Read the NFRC label, not the brochure
U-factor is heat flow — lower is better in heating climates. SHGC is how much solar heat gets in — lower helps cooling loads, higher can help passive solar in cold, sunny winters if you manage glare.
- Air leakage (AL) should be low; a great glass package with a leaky sash still drafts
- Visible transmittance (VT) is daylight, not efficiency
- Condensation resistance is comfort in cold weather, not a substitute for insulation
Low-E coatings and gas fills
Low-E coatings reflect infrared. The coating surface (inside vs outside of the gap) is chosen for heating vs cooling. Argon is standard; krypton appears in thinner triple-pane gaps. Spacers at the glass edge are a weak point — warm-edge spacers reduce condensation.
- South and west elevations in hot climates: control SHGC and add overhangs or blinds
- North elevations in cold climates: low U-factor matters most
- Do not skip insulation and air sealing around the new frame
Install quality is half the efficiency
A high-performance unit in a gaping rough opening with missing backer rod and sealant will still leak air. Full-frame replacement lets the crew flash the opening; inserts are faster but keep the old frame if it is square and sound.
- Ask how they flash sills and integrate with housewrap
- Interior and exterior sealant joints should be specified, not “as needed”
- Keep the sticker until you confirm the delivered unit matches the bid
Air leakage and install beat a tiny U-factor gap
A slightly better glass package in a sloppy insert install loses to a simpler unit with a pan flashing and a dense-packed gap. Ask for the whole-window NFRC numbers and the install method. SHGC should match your climate, not a national “lowest is best” myth.
- Hot climates often want lower SHGC on west glass
- Cold climates care more about U-factor and a warm edge spacer
- Use the cost guide for dollars; use this page for glass and frame performance