Windows do more than let light in. They're active thermal boundaries, daylighting tools, and code compliance checkpoints rolled into one assembly. Get the glazing selection wrong and you're fighting comfort complaints, blown energy budgets, and potential code violations for the life of the building.

Three performance numbers drive almost every glazing decision on the ARE: U-value (thermal transmittance), solar heat gain coefficient (SHGC), and visible light transmittance (VLT, sometimes called visible transmittance or Tvis). Each one measures a different aspect of how energy and light move through glass. The frame material ties directly into all three, because a high-performance glazing unit in a thermally unbroken aluminum frame can lose most of its advantage at the edges.

Energy codes like ASHRAE 90.1 and the IECC set prescriptive maximums for U-value and SHGC by climate zone. The National Fenestration Rating Council (NFRC) provides the testing and labeling system that verifies manufacturer claims. When NFRC labels aren't available, default table values apply, and those defaults are deliberately conservative, representing worst-case performance for any given window description.

For PPD, the task isn't just knowing these metrics. You need to evaluate trade-offs between competing performance goals, select assemblies that satisfy program requirements alongside code limits, and assess how glazing decisions ripple through mechanical system sizing, occupant comfort, and construction cost.