What Is a Good U-Factor for Windows?
The rate of heat loss is indicated in terms of the U-value. The lower, the greater a window’s resistance to heat flow and the better its insulating properties.
U-factor measures how well a product prevents heat from escaping a home or building. U-factor ratings generally fall between 0.15 and 1.20. Products with lower U-value are good at keeping heat inside the building. U-factor is particularly important during the winter season in colder climates. This label displays U-factor in U.S. units. Labels on products sold in markets outside the U.S. may display U-factor in metric units.
What’s the Difference between U-factor and R-value?
The biggest difference between U-factor and R-value is that U-factor measures the rate of heat transfer (or loss) while R-value measures the resistance to heat loss. R-value is a measure of conductance and resistance. A product with high conductance will conduct heat quickly, like a hot pan on the stove or a single pane of glass on a cold day. U-factor, on the other hand, takes into account more than conductance. It is also affected by the airflow (convection) around the window and the emissivity (radiated or reflected heat) of the glass.
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Typical U-factors of our windows
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uPVC tilt and turn windows: from 0.24 for standard double glazing down to 0.13 with triple glazing
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Energy Plus uPVC: 0.21
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Energy Ultra uPVC with triple glazing: as low as 0.14
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Aluminum windows: MB-79N 0.25 to 0.39, MB-86 0.16 to 0.35, MB-104 Passive 0.14 to 0.21, MaxLight 0.15 to 0.37
The exact value depends on the profile, the glass and the size of the window. Every quote lists the U-factor of each unit.
How to read U-factor on a window label
U-factor on an NFRC label is given for the whole window, frame and glass together. A glass-only value (Ug) is always lower than the whole-window value. When you compare products, compare whole-window numbers.
U-factor and R-value at a glance
R-value is roughly the inverse of U-factor. A window with U = 0.20 has an R-value of about 5.
Window performance guide