Thermal conduction and building heat loss
Principles of energy • Conservation and dissipation of energy
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Thermal conductivity: definition and effect
Thermal conductivity is a property of a material that measures how readily thermal energy passes through it. Higher thermal conductivity causes a higher rate of energy transfer by conduction across a material because particles or electrons pass energy more effectively. The higher the thermal conductivity of a material, the higher the rate of energy transfer by conduction across the material.
Heat transfer by conduction: controlling variables
Rate of conductive heat transfer across a flat layer depends on thermal conductivity, cross-sectional area, temperature difference, and thickness of the layer. Rate increases when thermal conductivity increases, when the area increases, or when the temperature difference increases. Rate decreases when the thickness of the layer increases. For a uniform wall, heat transfer by conduction is inversely proportional to wall thickness and directly proportional to thermal conductivity and temperature difference.
Wall thickness and building cooling
Thicker walls slow the rate of energy loss because the same temperature difference must be conducted across a longer path; increasing thickness increases thermal resistance. Thinner walls allow faster conduction and faster cooling of the interior. Modern building methods use layers and cavities so that the effective thickness for conduction is larger and the thermal resistance is higher. The thicker the walls, the slower the rate of energy loss.
Insulation and limiting factors
Insulation reduces conduction by introducing materials or trapped pockets of air with much lower thermal conductivity than brick or glass. Cavity wall insulation fills the gap between layers of wall with foam or trapped air to reduce heat flow. Limiting factors for the effectiveness of insulation include gaps, thermal bridges (materials that bypass insulation), moisture in the wall, and surface area of exposed walls or windows. Cavity insulation and trapped air reduce conduction because air has a much lower thermal conductivity than brick or glass.
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