U-Value: How Much Heat a Building Element Lets Through

U-value, or thermal transmittance, is the rate at which heat passes through a building element per unit area for each degree of temperature difference across it. Lower is better insulated. It is the inverse of R-value, and it describes conducted heat only — not the solar radiation passing through glass, which is measured separately as SHGC.

By Dhruv Agarwal · · 2 min read

Why it matters on a project

U-value is the number that appears in envelope compliance documents, and it is routinely misapplied in two ways: quoted for a product when the requirement is for an assembly, and treated as the only thermal number when for glazing it is usually the less important one.

Getting it right matters because the envelope is built once. An element that underperforms is corrected by replacing it, not by adjusting it.

What U-value does and does not cover

CoversDoes not cover
Conducted heat through the elementSolar radiation through glazing (see SHGC)
The combined effect of layers in an assemblyAir leakage through gaps and joints
Frame, glass and edge effects in a windowVentilation heat exchange
Thermal bridging, when assessed on the assemblyOccupant and equipment heat gains

Where the number goes wrong in practice

Thermal bridges. Fixings, brackets, structural members and junctions conduct heat around the insulation. On a facade with closely spaced brackets, the effect on the assembly figure is not marginal.

Gaps and compression. Insulation that does not fill its space, or is compressed to fit, does not deliver its declared value. The declared figure assumes correct installation.

Product substitution. A panel or glazing unit swapped late for a commercially similar one can change the assembly figure, and compliance was demonstrated on the original.

Calculation methods for building components are set out in ISO 6946, and for windows and doors in ISO 10077.

Common mistakes

  • Quoting glass U-value as window U-value. The frame and edge are part of the window and usually make it worse.
  • Comparing products on U-value alone for glazing. SHGC generally governs cooling load in Indian conditions.
  • Ignoring thermal bridging. The field of a wall is rarely where the problem is.
  • Accepting a substitution without rechecking. Compliance was calculated on a specific build-up.
  • Forgetting air leakage. A well-insulated envelope that leaks air performs worse than its U-values suggest, and leakage is not part of the U-value at all.

Standards referenced

Envelope requirements including U-factor criteria in ECBC 2017; calculation of thermal resistance and transmittance in ISO 6946; window and door thermal performance in ISO 10077. Applicable limits and compliance for a specific building must be established by the project's energy consultant.

Standards referenced

  • ECBC 2017 — Building envelope — U-factor requirements (Bureau of Energy Efficiency)
  • ISO 6946 — Building components — thermal resistance and transmittance (ISO)
  • ISO 10077 — Thermal performance of windows, doors and shutters (ISO)

Frequently asked

They describe the same property from opposite directions. R-value is thermal resistance — how well a material resists heat flow, so higher is better. U-value is transmittance — how readily heat passes, so lower is better. One is the reciprocal of the other. Insulation products are usually quoted in R-value or thermal conductivity, while whole assemblies such as walls, roofs and windows are usually quoted in U-value.

Not usually, for glazing. In a cooling-dominated climate the dominant heat gain through a window is solar radiation, which U-value does not describe at all — that is SHGC. U-value remains important for opaque walls and roofs, and for keeping the inner surface of glazing near room temperature so it does not become a condensation surface. Both numbers matter; they are just answering different questions.

Because the assembly includes everything the product does not — frames, fixings, structural members, junctions and any gap in the insulation. These form thermal bridges that conduct heat around the insulated area. A wall with excellent insulation and poorly detailed junctions can have a whole-assembly figure noticeably worse than the insulation suggests, which is why compliance is assessed on the assembly.

It combines the glass, the frame and the edge of the glazed unit, each weighted by its area. That is why frame material and the spacer at the glass edge both affect the result, and why a window cannot simply inherit the U-value of its glass. Calculation methods for windows are set out in standards such as ISO 10077.

ECBC 2017 sets envelope performance requirements for commercial buildings, including U-factor criteria, and these vary with climate zone and building type. What applies to a specific project must be determined by the project's energy consultant against the applicable version and the building's classification.

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