Thermal Performance | GlasFëns Information

Thermal Performance of Windows

A window is usually the weakest part of a building’s envelope. Even a good one loses heat several times faster than the insulated wall around it, so windows have an outsized effect on energy use, comfort and condensation. This page explains how window heat loss works, how it is rated, how to read performance claims, and what each part of the window contributes.

How heat moves through a window

Heat crosses a window in three ways:

Conduction through solid materials: the frame, the spacer at the edge of the glass, and the glass itself.
Convection of gas inside the cavities between panes, and of air against the inside and outside surfaces.
Radiation, the infrared heat every warm surface gives off. In a plain double pane, radiation is about two-thirds of the heat lost across the gap, which is why low-E coatings make such a difference.

Every part of a window plays a role, so its thermal performance is described in three zones: the center of the glass, the edge of the glass (the band about 2.5 in, or 63 mm, wide next to the frame, affected by the spacer), and the frame. The whole-window value combines all three by area.

U-factor and R-value

The U-factor is the rate of heat flow through the window per unit of area and temperature difference; lower is better. The R-value is its inverse; higher is better. In the U.S., U-factor is given in Btu/h·ft²·°F; elsewhere in W/m²·K (1 Btu/h·ft²·°F = 5.678 W/m²·K). See U-Value & SHGC for the full definitions.

How windows are rated

NFRC 100 (North America). The whole-window U-factor is calculated in LBNL THERM and WINDOW at a standard size for each product type, under standard winter conditions (0 °F outside, 70 °F inside), and validated by a guarded or calibrated hot-box test. NFRC ratings are what U.S. energy codes and ENERGY STAR accept. Commercial projects can also use NFRC’s Component Modeling Approach to rate the actual frames and glass of a specific building.
EN ISO 10077 (Europe and the Passive House Institute). Similar in principle, but with different standard sizes, boundary conditions and rules, so results differ from NFRC for the same window.
Phius accepts either method through its window certification program.

See NFRC & ENERGY STAR and Passive House & Phius.

Reading performance claims carefully

Window U-values are easy to present in a flattering way. Before comparing products, check:

Center of glass or whole window? The center-of-glass value is always the best number a window has. Codes, energy models and comfort depend on the whole window. Our own ZERO 04 glass is R-23 at the center of the glass; the GR85 fixed window that holds it is R-11.8 for the whole window, and we publish both, clearly labeled.
Which units? A U-value of 0.80 is excellent in W/m²·K and poor in Btu/h·ft²·°F. Many European figures are quoted in SI units without saying so.
Which method and size? NFRC and European values are not interchangeable, and larger windows usually rate better than small ones because they have proportionally more glass and less frame. Compare at the same standard size, by the same method.
Tested, certified or calculated? A certified rating has been simulated, tested and independently inspected. A calculated value is a model. Both are useful, as long as they are labeled; every number on our site says which it is.
Which glass package? A frame series is often advertised with its best glass, while quotes come with standard glass. Ask for the value of the window you are buying.
Will it last? Gas fills slowly escape from insulating units over the years, and low-E coatings vary in durability. Ask about gas retention and seal warranties, not just day-one numbers.
Does the difference matter? Differences of a few hundredths in U-factor are within the uncertainty of modeling and testing. They should not decide between two products on their own.

The frame

The frame is typically 20 to 30% of a window’s area, and in a small operable window even more. Its material and design set how much heat it carries:

Aluminum conducts heat roughly 1,000 times faster than wood or vinyl, so it needs a thermal break, a strip of low-conductivity polyamide that splits the frame into inside and outside halves.
Vinyl and wood insulate well, but most vinyl frames need galvanized steel reinforcement inside, which conducts heat.
Fiberglass insulates without a thermal break or steel. Our pultruded polyurethane fiberglass measured λ 0.114 W/m·K in testing by Intertek, lower than rigid PVC and softwood, and our profiles are foam-filled so heat finds no open air path through them.

Frame depth, the number of chambers, and how deeply the glass is set into the frame all matter as well. See Material Choices.

The edge of the glass

The spacer that separates the panes at their edges is a thermal bridge around the whole perimeter of the glass. Its effect is measured by the linear thermal transmittance Ψ_g in W/m·K.

Aluminum spacers, once standard, carry heat straight around the insulating glass, leaving a cold band at the bottom of the glass where condensation forms.
Stainless steel and hybrid spacers conduct less, but still contain metal.
Non-metal warm-edge spacers, such as silicone foam or thermoplastic (TPS) spacers, perform best. We use Super Spacer® Premium by Quanex, a silicone-foam spacer with no metal; the Passive House Institute certified our GR85 glass edge at Ψ_g 0.020 W/m·K.

