Window performance is defined by four measured properties — U-factor, solar heat gain coefficient (SHGC), air leakage, and visible transmittance. Each affects a different dimension of comfort and energy use, and the right balance depends on climate zone, home orientation, and what you are trying to improve. Understanding these ratings helps you evaluate window specifications before brand selection begins.
U-factor: the heat-flow number
U-factor measures heat transfer through the window assembly per unit of temperature difference. In Minnesota’s climate zone 6, code requires a maximum of 0.32. High-performance windows achieve 0.20–0.27. The practical difference between a 0.32 and a 0.22 U-factor window shows up on the coldest January nights — the interior glass surface is warmer with lower U-factor, which eliminates the cold-radiant effect that makes large windows feel uncomfortable in winter even in a heated room. For windows larger than 6 square feet, U-factor is the most important specification.
SHGC: the solar gain lever
SHGC ranges from 0 to 1. A value of 0.40 means 40% of solar energy passes through the glass. Higher SHGC captures passive solar heat in winter — valuable on south-facing windows in Minnesota’s heating-dominated climate. Lower SHGC reduces summer cooling load on west and southwest exposures. The specification decision is orientation-dependent: a single SHGC value applied to all windows in a project is usually a compromise. We specify by elevation when glazing area is significant.
Air leakage: how windows actually feel drafty
Air leakage (AL) is tested at a standardized pressure differential and expressed in cubic feet per minute per square foot of frame. Code maximum is 0.30 cfm per square foot, but that threshold is not difficult to achieve — most decent windows test at 0.10 or below. Premium fiberglass windows from Marvin test at 0.02–0.06. The difference between 0.30 and 0.05 is felt as draft near the window on windy days — not visible in an energy model, but a real comfort complaint in Minnesota winters.
Visible transmittance: the daylight trade-off
VT measures the fraction of visible light that passes through the glass. High VT (0.60+) keeps rooms bright. Low-emissivity coatings that improve U-factor and SHGC can reduce VT — especially triple-pane or high-solar-control glazing. Most projects balance this at 0.40–0.55. Rooms that rely on natural light for function — studios, home offices — should specify VT explicitly rather than accepting the default for a given efficiency package.
Reading a window specification correctly
NFRC-certified window labels report whole-window performance — the rated values include both the center-of-glass and edge conditions. Some manufacturers report center-of-glass U-factor in marketing materials because it is more favorable. Always use NFRC whole-window values for comparison. When evaluating competitive bids, confirm that all suppliers are quoting the same NFRC ratings for the specified unit — not a comparable unit with a lower price and a higher U-factor.
Related planning resources
If you are specifying windows for a remodel or new build and want to review performance targets before the order is placed, we can walk through the options by elevation.
Related: Windows and Energy Efficiency in Minnesota Homes • Insulation Planning for Minnesota Homes • Exterior Renovation Services