NFRC ratings: U-factor, SHGC and how to read them
The NFRC label is the US energy rating for a fenestration product, and it is what the model energy code reads to prove a window meets the code. It reports the whole assembly, not the glass alone: the U-factor (how fast heat moves through the unit), the solar heat gain coefficient (how much sun heat the unit lets in), the visible transmittance, and often air leakage and condensation resistance. Specify against the numbers the code and the climate zone ask for, not a brand claim.
The numbers on the label
- U-factor: heat flow through the whole unit — frame, spacer, glass and edge — in BTU per hour per square foot per degree Fahrenheit. Lower is better insulated.
- SHGC: the fraction of solar heat that enters, from 0 to 1. Lower blocks more sun heat; the right value depends on climate — a cooling climate wants it low, a heating climate accepts it higher.
- VT: visible light transmitted. This is the daylight number, separate from the heat numbers.
- Air leakage and condensation resistance where the product is rated for them: the airtightness of the assembly, and how it handles cold-weather condensation.
Why it is the whole unit, not the glass
A pane can measure well in a laboratory and still underperform in a frame: the edge of the IGU, the spacer and the frame material are all part of the heat path, and on a poorly detailed unit the edge is where the heat goes. NFRC rates the assembled product — frame, glazing, spacer and operating hardware together — which is why the rating is the honest number for a schedule. The configured unit is what is rated, so a change of glass or frame changes the label.
How it meets the code
The model energy code sets the target U-factor and SHGC for a window in each climate zone, and the code official accepts the NFRC label as the proof. A US project therefore specifies to the zone: the cooling zones lean on a low SHGC and a low U-factor; the heating zones trade the SHGC back for winter gain. The drawing office carries the zone targets and configures the unit to meet them, and the label for that configuration travels with the shipment.
What to put on the schedule
- The climate zone and the code year the project is on.
- The target U-factor and SHGC for that zone, as the code states them.
- The configuration that achieves them — glass build, spacer, fill — so the rated unit and the shipped unit are the same.
- Ask for the NFRC certificate for the configured product, not a general value.
Questions
For insulation, yes — lower means less heat moves through. But the SHGC has to be balanced with the climate: a very low SHGC in a cold zone throws away useful winter gain.