Solar Absorption and Heat on Surfaces
Building materials and their surfaces are continuously exposed to environmental factors such as temperature fluctuations, solar radiation, and wind. These elements contribute to the heating and cooling of exterior components, influencing the thermal performance of a building. This article explores how surface properties, material choices, and environmental conditions affect solar absorption and heat retention on building surfaces.
Understanding Solar Absorption and Surface Temperature: The surface temperature of external building components results from multiple factors, including material conductivity, temperature differences between the inside and outside of the structure, surface condition (e.g., color and texture), wind speed, and the type of solar radiation (direct or diffuse). These influences cause materials to absorb and emit heat, leading to temperature-dependent expansion and contraction, which can cause damage over time if not managed effectively.
Key Factors Influencing Surface Temperature:
Material Conductivity: Different materials conduct heat at varying rates. High-conductivity materials like metal transfer heat quickly, while low-conductivity materials like wood or insulation retain heat for longer.
Reflectivity and Absorbency: Surfaces reflect some solar radiation, while the rest is absorbed and converted into heat. This heat is then conducted into the material, affecting surface temperature. The degree of absorption depends on the surface's color and texture.
Surface Color and Finish: Dark-colored surfaces absorb more heat compared to lighter-colored ones. For example, a dark blue or black surface can absorb up to 90% of solar radiation, while a white surface may only absorb 25-40%. The color and texture of a material's finish play a significant role in its heat absorption capacity.
Environmental Influences: Wind, rain, and convection also affect surface temperatures. Wind speeds can cool surfaces through convection, while rain temporarily cools surfaces but may lead to thermal shock and material degradation over time.
Solar Absorption Values for Common Materials: The solar absorption capacity of materials varies significantly based on color and surface condition. Here are rough guide values for solar absorption (source: PHPP V10.4aEN):
White, smooth surfaces: 0.25 - 0.40
Grey to dark grey: 0.40 - 0.50
Green, red, brown: 0.50 - 0.70
Dark brown and blue: 0.70 - 0.80
Dark blue to black: 0.80 - 0.90
Tile (dark red): 0.8
Copper (oxidized): 0.64
Concrete: 0.6
Wood (untreated): 0.6
Iron (galvanized): 0.6
For ventilated façades and rear-ventilated roof coverings, these values must be multiplied by a factor of 0.9 to account for the ventilation effect.
How Surface Color and Material Impact Heat Absorption: The color and material of a surface significantly influence its solar absorption. Darker colors, such as black or dark brown, absorb a higher percentage of solar radiation and therefore heat up more than lighter colors like white or grey. This heat absorption is critical when considering materials for exterior cladding, roofing, and façade systems.
Impact of Heat on Material Expansion: When materials absorb heat, they expand, and when they cool, they contract. These temperature-dependent changes in length can lead to material degradation, cracking, and structural issues over time. Using materials with low thermal expansion or providing adequate ventilation can help mitigate these risks, especially in climates with extreme temperature variations.
The solar absorption and heat retention properties of building materials directly impact a building's thermal performance and durability. By understanding how material choices, colors, and environmental conditions influence surface temperatures, builders and architects can make informed decisions that enhance energy efficiency and reduce the risk of damage over time. Selecting the right materials and surface finishes is crucial to managing heat absorption, preventing structural issues, and maintaining building longevity.
