Energy efficiency
What the glass numbers are for.
Energy-efficient replacement windows are chosen for comfort and heat control. Savings are not guaranteed. Ratings belong to a specific window, size, and glass package, not to a brand name alone.
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U-factor
U-factor describes heat transfer through the whole window. A lower number means the unit insulates better. The ordered window’s NFRC label is the number that counts.
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Solar heat gain
SHGC describes how much solar heat passes through the glass. In a cooling-heavy climate, a lower solar heat gain coefficient is often part of the glass conversation.
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Light
Visible transmittance describes how much daylight comes through. Glass that controls heat can also change the light in the room.
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Installation still matters
A strong glass package does not finish the job. Fit, fastening, and how the opening is prepared are part of what a Window Specialist evaluates.