Setting the glass deeper into the frame also shields the edge and reduces its heat loss.

The glass

The glass is most of the window, so it sets most of the U-factor. Its performance comes from the number of layers, low-E coatings, the gas or vacuum between the panes, and the spacer.

Glass type	Typical center-of-glass U, Btu (W/m²·K)	Notes
Single pane	about 1.0 (5.7)	No longer allowed in conditioned buildings by most codes.
Double, clear, air	about 0.48 (2.7)	Common before low-E coatings.
Double, low-E, argon	about 0.20–0.25 (1.1–1.4)	Today’s typical code-minimum glass.
Triple, two low-E coatings, argon	about 0.09–0.12 (0.5–0.7)	The cold-climate and Passive House standard for many years.
Vacuum insulating glass (VIG)	about 0.07–0.12 (0.4–0.7)	As good as triple glazing, in a unit only about 6 to 10 mm thick.
GlasFëns ZERO 04 hybrid vacuum glass	0.044 (0.25), R-23	An argon insulating unit and a vacuum unit combined.

Low-E coatings are microscopically thin, transparent metal or metal-oxide layers that reflect infrared heat. Where they sit matters: surfaces are numbered from the outside in, and coatings on different surfaces tune insulation and solar heat gain separately. Gas fills such as argon, or krypton in narrow gaps, conduct less heat than air.

Triple glazing adds a third pane and a second gas-filled gap. It roughly halves heat loss compared with double low-E glazing and keeps the inside surface warm, but it is about 50% heavier and much thicker, so it needs deeper, stronger frames and hardware to carry it.

Vacuum insulating glass replaces the gas with a vacuum about 0.3 mm thick between two panes, held apart by a grid of tiny pillars and sealed hermetically at the edge. With no gas to conduct or convect, only radiation is left, and low-E coatings on the vacuum faces control it. A vacuum unit insulates like triple glazing at a fraction of the thickness and weight, and its performance does not change with altitude or the angle of the glass.

Hybrid vacuum glass combines both: an outer pane and an argon-filled gap in front of a vacuum unit. Our ZERO packages are built this way. The argon unit protects the vacuum unit from the full temperature swing outdoors, and together they reach R-23 at the center of the glass, in a unit that still fits a standard glazing pocket. See the glass.

Installation and thermal bridges

A window’s rating describes it on its own. Once installed, heat also flows around it, through the joint between frame and wall. Passive House designers measure this as the installation thermal bridge, Ψ_install. It is smallest when the window sits within the wall’s insulation layer and the insulation laps over the frame. The Passive House Institute certified installation values for our GR85 window in three typical high-performance walls. See Installation and Air Barrier Interface.

Comfort and condensation

Thermal performance is also about people. The colder the inside surface of a window, the more radiant chill and downdraft people feel near it, and the higher the risk of condensation and mold. Two measures describe this:

Condensation resistance (CR), NFRC 500, rated from 1 to 100; higher is better.
Temperature factor f_Rsi, used by the Passive House Institute: the coldest inside surface temperature relative to indoor and outdoor temperatures. Passive House requires at least 0.70 for cool, temperate climates and 0.75 for cold ones; our GR85 is certified at 0.77.

See Condensation Resistance.

Balanced design

U-factor is one part of a window’s energy performance. Solar heat gain (SHGC), visible light (VT), air leakage, durability and condensation resistance all have to be balanced for the climate, the orientation and the building type. In homes and in Passive House buildings, heating usually dominates in cold climates and a low U-factor pays back every winter. In large commercial buildings with high internal loads, cooling can dominate, and solar control and daylight matter as much as insulation. A good energy model will show where the balance lies; see Air Infiltration and Daylighting.

Our numbers

GlasFëns value	Result	Basis
ZERO 04 glass, center of glass	U 0.044 Btu (0.25 W/m²·K), R-23	Calculated
GR85 fixed window, whole window	U 0.085 Btu (0.48 W/m²·K), R-11.8	Calculated, NFRC 100 size
GR85 dual action window, whole window	U 0.093 Btu (0.53 W/m²·K), R-10.8	Calculated, NFRC 100 size
GR85 frame, U_f	0.72 W/m²·K	Certified, Passive House Institute
GR85 glass edge, Ψ_g	0.020 W/m·K	Certified, Passive House Institute
Polyurethane fiberglass, λ	0.114 W/m·K	Tested, Intertek

Every system is on the performance page, and energy consultants can ask us for THERM files.