Windows 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.