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U-factor
U-factor measures how easily heat moves through a window or door. Lower is better.
Why it matters
U-factor affects comfort near glass and affects heating and cooling costs.
Where people get this wrong
U-factor is not R-value. They are related but flipped. Lower U is better. A great U-factor does not fix a leaky install.
Real-world example
You sit near a big window in winter and feel cold even though the thermostat says 72. A lower U-factor window helps reduce that cold radiating feeling.
Where this hits your build
This comes up during selections and installation. Performance depends on the product and the install. A good window installed poorly still leaks.
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