E2 Heat Transfer in Buildings

Topic

Heat moves through a building via the same three combined pathways: conduction through solid materials—which occurs faster the more conductive the material is; convection at surfaces in contact with air—which intensifies as air movement increases; and infrared radiation between surfaces—which rises rapidly with the absolute temperature of the emitting surface and depends on that surface's heat-emitting efficiency.

These three pathways combine into a single parameter summarizing a building envelope's heat transmission characteristics: each wall layer offers its own resistance to heat flow, and these resistances add up like resistors in series; consequently, the higher the total accumulated resistance, the lower the heat loss for a given indoor-outdoor temperature difference. This means that adding just one layer of good insulation to a wall drastically reduces heat loss, even if the original wall was already thick; what matters is not the total thickness, but the resistance effectively added.

In addition to this steady-state heat transmission, a building's material mass and specific heat capacity provide inherent thermal inertia: the building takes a characteristic amount of time to respond to outdoor temperature changes, and this lag dampens daily heat or cold spikes rather than immediately transferring them to the interior.

Thus, this module—which begins by outlining the three physical pathways of heat transfer—concludes by demonstrating that neither of the resulting parameters is sufficient on its own: it is the combination of thermal resistance (determining *how much* heat is lost) and thermal inertia (determining *when* it is lost) that collectively defines whether an envelope is energy-efficient.

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