How the two differ in kind
Every window carries two performance numbers and they answer different questions. Treating them as a single 'better glass' axis is how a specification ends up wrong for its climate.
The U-FACTOR describes CONDUCTION: how much heat passes through the assembly per unit area for each degree of temperature difference across it. It covers the glass, the gas fill, the spacer and the frame together, and lower is better in every climate — it reduces loss in winter and gain in summer alike. It is also the number that decides whether the inner surface is warm enough to avoid condensation, and whether sitting near the window is comfortable.
The SOLAR HEAT GAIN COEFFICIENT describes SOLAR energy: the fraction of incident sunlight that ends up as heat inside, counting what passes straight through plus the share of what the glass absorbs and re-radiates inward. Here 'lower is better' is conditional. In a cooling-dominated climate a low SHGC keeps out the sun that the air conditioning would otherwise have to remove, and it is one of the most effective specification choices available. In a heating-dominated climate the same low SHGC rejects winter sunlight that was free heat, on the elevation that receives most of it.
That is the trap: specifying the lowest available figure for both because both are performance numbers. Low U-factor is right everywhere. Low SHGC is right where cooling governs and costly where heating does.
The second point is that SHGC is an ORIENTATION question, not a building one. An equator-facing elevation receives useful winter sun at a high enough altitude to be shaded in summer by a modest overhang, so it often wants a moderate-to-high SHGC. East and west elevations receive low-angle sun that no horizontal shading excludes and that arrives when the building is already warm, so they usually want a low one. A single whole-house specification cannot be right for both.
The factors that actually differ
Show
| U-factor | Solar heat gain coefficient | |
|---|---|---|
| What it measures | Conduction through the whole assembly — glass, gas, spacer and frame. | The fraction of incident solar energy that ends up inside. |
| Is lower always better | Yes, in every climate and on every orientation. | No. Lower is better where cooling dominates and harmful where heating does. |
| Climate dependence | None in direction; the value of improving it rises with the severity of the climate. | Total. It is the number that flips with the climate. |
| Orientation dependence | None. | Strong. Equator-facing glass often wants a higher figure; east and west want a lower one. |
| What improves it | More panes, low-emissivity coatings, inert gas fill, warm-edge spacers, and a better-insulated frame. | Spectrally selective coatings, tinted or reflective glass — and external shading, which acts before the glass. |
| Comfort effect | Large. A cold inner surface radiates away from the occupant and creates a draught by convection. | Large in the other direction — direct sun on a person is uncomfortable regardless of air temperature. |
| Condensation | Directly related. A warmer inner surface resists condensation, which is why the frame and spacer matter. | Not related. |
| Daylight | Unaffected. | Related but not identical — visible transmittance is a separate figure, and a spectrally selective coating separates the two deliberately. |
| Alternative route | There is none; it is a property of the assembly. | External shading, which rejects solar gain before the glass and can be seasonal where fixed SHGC cannot. |
| Where it is specified | Whole building, usually one target. | Ideally by orientation, which a single whole-house spec cannot do. |
Which one, and when
Choose u-factor when…
- A heating-dominated climate, where conduction loss over a long winter is the dominant term.
- Condensation or cold-surface discomfort is a concern, which the U-factor governs.
- Comparing whole windows rather than glass, since the frame and spacer are in the figure.
- Always — a lower U-factor is never the wrong direction.
Choose solar heat gain coefficient when…
- A cooling-dominated climate, where solar gain is the dominant load.
- East and west elevations in any climate, where low-angle sun cannot be shaded horizontally.
- Large glazed areas, where the solar gain through them governs the cooling plant.
- Assessing shading, since the effective SHGC of a shaded window is what actually matters.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why is a low SHGC bad in a cold climate?
- Because it rejects heat you wanted. In a heating-dominated climate, sunlight entering through an equator-facing window in winter is useful energy arriving at no cost — it offsets heating that would otherwise be bought, on the days when heating demand is highest. A low-SHGC glass on that elevation rejects a large share of it, so the building loses free heat all winter to avoid a summer gain that a modest overhang could have excluded geometrically. The effect is real enough that passive solar design deliberately specifies HIGH SHGC glass on the sun-facing elevation and controls summer gain with shading instead. The nuance is that the argument is orientation-specific: the same house wants low SHGC on its east and west glass, where the sun arrives low and cannot be shaded.
