{"id":459425,"date":"2021-05-17T18:24:49","date_gmt":"2021-05-17T22:24:49","guid":{"rendered":"https:\/\/www.finehomebuilding.com\/?p=459425"},"modified":"2024-12-06T15:16:04","modified_gmt":"2024-12-06T20:16:04","slug":"understanding-window-efficiency","status":"publish","type":"post","link":"https:\/\/www.finehomebuilding.com\/project-guides\/windows-doors\/understanding-window-efficiency","title":{"rendered":"Understanding Window Efficiency"},"content":{"rendered":"

R-value, the ability of a material to resist heat flow through it, became a household word in the 1970s, when soaring energy costs led to a boom in home insulation. The higher the R-value, the better, we learned. Given the push to make houses ever more energy-efficient, the next frontier was windows because single-pane glass is a terrible insulator. Shortly, there was a blizzard of energy-saving technologies applied to windows, doors, and skylights\u2014and a flurry of new terms to describe them.<\/p>\n

Chief among this thicket of terms is the U-factor, an aggregate measure of how well nonsolar heat flows through a window\u2019s glazing and frame. Simply put, U-factor is the inverse of R-value (1 divided by the R-value), so the lower the U-factor, the better. The U-factor is a rough measure of conductive heat loss and gain. But the more elaborate window science became, the more crowded labels grew\u2014as you can see in the drawing below. This information is all good, but to figure out which window features make economic sense for your region, have a look at www.efficientwindows.org<\/a><\/strong>.<\/p>\n

Well, that will get you started. And, of course, there\u2019s a wealth of professionals, from architects to energy consultants, who can help you balance the competing claims of cost, performance, and a plethora of features that there\u2019s no room for here\u2014including impact-resistant glazing that\u2019s required in storm-prone areas, acoustic windows whose ability to reduce noise is rated with a sound transmission coefficient (STC), and even \u201cself-cleaning\u201d coatings that claim to slow the buildup of dirt on glass.<\/p>\n

Energy-Efficient Ratings<\/strong><\/p>\n

Reading the NFRC label on a window is an education in itself. NFRC, for starters, is the National Fenestration Rating Council, a nonprofit organization that independently rates the energy efficiency of windows, doors, and skylights. Here’s what the label is telling you.<\/p>\n

Double glazing: <\/span><\/strong>Today\u2019s standard insulated glass unit, consisting of two\u00a0glass panes separated by spacers, with a sealed, airtight space between\u00a0them. Triple glazing is often specified for cold climates. Glazing<\/span>, by the\u00a0way, is the fancy name for glass panes, the space between them, their\u00a0airtight seals, and the putty or caulk that seats them in the frame.<\/em><\/p>\n

Gas filling:<\/strong> <\/span>Clear, inert, nontoxic gas that\u00a0insulates between panes. Argon and krypton,\u00a0for example, are less conductive than air and\u00a0hence better insulators.<\/em><\/p>\n

Low-emissivity (low-e) coatings:<\/strong> Thin,\u00a0<\/span>nearly invisible window coatings that\u00a0selectively reflect heat back into a\u00a0room (to conserve energy) or block\u00a0sunlight to reduce solar gain. Low-e\u00a0coatings also filter damaging UV rays.<\/em><\/p>\n

\"Read<\/a><\/p>\n

U-factor: <\/span><\/strong>The sum of a\u00a0window\u2019s insulating values,\u00a0drawn from all its parts\u00a0(glazing, frame, sashes).\u00a0Measuring nonsolar heat flow,\u00a0the smaller a U-factor\u00a0number, the better.<\/em><\/p>\n

Solar heat gain coefficient (SHGC):\u00a0<\/strong>The percentage of solar heat that passes through the glazing. Higher SHGC numbers indicate greater passive solar gain\u2014desirable in cold regions. Conversely, lower SHGC numbers mean lower air-conditioning bills in hot climates.<\/em><\/p>\n

Visible transmittance (VT):\u00a0<\/strong>The amount of visible light\u00a0that enters. Values range from\u00a00 to 1; higher is better. Wide\u00a0sashes and frames block\u00a0light, lowering VT numbers.<\/em><\/p>\n

Air leakage (AL):<\/strong> The movement\u00a0<\/span>of air through a window system,\u00a0measured in cu. ft.\/sq. ft. Lower\u00a0is better; <0.03 is optimal.<\/em><\/p>\n

Condensation resistance (CR):\u00a0<\/strong>Higher ratings (from 0 to\u00a0100) predict less\u00a0condensation.<\/em><\/p>\n

\n

More About Energy-Efficient Windows<\/h3>\n\n\n\n\n
Insulating Glass Keeps Getting Better<\/a> –<\/strong> Windows are still a weak link in a home’s thermal envelope, but innovative insulating glass units promise better performance in the future.<\/td>\n\"insulated<\/a><\/td>\n<\/tr>\n
Low-E Glass<\/a> – <\/strong>Low-emissivity glass is the go-to choice for energy-efficient windows.<\/td>\n\"how<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
\n

\"Renovation<\/p>\n

Excerpted from\u00a0Renovation, 5th Edition<\/i><\/strong> (The Taunton Press, 2019) by Michael Litchfield and Chip Harley<\/p>\n

Available at Amazon.com<\/a>.<\/strong><\/p>","protected":false},"excerpt":{"rendered":"

R-value, the ability of a material to resist heat flow through it, became a household word in the 1970s, when soaring energy costs led to a boom in home insulation. 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