Q: Windows-What is a U-value?
A: Who would think that understanding how windows would perform is so complicated. AAMA (American Architectural Manufacturers Association) has a product certification that helps. Their website is www.aamanet.org/general/2/127/product-certification.
Cars have MPG (miles per gallon) to rate their efficiency. Insulation has an R- value. Homes have the HERS (home energy rating system). Just about every product for a home has some sort of grading system. Windows have the U- value.
What is good? Good for the U- value and HERS rating is when the number is lower. MPG and R- value are better when the number is higher.
The rate of heat loss is indicated in terms of U-factor (U-value) of a window assembly. The lower the U-factor, the greater a window’s resistance to heat flow. The better its insulating properties.
The U-factor is used to express the insulation value of windows. R- value is used for the insulation in most other parts of the building envelope (walls, floors and roofs). To compare R-value and the U-factor-divide 1 by the U-factor number. For example-What is the R-value of a window that has a U-value of .25? Answer-1/.25= 4. R-value being 4.
What does all this mean? Building codes vary by area. Some require Windows with a U-value of .35. Others .32. A good green building program has windows at .30 or lower.
A good place to learn more about what is on a window label is: www.nfrc.org/label.aspx.
Keep in mind-The U-value is important-how that window is installed is also. When trying to achieve efficiency-it’s great to purchase an efficient product. It reminds of somebody buying a fuel efficient car. The desire is to spend less on fuel yet they forget about maintaining proper tire pressure in their tires.
Windows are no different. Next week’s topic will be proper window installation.
Build well and enjoy life.
Please submit questions to firstname.lastname@example.org Jon can be reached direct at 360.907.5800. Tune into Green Building with Jon Girod on 1550 a.m. on Saturdays at 9 am. See you next week!