Thermal Radiation

What is thermal radiation?

Thermal radiation is the transfer of heat via electromagnetic waves, primarily in the infrared wavelength range (760 to 3,000 nanometers). Unlike heat conduction, which requires direct contact between materials, or heat convection, which moves heat through fluids, thermal radiation requires no medium and can travel through empty space. Every object with a temperature above absolute zero emits thermal radiation; hotter objects emit more intensely and at shorter wavelengths. In building physics, thermal radiation is the third fundamental heat transfer mode and often dominates energy exchange in interior spaces.

How do the three heat transfer modes differ in buildings?

Conduction transfers heat through solid materials by molecular contact, dominating heat flow through walls, roofs, and floors. Convection moves heat via air or water circulation; warm air rises while cool air sinks, creating currents that distribute thermal energy throughout spaces. Thermal radiation travels directly between surfaces as electromagnetic waves, independent of air movement or solid contact. In typical room conditions, radiative exchange between surfaces accounts for roughly 50 percent or more of total heat transfer, often exceeding convective effects, yet this mode remains underemphasized in standard building codes.

How does emissivity control thermal radiation?

Emissivity, expressed as a value from 0 to 1, describes how effectively a surface emits thermal radiation compared to an ideal blackbody. Most building materials (brick, concrete, drywall, glass) have high emissivity of 0.85 to 0.95, meaning they both absorb and emit infrared radiation readily. Polished metals and reflective coatings have much lower emissivity, typically 0.04 to 0.10, reflecting most incoming thermal radiation instead of absorbing it. Low-emissivity (low-e) coatings on windows use this principle: they allow visible light through but reflect far-infrared radiation, reducing heat loss in winter. Passive low-e coatings reflect indoor radiant heat back into the room; solar-control low-e coatings reflect incoming solar infrared to reduce summer cooling loads. The choice of coating type depends on your climate zone and whether your priority is winter retention or summer rejection.

Why are reflective foils and radiant barriers often overstated?

Reflective foil barriers beneath roofs or in attics promise dramatic summer cooling benefits, but real-world performance is far more modest than marketing claims suggest. Aluminum foil reflects 94 to 97 percent of thermal radiation, so in theory it should be highly effective. However, reflective foils work only when facing an air gap; placing foil directly against insulation or drywall eliminates the air space and reduces effectiveness dramatically. Additionally, field studies show actual energy savings of 7 to 12 percent annually in cooling electricity, not the 30 to 50 percent reductions sometimes advertised. Claims often ignore maintenance (foil degrades if dust accumulates), installation challenges (air gaps must be maintained), and climate dependence (barriers are most effective in hot, dry climates). Combined with roof ventilation and adequate insulation, reflective barriers contribute to summer heat control; alone, they rarely justify their cost.

What is radiant temperature and how does it affect comfort?

Radiant temperature, or mean radiant temperature (MRT), represents the average temperature of all surfaces surrounding a person, weighted by their angles relative to the person's position. Because humans continuously radiate thermal energy, they feel cold when surrounded by cold surfaces even if air temperature is comfortable. A large uninsulated window or exposed concrete wall can reduce comfort significantly through radiant cooling. Conversely, warm sunny surfaces increase radiant warmth. Passive house design minimizes radiant asymmetry by keeping all interior surfaces close to the air temperature, avoiding cold spots and drafts. This principle explains why thermal comfort depends on more than air temperature alone; the building envelope quality directly shapes occupant sensation through radiant exchange.

How does thermal radiation interact with building mass?

Thermal mass materials like concrete, brick, and stone have high thermal capacity and high emissivity. They absorb solar radiation and convective heat during the day, then emit that stored energy as thermal radiation at night, moderating indoor temperature swings. In climates with large daily temperature differences, thermal mass combined with night cooling ventilation provides passive temperature buffering. However, in air-conditioned buildings with minimal day-night swings, thermal mass's role in thermal radiation becomes less critical. In passive house design, thermal mass is sized carefully to absorb intermittent solar and internal gains and re-emit them when needed, reducing mechanical heating and cooling demands.

What is the practical role of thermal radiation in building design?

Thermal radiation must be addressed alongside conduction and convection for accurate energy modeling and comfort design. High-performance window u-values are achieved largely by controlling thermal radiation through low-e coatings; conduction through the frame and convection in air gaps play supporting roles. In roof design, summer heat gain is driven more by solar radiation absorption and thermal radiation re-emission than by conduction alone. Interior surface temperatures, influenced by radiant properties and exposure to solar and internal heat sources, determine both comfort and interior humidity levels. Proper building physics requires treating thermal radiation not as a minor factor but as a co-equal mechanism alongside conduction and convection.