What Emissivity Really Describes
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The emissivity value appears on every datasheet for an insulating coating, usually as a small figure between 0 and 1. What it describes is precisely defined. What follows for a building component depends on geometry — and that is exactly where things get imprecise in practice.
The definition sits inside a power of four
Every surface above absolute zero emits heat as electromagnetic radiation. The Stefan-Boltzmann law describes how much:
M = ε · σ · T4 [W/m²]
σ is the Stefan-Boltzmann constant at 5,67 · 10−8 W/(m²·K4), T the absolute temperature in Kelvin. The emissivity ε is dimensionless and states what proportion of the radiation of an ideal black body the surface actually emits.
Two things follow immediately:
- The fourth power is the decisive term. Between 20 °C (293 K) and 5 °C (278 K) there are only 15 Kelvin, yet the radiative flux changes by around 20 per cent. At industrial surfaces of 200 °C and above, the difference becomes dramatic — there, radiation is the dominant loss path.
- ε and absorptance are identical at the same wavelength. This is Kirchhoff’s law of radiation. A surface that radiates poorly also absorbs poorly at the same wavelength.
Typical values, and why they surprise
| Surface | ε (long-wave) |
|---|---|
| Black body (model) | 1,00 |
| Mineral render, masonry, timber | 0,90 to 0,95 |
| Emulsion paint, whatever the shade | around 0,90 |
| Window glass, uncoated | around 0,84 |
| Low-E coating on glass | 0,03 to 0,17 |
| Bare aluminium foil | 0,03 to 0,05 |
The most important value in this table is the second one. Almost all building materials sit around 0,9. A brick, a lime-cement render and a white wall paint are practically indistinguishable in long-wave emissivity.
And a point that regularly causes confusion: the visible colour says nothing about the long-wave emissivity. A black and a white emulsion paint both sit at around 0,9. Colour affects the short-wave absorptance — the uptake of solar radiation — an entirely different quantity for an entirely different wavelength range.
Where a low emissivity demonstrably matters
There are three well-documented applications, and they share one feature: the low-ε surface faces an air space or a large temperature gradient.
Insulating glazing
In the cavity of double glazing, a substantial share of the heat flow travels as radiation between the two glass surfaces. A Low-E coating on the room-side pane reduces this share drastically. The effect is large, measurable, and set out in the product standard — the step from uncoated to coated thermal-insulation glazing is one of the biggest leaps building physics has made in recent decades.
Radiant barriers in front of an air layer
An aluminium facing only acts as a radiant barrier when it faces a still air layer. If the same foil is laminated or rendered in directly, the effect disappears entirely: without an air gap there is no radiative exchange to reduce, and the heat flow continues as conduction.
This is the most common application error in this field — and the reason the installation situation belongs in every calculation.
Hot surfaces in plant engineering
On pipework, vessels and boilers with surface temperatures well above room temperature, the radiative share dominates because of the fourth power. This is the classic field of application for an emissivity-reducing coating.
Where it gets complicated at a building component
At a room-side wall surface, the situation differs from that in the glazing cavity, for a simple reason: radiation and convection run in parallel there, and the standard combines both into one figure.
The surface heat transfer resistance Rsi, with the standard value 0,13 m²K/W, is not a pure radiation quantity. It contains the convective share — the air movement at the wall — just as much as the radiative exchange with the other surfaces in the room. Reducing the radiative share changes Rsi, but not in the proportion a pure radiation calculation would suggest.
There is a further point: the radiative exchange takes place between the wall and the other surfaces of the same room, not against the cold exterior. The temperature difference is therefore small — a few Kelvin rather than the 25 Kelvin used in the calculation under DIN 4108-2. And it is precisely this small difference that enters the fourth power.
What follows for the surface temperature of a specific wall build-up cannot be stated in general terms. It depends on the ratio of resistances across the whole component, on the room geometry and on how the room is used. A percentage figure without the associated wall build-up is therefore not a statement.
What this means for a datasheet
Three questions to ask of any emissivity figure:
- For which wavelength range does the value apply? Long-wave (thermal radiation at room temperature) or solar (short-wave)? Both are ε values, but they describe different processes and have different orders of magnitude.
- By which method was it measured? For long-wave emissivity, measurement to EN 15976 or with an emissometer to ASTM C1371 is standard. A value without a stated method cannot be verified.
- Against which surface and at what distance? Without the installation situation, the radiative exchange cannot be calculated — and without an air gap, it may not exist at all.
And the U-value?
For requirements under the Gebäudeenergiegesetz and for funding programmes, what counts is not the emissivity but the design value of thermal conductivity under DIN 4108-4, a general building authority approval, a European Technical Assessment or a harmonised product standard. A low ε value does not replace this evidence, however plausible the underlying physics may be.
Anyone arguing with emissivity in a customer conversation is therefore arguing from a real physical effect — but not from a figure that can justify a funding application.
Sources
- Stefan-Boltzmann law and Kirchhoff’s law of radiation — fundamentals of heat transfer
- DIN EN ISO 6946 — Thermal resistance and thermal transmittance, surface heat transfer resistances
- DIN 4108-2 — Minimum requirements for thermal insulation, boundary conditions of the calculation
- DIN 4108-4 — Thermal and moisture protection design values
- DIN EN 15976 — Determination of emissivity; ASTM C1371 — emissometer method
- Gebäudeenergiegesetz (GEG) and BEG-EM FAQ on the recognition of U-value evidence
This article describes general physical relationships. It contains no statement about the effect of a specific product. What has been measured on individual buildings, together with the relevant boundary conditions, is set out on the References and Measurements page.