Confused about R-values and other window and door terms?

If you’re shopping for energy-efficient windows or doors, you’ve probably seen many unfamiliar terms – from R-values and U-values to solar heat gain coefficient and ENERGY STAR® labels. Decoding these descriptions to find the right products for your home can be a challenge. Here’s a Pella guide to help get you through the technicalities.

R-value, U-value or U-factor – R-value indicates the resistance to heat flow through a window. U-value, the inverse of R-value, indicates a window’s tendency to transfer heat. Both R-value and U-value are measures of heat flow through a window.

Solar heat gain coefficient (SHGC) – The measure of how well a window handles heat from sunlight. The SHGC is the fraction of the heat from the sun that enters through a window. The lower a window’s SGHC, the less solar heat it transmits.

Low-E – Low-Emissivity or Low-E glass works by reflecting heat back to its source. Low-E glass utilizes an ultra thin metallic coating on the glass surface. Some Low-E coatings also can help protect interior furnishings from fading caused by the sun’s ultraviolet (UV) rays.

ENERGY STAR label – ENERGY STAR is a government-backed program helping businesses and consumers protect the environment through the use of high-efficiency products. ENERGY STAR designations on windows and doors mean these products use less energy, save money and help protect the environment. In the U.S., ENERGY STAR is administered by the Environmental Protection Agency (EPA).

Why care about R-value?
R-value is the measure of thermal resistance – how much resistance there is to energy that is leaving or entering a house, said Martin Wesemann, Pella’s director of advanced technology. Walls, doors and windows all help prevent thermal energy from leaving buildings, and therefore, help conserve the amount of energy used to heat or cool a home or commercial building. The higher a window or door’s R-value, the more energy that is saved.

How does it work?
Thermal energy attempts to flow from hot to cold, Wesemann said. For example, energy leaves a house in the winter because outdoor temps are colder than indoor temps. But in the summer, energy enters a house because outdoor temps are warmer than the air-conditioned rooms inside.

The rate at which the energy moves through your home depends on the temperature difference, Wesemann said. Extreme climates – with very cold winters and very hot summers where there are large differences between outdoor and indoor temperatures – cause the thermal energy to move more quickly, and when that happens, the heater or air-conditioner must work harder to compensate. In extreme climates, higher R-values slow the rate of heat transfer through doors and windows.

“A good wall – say it’s one that is in a highly insulated, energy-efficient home – has an R-value of R-19,” Wesemann said.

Now compare that solid wall to energy-efficient windows. Until recently, the highest efficiency windows in the market offered an R-value of 3.3. “It’s one-sixth of the resistance of heat going out of the house through the wall in winter,” Wesemann said. “However, unlike the wall, the window allows solar heat to enter the house and reduces how hard the heater must work.”

R-5 windows
Advanced technology is helping manufacturers improve R-values. Pella® Windows & Doors now offers windows with R-5 value.

“Going to R-5 is a 50 percent increase over R-3.3,” Wesemann said. “It’s 50 percent more resistant to the heat flow than a current U.S. ENERGY STAR product. That’s about one-fourth the value of the wall, with the added benefit of natural daylighting and potential ventilation provided by the window.”

Benefits of R-5 windows include:

  • Reduced energy consumption to heat your home in the winter
  • Potential for reduced solar heat gain – the energy from the sun that comes through windows in the summer to heat your home
  • Reduced potential energy costs
  • Improved temperature uniformity and enhanced room comfort

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