Ask for the thermal performance of a window or curtain wall and you will usually get one number: the U-value. It looks simple and it is easy to compare. But the same word is used for the glass in the middle of a pane, for a frame profile, and for a complete installed unit. They are different measurements with different results. Comparing a centre-of-glass figure with a whole-window figure leads to bad decisions. This article explains what is included in each.
What a U-value measures
A U-value describes how much heat passes through a building element for each degree of temperature difference across it, per unit of area. A lower number means the element resists heat flow better. It is a steady-state measure: it does not describe how fast a wall warms up, nor does it include sunlight, which is handled by a separate figure.
For a facade, the practical meaning is simple. A low U-value reduces heat loss in cool weather and reduces heat gain from outside warmth in hot weather. In an air-conditioned building, the effect shows up as cooling load and comfort next to the glass. The number is useful, as long as you know what it covers.
Three figures that are often confused
The first is the centre-of-glass value, which describes only the middle of the glazing unit, away from the edges. It is the most flattering figure and the one most often printed in glass brochures. The second is the frame value, describing the profile alone. The third is the whole-window or whole-assembly value, which combines glass, edge, frame and sometimes the way the unit is installed.
The whole-assembly number is almost always less favourable than the centre-of-glass number, because heat is lost faster through frames and glass edges. It is also the number that matches what the building experiences. When two suppliers quote different figures, check first that both are describing the same thing.
Why the edge and the frame matter
Glass edges are thermal weak points. The spacer that separates the two panes of a double-glazed unit conducts heat more readily than the gas fill, so the band near the edge performs worse than the centre. The effect is larger in small panes, where edge is a bigger share of the area, and smaller in large ones.
Frames matter even more in aluminium systems, because metal conducts heat strongly. A thermal break is a barrier of low-conductivity material placed between the inside and outside parts of the frame, cutting that path. Our article on thermal breaks in aluminium frames explains how it works. Without one, the frame can undermine good glass.
Glass build-up changes the figure
Within a double-glazed unit, several choices alter the U-value: the width of the gap, the gas in it, the coatings on the surfaces and the type of spacer. A coating that reflects heat radiation lowers the figure, and so can a gas that conducts less heat than air. Triple-glazed units reduce it further, with a cost in weight and thickness.
Our guide to low-E coatings and the article on what is inside an insulated glass unit give more detail. The key point for specifiers is that the build-up must be written in full, with the coating surface identified. A figure without a build-up cannot be checked.
Spacers deserve a mention of their own. The material and design of the edge spacer changes how much heat passes at the perimeter of the glass, and some spacers are designed specifically to reduce it. If edge performance matters on your project, ask which spacer is proposed and whether the supplier's figure includes its effect. A specification that names only the glass layers and the gas may leave this unstated, and the difference shows up in the whole-window figure.
Size and configuration are not neutral
A quoted whole-window figure normally refers to a standard test size or a calculation for a defined configuration. A real window may be much larger, much smaller, or have a different split between fixed and opening parts. Each change alters the proportion of frame to glass and so alters the result.
A casement window with a wide opening sash and a visible frame will usually perform differently from a large fixed pane in a curtain wall. For a project, ask the supplier to state the figure for the configurations that actually appear in the design, or to give a calculation method that covers them.
Spandrels and opaque panels
Not all of a curtain wall is vision glass. Between floors there are often opaque panels, called spandrels, that hide slab edges and services. Their U-value depends on the glass or metal skin, the insulation behind it and the way it is supported, and it is a separate figure from the vision glass. Our glossary entry on spandrel explains what they are.
A specification that quotes only the vision glass figure may hide a weak spot. If the opaque area is a large share of the elevation, its insulation and the continuity of that insulation at joints will shape the overall performance of the facade. Ask for the opaque build-up to be stated with the same care as the glass, and check that the insulation is shown continuous across the frame rather than interrupted at every transom.
Thermal bridges are the other part of this. A metal bracket, a fixing or a mullion that passes through insulation carries heat across it. The effect is not captured by a centre-of-panel figure, so ask how the supplier has accounted for them.
Where U-value stops being the whole story
U-value says nothing about sunlight. A pane can have an excellent U-value and still let in a lot of solar heat, which is a separate property. In sunny climates, the solar factor and the way shading is arranged often matter more for comfort and energy use than a small improvement in U-value.
Other qualities sit outside the number too: daylight transmission, reflectance, sound reduction and air leakage through joints. Our article on air leakage in facades explains why a well-insulated unit can still lose energy if joints are poorly sealed. Thermal performance is a set of properties, not one.
Different climates change which number to chase. In a cool climate, a low U-value reduces heating demand and keeps the room edge comfortable. In a hot, sunny climate, the building mostly wants to keep heat out, so solar control and shading usually deserve more attention than a marginal improvement in insulation. Our guide on glass reflectivity and glare shows how those choices also change the appearance of the facade.
Condensation and the inside surface
A better U-value also raises the temperature of the inside surface in cool weather, which reduces the chance that moisture condenses on it. That is a practical comfort and durability gain, particularly on frames. Even so, condensation depends on indoor humidity, ventilation and local airflow, so a low figure does not remove the risk altogether. Our article on condensation on windows and facades looks at the causes.
A practical tip is to ask what happens to the figure at the edges of the facade. Where the window meets the wall, the connection, the insulation and the sealing all affect heat flow, and a good unit installed badly will not perform as quoted. The figure on the data sheet describes the product. The building experiences the installed result, including the interface with the wall. Coordinating that interface early is cheaper than chasing a thermal complaint after handover.
How to compare figures fairly
Before accepting a U-value in a specification or tender, put the numbers on a common basis. Ask each supplier to state what is covered, how it was derived and for what size and configuration. Where the figure comes from a test or simulation, the report should be available for review by the facade consultant.
- Does the figure refer to glass centre, frame or the whole assembly?
- What is the full glass build-up, including gap, gas, coating surface and spacer?
- What frame system and thermal break arrangement are included?
- For what window size and configuration is the figure given?
- Is it a test result or a calculation, and can the report be reviewed?
- Are solar factor and daylight figures stated alongside it?
A short table that lists these answers for each glass and frame type lets the design team see differences at a glance, and avoids surprises when the final product arrives.
General guidance for orientation, not a specification. Applicable standards, values and details are confirmed per project with the facade consultant.
