Thermal 2 – Building Heat Transfer & HVAC Vocabulary

Vocabulary flashcards reviewing key terms from the Thermal 2 lecture, including building heat transfer concepts, HVAC calculations, and material properties.

Solar Heat Gain Coefficient (SHGC)

Fraction (0–1) of incident solar radiation that enters a window as heat; high SHGC admits more solar heat, low SHGC blocks it.

Skin-load dominated building

Structure whose energy use is controlled mainly by heat gains/losses through its envelope because of large surface-to-volume ratio.

Internal-load dominated building

Building where occupant, lighting, and equipment loads outweigh envelope effects; typical of offices, factories, high-rises.

Solar insolation

Amount of solar radiant energy received per unit area, usually expressed in Btu/ft² or kWh/m².

British Thermal Unit (BTU)

Heat required to raise 1 lb of water 1 °F; basic U.S. unit of heat energy.

Psychrometry

Study of the thermodynamic properties of moist air, including temperature, humidity, and enthalpy.

Relative humidity (RH)

Percentage ratio of actual water vapor in air to the maximum it could hold at the same temperature.

Dew point

Air temperature at which water vapor condenses into liquid when air is cooled at constant pressure.

Wet-bulb temperature

Lowest temperature achievable by evaporative cooling; indicator of humidity levels.

Enthalpy (of air)

Total heat content of air, combining sensible and latent heat, usually expressed in Btu per pound of dry air.

Low-e glass

Glazing treated with a low-emissivity coating that reduces infrared heat transfer while maintaining visible light transmittance.

Balance point temperature

Outdoor temperature at which a building's internal gains exactly equal its heat losses, requiring no heating or cooling.

Heating Degree Day (HDD)

Cumulative measure of how cold weather is; for each day, HDD = base temperature – average outdoor temperature when below base.

Thermal conductivity (k-value)

Material property expressing rate of heat flow through a 1-inch thickness for a 1 °F temperature difference (Btu·in/hr·ft²·°F).

Thermal resistivity (r-value per inch)

Reciprocal of conductivity; resistance to heat flow offered by 1-inch thickness of material.

Conductance (C)

Heat transfer rate through a given thickness of a specific material (k divided by thickness).

Thermal resistance (R-value)

Measure of a component’s opposition to heat flow; R = thickness ÷ k, or R = 1 ÷ C.

U-value

Overall heat-transfer coefficient of an assembly; U = 1 ÷ (sum of R-values). Lower U means better insulation.

Conduction

Heat transfer within or between solid materials in direct contact.

Convection

Heat transfer by the movement of fluids (air or water) across a surface.

Ventilation

Planned, controlled introduction of outdoor air to maintain indoor air quality.

Infiltration

Unintended leakage of outdoor air through cracks and openings in the building envelope.

CFM (Cubic Feet per Minute)

Unit of volumetric airflow rate; used to size ventilation and infiltration loads.

Thermal bridging

Flow of heat through elements with higher conductivity (e.g., metal studs) that bypass insulation layers.

Absorptance

Fraction of incident radiant energy a surface absorbs; dark, matte surfaces have high absorptance.

Emittance (emissivity)

Ability of a surface to re-radiate absorbed heat; low-e materials have low emittance.

Radiant heat transfer

Exchange of thermal energy via electromagnetic waves between surfaces at different temperatures.

Angle of view (view factor)

Geometric relationship determining proportion of radiant energy exchanged between two surfaces.

Thermal mass

Capacity of a material (e.g., concrete, brick) to store and release heat, moderating indoor temperature swings.

Porous material

Material containing air voids that increase thermal resistance (e.g., fiberglass insulation).

Dense material

Material with high mass and typically higher conductivity; provides lower R-value per inch (e.g., concrete).

Passive solar heating

Design approach that uses building orientation, glazing, and thermal mass to collect, store, and distribute solar heat without mechanical systems.

Reflective air space (double shiny sides)

Unvented cavity with foil or shiny surfaces on both faces; provides about R-10 by reducing radiant heat transfer.

Unvented air space

Plain air cavity without reflective surfaces; offers roughly R-1 thermal resistance.

Latent heat

Energy absorbed or released during phase change of water (vapor ↔ liquid); contributes to humidity loads.

Sensible heat

Heat that causes a temperature change in air or materials without a phase change.