- Why can't horizontal shading handle east and west?
- Because those elevations see the sun at low altitude, and a horizontal projection shades by intercepting sunlight from above. In the morning and evening the sun is close to the horizon and its rays arrive almost parallel to the ground, passing under any overhang however deep. That is also when they hit east and west glass — so the orientation that most needs shading is the one geometry cannot shade. The consequences follow directly: east and west glazing is where a low SHGC specification earns its place, where vertical fins rather than overhangs are the shading device, and where reducing the glazed area is a legitimate design response. West is the worst of the two, because its gain arrives in the afternoon when the building has already absorbed a day of heat.
- How do low-emissivity coatings affect both numbers?
- They improve U-factor and they can be tuned to do very different things to SHGC, which is why 'low-E' is not a single product. All low-emissivity coatings reduce radiant heat transfer across the cavity, lowering the U-factor. Where they differ is in their treatment of the solar spectrum. A coating optimised for a heating climate is placed to reflect interior heat back inward while admitting a high proportion of solar energy — high SHGC, low U-factor. One optimised for a cooling climate is spectrally selective, reflecting the infrared part of sunlight while transmitting visible light — low SHGC with a high visible transmittance, so the room stays bright without the heat. Specifying 'low-E glass' without saying which is how a cooling-climate coating ends up on a heating-climate building.
- Does the frame count?
- Substantially, and it is the reason a whole-window figure differs from a centre-of-glass one. The U-factor quoted for a window is normally the whole assembly — glass, spacer and frame area-weighted together — while the centre-of-glass value describes only the middle of the pane, away from the edges. The difference is not small: a thermally poor frame and a conventional aluminium spacer conduct far more than the glass they surround, and on a small window the frame is a large proportion of the area. That is why warm-edge spacers exist and why frame material and thermal break design matter. When comparing products, the figures have to be the same kind: a centre-of-glass number set against a whole-window number flatters the first considerably.
- How does shading relate to SHGC?
- It reduces the EFFECTIVE solar gain without changing the glass, which is often the better lever. An external device — an overhang, fins, louvres, a shutter, a tree — rejects sunlight before it reaches the glass, so the energy is released outdoors rather than entering as heat. The effective SHGC of a shaded window is therefore the glass's own figure multiplied by the fraction of sun reaching it, which for a well-designed overhang on an equator-facing facade can be very small in summer and close to unity in winter. That seasonal behaviour is exactly what a fixed glass specification cannot provide, and it is why the strongest arrangement in a heating climate is high-SHGC glass with geometric shading rather than low-SHGC glass all year.
- Should the specification differ by elevation?
- Ideally yes, and the reason a single whole-house specification persists is procurement rather than physics. An equator-facing elevation in a heating climate wants a high SHGC and shading; east and west want a low SHGC; and the pole-facing elevation gets essentially no direct sun, so its SHGC barely matters while its U-factor matters most. Specifying by orientation captures all of that. What stops it is that ordering two or three glass specifications for one building costs more, complicates the supply, and creates a risk of units being installed on the wrong elevation — which is a real failure mode and one that is invisible afterwards. Where the glazed area is large enough for the energy difference to be significant, the added complexity is usually worth it; on a small house it frequently is not.
- What about visible transmittance?
- It is a third number, related to SHGC and deliberately separable from it. Visible transmittance is the fraction of visible light the glazing admits, and it governs daylight and the view. A simple tinted or reflective glass reduces SHGC and visible transmittance together, which darkens the room — historically the reason heavily tinted buildings need their lights on all day. A spectrally selective coating separates them, reflecting the invisible infrared that carries most of the solar heat while transmitting the visible part, so the room stays bright with a much lower solar gain. The ratio between the two is sometimes quoted as a light-to-solar-gain figure, and it is the number to look at when the requirement is a cool room that is still daylit.
- Which matters more?
- It depends on the climate and on how much glass there is, which is why energy codes set both and weight them by climate zone. In a severe heating climate with modest glazing, the U-factor dominates: conduction loss acts through every hour of a long winter, day and night, on every elevation. In a cooling-dominated climate with substantial glazing, SHGC dominates: solar gain through glass is frequently the largest single component of the cooling load. Mixed climates are the difficult case and are where orientation-specific specification and shading pay most. The practical way to settle it for a specific building is the load calculation that would be run anyway, with the glazing specification varied — which turns a general argument into a number for that building on that site.