How to Simulate Green Roofs in DesignBuilder: Plant Data & Properties

Heat transfer occurs through three fundamental mechanisms: conduction (direct contact transfer), convection (fluid movement driven by density changes), and radiation (electromagnetic wave transfer without medium). Heat naturally flows from hot to cold because molecules at higher temperatures have greater kinetic energy and transfer energy through molecular collisions. The rate of conductive heat flow follows Q/t = K × A × ΔT / L, where K is thermal conductivity, A is cross-sectional area, ΔT is temperature difference, and L is length. The R-value quantifies thermal resistance as R = L/K, with higher values indicating better insulation. Materials like silver conduct heat efficiently (high K, low R), while fiberglass provides excellent insulation (low K, high R).

This section covers the three modes of heat transfer. Conduction is heat transfer through stationary material without bulk movement of molecules, occurring in solids. Convection is heat transfer through fluid movement where hotter, less dense fluid rises and cooler, denser fluid sinks. Natural convection occurs from density differences, while forced convection uses fans or pumps. Radiation is heat transfer through electromagnetic waves without requiring a medium. Applications include solar heaters (radiation), car radiators (forced convection), and building insulation. Thermal pollution from power plants and industries increases water and air temperatures, disrupting ecosystems.

This section covers three fundamental heat transfer mechanisms. Heat always transfers from hotter to colder objects until thermal equilibrium is reached, where temperatures equalize and heat flow stops. Conduction transfers heat through solids without material movement via particle vibration and collision. Good thermal conductors (copper, aluminum, silver) transfer heat quickly, while poor conductors (wood, plastic, glass) transfer heat slowly. Convection transfers heat through fluids by fluid movement: heated fluids expand, become less dense, and rise while cooler fluid sinks, creating convection currents. This explains sea breezes, birds flying over warm water, and heating/cooling systems. Radiation transfers heat through electromagnetic waves (primarily infrared) without requiring a material medium, traveling through vacuum as demonstrated by the Sun heating Earth. Dark-colored objects absorb more radiation than light-colored objects. Thermal imaging cameras detect infrared radiation for night vision and temperature visualization.

Heat transfer occurs through three primary mechanisms: conduction (transfer of thermal energy through direct contact between particles in solids, where vibrating atoms transfer energy to neighboring atoms), convection (transfer of heat through bulk motion of fluids, where heated fluid becomes less dense and rises while cooler fluid sinks, creating circulation currents), and radiation (transfer of thermal energy through electromagnetic waves without requiring a medium, where all objects emit radiation proportional to their temperature). Thermal equilibrium exists when net heat flow between objects is zero, though heat continues to transfer bidirectionally at equal rates.

Heat transfer occurs through three fundamental mechanisms: conduction (heat transfer through solids via atomic vibration without material movement, governed by Fourier's Law), convection (heat transfer through fluids via particle movement, including natural and forced convection with efficiency measured by the Nusselt number), and radiation (heat transfer through electromagnetic waves without requiring a medium, governed by the Stefan-Boltzmann Law where power is proportional to the fourth power of absolute temperature).

Extensive green roof: substrate depth <15cm, low maintenance, low cost, low plant diversity. Intensive green roof: substrate depth >15cm, higher maintenance, higher cost, higher plant diversity. Substrate depth requirements: Low-growing succulents (50-150mm), Annuals/herbaceous (>150mm), Small shrubs/turfs (>250mm), Shrubs up to 2m (>500mm). Peat-based media preferred over soil for roof gardens.

Green roofs are categorized into extensive and intensive types based on their design and purpose; extensive green roofs feature shallow substrate layers (100-200mm) supporting resilient vegetation like succulents and grasses, requiring minimal maintenance and weighing 60-150 kg/m², while intensive green roofs have deeper soil layers (up to 1m+) accommodating larger plants like bushes and trees, requiring more maintenance and weighing 200+ kg/m², with both types providing water retention benefits but differing significantly in cost, accessibility, and structural impact.

