U-Value & SHGC | GlasFëns Information

U-Factor, R-Value and Solar Heat Gain Coefficient (SHGC)

Three numbers describe how a window handles heat. The U-factor tells you how much heat flows through it. The R-value is the same information turned upside down. The solar heat gain coefficient (SHGC) tells you how much of the sun’s heat it lets in. Together with visible transmittance (VT), they are the numbers energy codes, Passive House and PHIUS designers, and energy modelers work with.

What is the U-factor?

The U-factor (in Europe, the U-value) is the rate of heat flow through a window for each degree of temperature difference between inside and outside. The lower the U-factor, the better the window insulates and the less heat it loses in winter.

In the United States it is given in Btu/h·ft²·°F. Europe and the rest of the world use W/m²·K. To convert, 1 Btu/h·ft²·°F = 5.678 W/m²·K. Our GR85 fixed window, for example, has a whole-window U-factor of 0.085 Btu/h·ft²·°F, which is 0.48 W/m²·K.

Because a window is made of different parts, you will see several U-values:

Term	What it describes
U_w (whole window)	The complete window: glass, edge of glass and frame together. This is the number that matters for the building.
U_g (center of glass)	The glass alone, measured at its center, away from the frame. Always lower than the whole-window value.
U_f (frame)	The frame alone.
Ψ_g (glass edge)	The extra heat lost along the edge of the glass, where the spacer sits. Measured in W/m·K, per length of edge.

U-factors are only comparable when they are rated the same way. In North America, whole-window U-factors are rated under NFRC 100 at standard sizes and winter conditions. In Europe and for Passive House certification they are calculated under EN ISO 10077, at different sizes and conditions. Always compare NFRC with NFRC, and certified with certified.

What is the R-value?

The R-value is thermal resistance, the inverse of the U-factor: R = 1 / U. The higher the R-value, the better. Windows are normally rated by U-factor, but the R-value makes it easy to compare a window with the insulated wall around it.

In U.S. units, R-value is given in h·ft²·°F/Btu. In SI units it is m²·K/W; multiply an SI R-value by 5.678 to get the U.S. value. Our ZERO 04 hybrid vacuum glass, for example, has a center-of-glass U-factor of 0.044 Btu/h·ft²·°F, which is R-23. Be careful to note whether an R-value is for the center of the glass or for the whole window: our GR85 fixed window with that glass is R-11.8 for the whole window.

How the frame affects the U-factor

The frame is typically 20 to 30% of a window’s area, so the material it is made of matters. Thermal conductivity, λ, measures how easily heat moves through a material; the lower, the better.

Frame material	λ, W/m·K	What it means for the window
GlasFëns polyurethane fiberglass	0.114	Insulates on its own, with no thermal break and no steel reinforcement. Measured by Intertek.
Softwood	0.13	Insulates well, but needs regular finishing and moves with moisture.
Rigid PVC (vinyl)	0.17	Insulates well, but most vinyl frames need galvanized steel inside for strength.
Hardwood	0.18	Insulates well, with the same upkeep as softwood.
Typical polyester fiberglass	0.40	Insulates far better than metal, but more than three times as conductive as ours.
Steel	50	Conducts heat readily; cold frames and condensation.
Aluminum	160	Conducts heat very readily; needs a plastic thermal break.

All values except ours are design values from EN ISO 10077-2, Annex A, the international standard for calculating window frame heat loss. Ours is an independent laboratory measurement by Intertek.

The profile design matters as much as the material. Our pultruded fiberglass profiles are foam-filled, so heat finds no open air path through the frame. The Passive House Institute certified our GR85 frame at U_f 0.72 W/m²·K, and it assumed a fiberglass conductivity of 0.27, more than twice what Intertek measured. See Material Choices for how our polyurethane fiberglass differs from other fiberglass.

How the glass affects the U-factor

The glass is most of the window, so it sets most of the U-factor. Four things bring it down:

More layers. Each additional pane and gap adds insulation.
Low-E coatings. Microscopically thin metal-oxide layers reflect radiant heat back where it came from.
Gas fill. Argon or krypton in the gap conducts less heat than air.
A vacuum. A vacuum gap cannot conduct or convect heat at all, so radiation is the only path left, and low-E coatings close it.

Glass	Typical center-of-glass U
Double glazing, low-E, argon	about 0.20–0.25 Btu (1.1–1.4 W/m²·K)
Triple glazing, two low-E coatings, argon	about 0.09–0.12 Btu (0.5–0.7 W/m²·K)
GlasFëns ZERO 04 hybrid vacuum glass	0.044 Btu (0.25 W/m²·K), R-23

The edge of the glass matters too. A metal spacer carries heat around the insulating glass; a warm-edge spacer does not. We use Super Spacer® Premium by Quanex, a silicone-foam spacer with no metal. See the ZERO glass packages for every option.

What is the solar heat gain coefficient (SHGC)?

The SHGC is the fraction of the sun’s heat striking a window that ends up inside the building, from 0 to 1. The lower the SHGC, the less solar heat the window lets in.

In hot climates, a low SHGC keeps out unwanted heat and cuts air-conditioning loads.
In cold climates, a higher SHGC on south-facing windows lets in free winter heat, which Passive House design puts to work.

The SHGC is set mainly by the glass and its coatings, and to a lesser degree by how much of the window is frame. In North America it is rated for the whole window under NFRC 200. Europe uses the g-value, which is rated for the glass alone under EN 410, so the two are not directly interchangeable.

Our ZERO packages tune the SHGC by climate: the coating on surface 2 is chosen for a high, neutral or low solar gain, while the coatings inside the vacuum keep the insulation the same.

What is visible transmittance (VT)?

Visible transmittance is the fraction of visible light that passes through the window, from 0 to 1. A higher VT means more daylight. Good low-E coatings keep VT high while controlling solar heat, so a window can be bright and still keep the heat out.

What is a thermally broken aluminum window?

Aluminum conducts heat very readily, so an aluminum frame on its own is a thermal bridge between inside and outside. A thermally broken aluminum frame is split into an inner and an outer section joined by a strip of low-conductivity material, usually glass-fiber reinforced polyamide, which slows the flow of heat through the frame. A thermal break makes a large improvement, but the aluminum on each side still conducts readily. Fiberglass needs no thermal break: the frame material itself is the insulator.