Window glossary

U-factor measures how readily heat escapes through the whole window assembly. Lower is better. NFRC lists the range as 0.20 to 1.20. SHGC measures how much solar heat the window lets in, on a 0 to 1 scale; low is good in hot sunny climates, less so in cold ones where free winter heat helps.

Visible transmittance (VT) is how much daylight comes through, 0 to 1, higher being brighter. Air leakage is infiltration through the assembly, with lower better and NFRC certifying at 0.3 or below. Condensation resistance is a comparative rating where higher performs better.

IGU is the insulated glass unit — two or three panes sealed together with a spacer and often an inert gas fill. When the seal fails you get permanent fog. Low-E is a microscopically thin coating that reflects heat; where it sits in the assembly changes whether it mainly blocks solar gain or retains indoor heat.

Sash is the moving frame holding the glass; glazing is the glass itself; weep holes are the drainage slots at the bottom of the frame; egress refers to the emergency escape and rescue opening requirements in the residential code your jurisdiction adopted.

How to choose

Ask for label numbers, not adjectives

"Energy efficient" is not a specification. U-factor, SHGC, VT and air leakage are.

Interpret SHGC by climate zone

ENERGY STAR criteria are set per climate zone. The right SHGC in Phoenix is wrong in Minneapolis.

Know insert vs full-frame vocabulary

"Pocket" or "insert" replacement keeps the existing frame. "Full-frame" or "new construction" removes it to the rough opening.

Understand what a spacer is

The spacer separates the panes in an IGU. It is a real durability factor, so ask what type is used, but be skeptical of unsupported longevity claims about it.

Watch for proprietary names

Manufacturers brand their glass packages. Ask what the NFRC-rated numbers are for that specific package so you can compare it to another brand.