Intensive green roofs feature thick growing mediums (150-200mm) with substantial soil weight (approximately 13 tons dry, increasing to 25 tons when wet), requiring structural engineering assessment. Extensive green roofs use much thinner profiles (around 20mm) with lightweight sedum plants that require minimal moisture and maintenance.

Extensive green roofs feature low construction height, minimal weight, and require only 1-2 maintenance visits per year with sedum plantings, making them suitable for flat to slightly sloped roofs up to 10°; intensive green roofs offer greater design flexibility with seating areas, lawns, and park landscapes but require significantly higher maintenance comparable to traditional gardening, and are best suited for flat roofs with minimal slope. Both types provide important benefits for climate change adaptation, biodiversity, fine dust reduction, and human well-being.

Green roofs come in two primary designs suited to different applications. Extensive systems use shallow 1-5 inch growing media with hardy succulents that store water in leaves and spread vegetatively, requiring minimal maintenance. Intensive systems use deeper 12+ inch media supporting diverse plants, crops, and even small trees, requiring regular irrigation, pruning, and structural reinforcement including 3.5-foot safety barriers. Both types require waterproof membranes, root barriers, drainage layers, and filter cloths, but intensive systems demand additional engineering considerations for load-bearing capacity and occupant safety.

In building energy simulation, thermal properties of materials are critical for accurate modeling: thermal conductivity (lambda, λ) measures a material's ability to conduct heat, with metals (40-200+ W/m·K) being conductors and insulators like cork or fiberglass (0.03-0.7 W/m·K) having low conductivity; surface color significantly impacts solar heat absorption, with white surfaces absorbing only 12.8% of solar radiation while black surfaces absorb 95%, leading to temperature differences of 15-20°C; for windows, three key properties define solar behavior—reflectance (7% for clear glass), absorptance (15% for clear glass), and transmittance (78% for clear glass)—with colored glass increasing absorptance and reducing transmittance, and double glazing reducing thermal transmittance from 5.7 to 2.7 W/m²·K.

Building energy simulation requires accurate material property definitions including thickness, thermal conductivity, density, and specific heat capacity. Concrete has conductivity of 1.75 W/(m·K), density of 2,200-2,400 kg/m³, and specific heat of 1 kJ/(kg·K). Radiant barriers use SHGC of 0.9 and VLT of 0.5. Windows require SHGC modeling with verification against manufacturer data. Glass properties include conductivity around 0.96 W/(m·K) with visible transmittance and solar heat gain coefficients. Data sources include NBR 15220-3 standard tables and manufacturer specifications. Building construction systems combine individual components (walls, roofs, floors, windows, doors) into assemblies with defined thermal performance. The Brazilian Construction Component Library provides pre-defined components including ceramic block walls, concrete walls, precast slabs, and roof assemblies. Complex assemblies can be simplified into equivalent components with averaged properties.

In building energy simulation, accurate thermal properties of construction materials—including thermal conductivity, density, specific heat capacity, and solar absorptance—are essential for predicting building energy performance; these properties vary by material type (e.g., ceramic blocks, bricks, insulation) and must be input into simulation software according to regional construction practices and Brazilian standards like NBR 15220.

Three key parameters describe thermal performance: conductivity (K) is an inherent material property measured in W/m·K, independent of thickness; conductance includes thickness effects and is measured in W/m²·K; U-value represents overall air-to-air thermal transmittance, calculated as the reciprocal of total thermal resistance. U-values range from 0.1-3.5 W/m²·K for typical building walls. These parameters are measured using standardized hot plate or hot box apparatus. The relationship between material density and thermal conductivity is strong—higher density materials conduct heat more efficiently.

Thermal transmittance (U-value) is calculated from thermal conductivity of building materials. The IRAM 11.601 standard provides technical properties of construction materials, including how to calculate total thermal resistance of building components. Each material manufacturer provides their own values through testing. The total resistance includes surface resistances (interior and exterior), material resistances (thickness/lambda for each layer), and air cavity resistances. The inverse of total resistance gives thermal transmittance, which is compared against maximum allowable values in IRAM 11.605.