coatings, which are transparent metallic oxides that reflect up to 90% of the long wave heat energy, while passing shorter wave, visible light on through. This helps keep your house or building cooler in the summer months. So remember, in cold climates, the coatings we’re speaking of reflect the long wave radiant heat back into the house while admitting visible light. The floors, walls and furniture absorb the shorter wavelength visible light, and then reradiates from them as long wave heat energy that the reflective low-e coating keeps inside the building. Low-e coatings work best in warm climates when you apply them to the internal surface of the interior panes. Conversely, in colder weather climates, they work better when applied to the inter pane surface of the exterior panes. Low-e coatings improve the insulating value of a window just about equal to adding an additional pane of glass. When you combine low-e coatings with low conductance gas fillings, like argon or krypton, you can boost your energy efficiency by nearly 100% over clear glass. Those inert gases will leak from the window over time, usually about 10% over 20 years, but they are “inert”, and not dangerous to you. And that reduction of gas only reduces the U value of the unit by only a few percentage points. The initial added cost for low-e coatings and low conductance gas fillings is only about 15% of the window’s overall cost…not much of a decision at all. Let’s talk briefly about VT (visible transmittance) of windows. Windows with higher “visible transmittance” are very easy to see through, and admit natural daylight. They can also save your energy because with windows like these, you need less artificial light in the room. Some of the tints and coatings that block heat also reduce visible transmission, so you want to be careful in these selection combinations. Some manufacturers list the “VT”s of windows as comparisons with the amount of visible light that would pass through an open hole in the wall the same size as the window. Sometimes they refer to “VT” as a “whole window” value, including the effect of the frame. Since it’s important to be able to see through the glass and not the frame, be sure the “VT” in residential glass windows extends from a shady 15% for some tinted glass up to about 90% for clear glass. To most people, glass with “VT” values above 60% looks clear, where values below 50% begins to look dark and/or reflective. It can even make a difference if you look through glass at an angle, which can change the way it looks to you. It’s best to look at a sample of the glass outdoors before you decide to order the window. See your local glass dealer for specific information about all of the considerations in buying your next window replacements. More next month…
 
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