HOW ACCURATE ARE "R" VALUES?

Read this news article that appeared in the Washington Post referencing window glass efficiency and testing.

Out-of-the-Ordinary Glass to Curb Heat Loss

By Mike McClintock- For The Washington Post

Column: HOME SENSE (ABBREVIATED)
Thursday, March 21, 1996; Page T07

Window efficiency is important because glass is the thermal weak link in a building's energy envelope. Heat passes through glass 10 to 12 times faster than it does through an insulated wall.

Ordinary windows are made with single-thickness glass, which can drip with condensation, create hot spots in summer and cold spots in winter, decrease your comfort level and increase your utility bills. A storm window will help solve these problems, but double glazing with a sealed airspace is better. Or you can opt for a high-tech super window.

To compare window efficiency, the best measure is a U-factor, which accounts for materials and construction – the whole package. Unfortunately, most claims are made in R-values, which can measure insulating values of the frame or the glass but can't combine them into one number.

Single-thickness glass is rated less than R-1.0. Double glazing with a sealed air space is approximately R-2.0. Double glazing using an inert gas and Low-E coating, a film that blocks heat from entering in summer and escaping in winter, is rated from about R-3.0 to R-4.5.

To put the numbers in perspective, a dense building material such as brick has a negligible R-value of R-0.2, while a batt of fiberglass insulation is rated at R-3.5 per inch of thickness.

Here are three high-tech alternatives to plain glass windows, including a gee-whiz unit that's wired to your home's electrical system.

Low-E glazing:
Low-E is short for low-emissivity, the name of an invisible metallic coating protected between two panes of glass. This coated layer is protected in a sealed space filled with an inert gas, usually argon, which is heavier than air and not as good a conductor of temperature.

The thin metallic film, barely the thickness of 200 atoms, screens out long-wave radiant- heat energy and ultraviolet (UV) rays that fade floors and fabrics while allowing shorter-wave, visible light to enter.
The Low-E coating offers benefits all year round. It blocks most UV rays without the need for blinds or tinted coatings. In winter, some of the radiant heat inside that would be lost through conventional glass is reflected back into living spaces, which reduces heating costs. In summer, heat radiating from driveways and patios that would pass into the house through ordinary glass panes is reflected, which reduces air conditioning costs. The energy saving is substantial because about two thirds of the heat lost through uncoated double-glazing is radiant heat.

When it comes to cost, the more energy efficient a window is, generally the higher the price tag. That tempts some suppliers to play numbers games by claiming unrealistic R-values taken from non-standardized measurements.

To avoid comparing apples and oranges, you can ask if the manufacturer subscribes to the rating procedures of the National Fenestration Ratings Council, based in Silver Spring. Its members include include such major manufacturers as Marvin, Hurd, Pella and Andersen. The council has a product directory, for $15, that lists all major window manufacturers' products.

There are more than 20,000 windows, of all sizes and types, that have been tested and certified. Write to the council at 1300 Spring St., Suite 120, Silver Spring, Md. 20910, or by calling 888-286-0732.