Building Physics Principles

Building physics principles are fundamental concepts that explain the behavior of buildings and their interaction with the external environment. These principles are critical in the Postgraduate Certificate in Building Performance Analysis, where students learn to analyze and optimize building performance. This explanation covers key terms and vocabulary in building physics, including heat transfer, moisture transfer, air flow, and daylighting.

Heat Transfer

Heat transfer is the process of energy movement from one body to another due to a temperature difference. There are three main types of heat transfer: conduction, convection, and radiation.

Conduction is the transfer of heat through a solid material due to the collision of particles. An example is a metal spoon conducting heat from a hot bowl of soup.

Convection is the transfer of heat through a fluid (liquid or gas) due to the movement of the fluid. An example is a room heater warming up a cold room by convection.

Radiation is the transfer of heat through electromagnetic waves, without requiring a medium. An example is the sun warming the earth through radiation.

Thermal conductivity (k) is a property of materials that measures their ability to conduct heat. High thermal conductivity materials, such as metals, conduct heat well, while low thermal conductivity materials, such as insulation, conduct heat poorly.

Thermal resistance (R) is a property of materials that measures their ability to resist heat flow. High thermal resistance materials, such as insulation, resist heat flow well, while low thermal resistance materials, such as metals, resist heat flow poorly.

Heat transfer coefficient (U) is a property of assemblies that measures their overall heat transfer rate. It takes into account the thermal conductivity, thickness, and area of the materials in the assembly.

Moisture Transfer

Moisture transfer is the process of water movement from one location to another due to a difference in water vapor pressure. There are two main types of moisture transfer: diffusion and convection.

Diffusion is the transfer of water vapor through a material due to a concentration gradient. An example is water vapor diffusing through a wall due to a higher concentration on the inside than the outside.

Convection is the transfer of water vapor through a fluid due to the movement of the fluid. An example is air carrying moisture from the outside to the inside of a building through cracks and gaps.

Moisture content (MC) is the amount of water in a material, expressed as a percentage of the material's dry weight.

Vapor permeability (δ) is a property of materials that measures their ability to allow water vapor to pass through. High vapor permeability materials, such as gypsum board, allow water vapor to pass through well, while low vapor permeability materials, such as polyethylene, allow water vapor to pass through poorly.

Air Flow

Air flow is the movement of air from one location to another due to a pressure difference. There are two main types of air flow: natural and mechanical.

Natural air flow is the movement of air due to natural forces, such as wind or buoyancy. An example is wind blowing air into a building through a window.

Mechanical air flow is the movement of air due to mechanical systems, such as fans or pumps. An example is a exhaust fan pulling air out of a bathroom.

Air change rate (ACH) is the number of times the air in a space is replaced with fresh air in a given time period.

Infiltration is the unintended flow of air into a building through cracks and gaps in the building envelope.

Exfiltration is the unintended flow of air out of a building through cracks and gaps in the building envelope.

Daylighting

Daylighting is the use of natural light to illuminate a building's interior. Daylighting can reduce energy consumption, improve visual comfort, and enhance occupant well-being.

Daylight factor (DF) is the ratio of indoor illuminance (Ei) to outdoor illuminance (Eo) on a horizontal surface.

Luminous efficacy (η) is the ratio of luminous flux (F) to power (P) of a light source. It measures the efficiency of a light source in converting electrical power into visible light.

Solar gain is the increase in temperature due to the absorption of solar radiation through windows or other openings.

Glare is the excessive brightness that impairs visual performance or comfort.

Practical Applications

Understanding building physics principles is critical in building performance analysis. For example, a building with poor insulation will have high heat loss, leading to increased energy consumption and occupant discomfort. By understanding thermal conductivity and resistance, analysts can recommend insulation materials and thicknesses to reduce heat loss and improve energy efficiency.

Similarly, moisture transfer can lead to mold growth, rot, and other building damage. By understanding moisture content, vapor permeability, and diffusion, analysts can recommend materials and construction practices to prevent moisture damage.

Air flow can affect indoor air quality, thermal comfort, and energy consumption. By understanding air change rate, infiltration, and exfiltration, analysts can recommend building envelope improvements and ventilation strategies to improve indoor air quality and energy efficiency.

Daylighting can reduce energy consumption and improve occupant well-being. By understanding daylight factor, luminous efficacy, solar gain, and glare, analysts can recommend window sizes, orientations, and shading strategies to optimize daylighting and reduce artificial lighting demand.

Challenges

Building physics analysis can be complex due to the interacting factors and variables involved. For example, heat transfer, moisture transfer, and air flow can affect each other, leading to complex interactions and feedback loops.

Moreover, building physics analysis requires accurate and detailed information about the building's construction, materials, and systems. Gathering this information can be time-consuming and challenging, especially for existing buildings.

Finally, building physics analysis requires specialized software and tools, which can be expensive and require training and expertise to use effectively.

In conclusion, building physics principles are essential in building performance analysis. Understanding heat transfer, moisture transfer, air flow, and daylighting can help analysts optimize building performance, reduce energy consumption, and improve occupant comfort and well-being. However, building physics analysis can be complex and challenging, requiring accurate information, specialized software, and expertise.