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
| Covers | Does not cover |
|---|---|
| Conducted heat through the element | Solar radiation through glazing (see SHGC) |
| The combined effect of layers in an assembly | Air leakage through gaps and joints |
| Frame, glass and edge effects in a window | Ventilation heat exchange |
| Thermal bridging, when assessed on the assembly | Occupant 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
- ISO 6946 — Building components — thermal resistance and transmittance
- ISO 10077 — Thermal performance of windows, doors and shutters