Every window datasheet leads with two numbers, and they are routinely read as if lower were better for both. For one of them that is true. The other one is a valve, not a leak — and setting it wrong costs real heating or cooling energy every year the building stands.
Two numbers, two different questions
U-value (thermal transmittance; U-factor in North America) answers: *when it is colder on one side than the other, how fast does heat conduct through?* Units are W/m²K in the SI world, Btu/h·ft²·°F in NFRC ratings (multiply IP by 5.678 to get SI). It acts 24 hours a day in both seasons — heat out in winter, heat in during a cooling season. Lower is unambiguously less loss. The whole-window number is assembled from glass, frame, and spacer contributions — [the EN ISO 10077-1 mechanics are here](/resources/blog/en-iso-10077-window-u-value-calculation).
SHGC (solar heat gain coefficient, per NFRC 200) answers: *of the solar energy hitting the window, what fraction ends up inside?* It is dimensionless, 0 to 1, and counts both directly transmitted radiation and the part absorbed in the glazing and re-emitted inward. A skylight at SHGC 0.60 is a heater whenever the sun is out; a curtain-wall unit at 0.20 is sunglasses. Whether high or low is "better" depends entirely on whether the building wants that free heat.
The same physics wears a different label in Europe: the **g-value** (total solar energy transmittance, EN 410). Numerically g and SHGC land within a few percent for the same glazing — but with a reporting trap: European datasheets usually quote g for the **glazing alone**, while NFRC's SHGC is a **whole-product** number that includes the frame, which admits no solar gain and therefore drags the value down. A European IGU at g 0.50 and an NFRC window at SHGC 0.40 may be the same glass in the same window. Confirm the basis before comparing.
Setting both by climate
Heating-dominated climates want minimum U and a usefully high SHGC: every watt of winter sun through south glazing is heat the boiler does not supply. ENERGY STAR Version 7.0 encodes this — the Northern zone prescriptive path requires **U ≤ 0.22 Btu/h·ft²·°F (≈ 1.25 W/m²K) together with SHGC ≥ 0.17**, an explicit floor on solar gain. Passive house practice pushes the same logic harder: in cool-temperate climates, certified projects typically pair triple glazing around Ug 0.5–0.7 W/m²K with g-values high enough that south windows net positive over the heating season — the [PHI requirement set is covered here](/resources/blog/passive-house-window-u-value-requirements).
Cooling-dominated climates flip the priorities. ENERGY STAR 7.0's Southern zone allows U ≤ 0.32 IP but caps **SHGC at ≤ 0.23**; South-Central requires U ≤ 0.28 with the same SHGC cap. In Riyadh, Singapore, or Phoenix, solar gain is the enemy and spectrally-selective low-E coatings that pass daylight while rejecting near-infrared do the heavy lifting.
Mixed climates are where single-number thinking fails hardest. North-Central ENERGY STAR (U ≤ 0.25, SHGC ≤ 0.40) is a compromise; serious projects go finer-grained — orientation-specific glazing, with higher SHGC on south faces shaded by overhangs and lower on east/west faces where low-angle summer sun is unshadeable. Phius's climate-zone-specific window data takes the same orientation-aware approach.
Three practical rules survive all three cases. Set U-value by climate severity and comfort (it also controls the interior surface temperature that decides [condensation behaviour](/resources/blog/aluminum-window-condensation-cold-climate)). Set SHGC by cooling load and orientation. And never chase one number with a product that wrecks the other — a triple-silver coating that hits SHGC 0.18 in a heating climate saves cooling the building did not need and forfeits solar heating it did.
Where the frame sits in both numbers
Coatings and gas fills live in the glazing, so SHGC tuning is mostly a glass decision — the frame only enters by occupying area (NFRC's whole-window SHGC falls as frame fraction rises). U-value is the opposite: on a typical window the frame is the thermally weakest zone and sets the floor under the whole-window number. A frame at Uf 2.5–4.0 W/m²K (thermally-broken aluminum territory) caps how far any glazing can carry the assembly; insulating frames — pultruded GFRP at roughly 0.3 W/m·K bulk conductivity, timber, uPVC — let premium glazing keep its rating at whole-window scale. The frame-material trade-offs, including why GFRP holds slim aluminum-like sightlines while insulating, are on the [FRP vs aluminum comparison](/technology/frp-vs-aluminum-windows); F1's PHI-certified GFRP-PU [fenestration systems](/products/fenestration-systems) are the applied case.
To see both numbers interact on a real build — your frame, glazing package, and spacer, computed to EN ISO 10077-1 with program pass/fail flags — run the free [whole-window U-value calculator](/technology/u-value-calculator), then sanity-check the SHGC against your climate zone's table above.
