Heat transfer: conduction / convection / radiation, and R-value vs U-value

One-line orientation

Heat moves through a building three ways — conduction, convection, radiation — and the exam tests which mechanism applies and how the conductive-loss metrics (R-value, U-value) relate.

Key points

• Conduction: direct heat transfer through a material in contact (the heat path through walls, glass, and structure).
• Convection: heat carried by a moving fluid — air or water.
• Radiation: heat transferred by electromagnetic waves; needs no intervening medium (e.g. solar gain through a window, controlled by SHGC).
• The conduction chain of properties: conductivity k (rate heat passes through a material) → resistivity r is its inverse (1/k) → applied to a specific element these become conductance C and resistance R.
• R-value = resistance to conductive heat flow; add R-values in series through an assembly to get total R. Higher R = better insulator.
• U-value (transmittance) = overall rate an assembly conducts heat = 1 / total R. Lower U = better. Windows are rated by U-value (assembly heat loss) plus SHGC and VLT.

Confusions / comparison

	What it measures	Direction of “good”	Relationship
Conductivity k	material property: heat-transfer rate per unit thickness	lower k = better insulator	r = 1/k
Resistivity r	material property: resistance to heat flow	higher r = better insulator	r = 1/k
R-value	a specific element/assembly’s resistance	higher R = better	R-values add in series
U-value	a specific assembly’s overall conductance (transmittance)	lower U = better	U = 1 / total R

	Conduction	Convection	Radiation
Carrier	material in contact	moving fluid (air/water)	electromagnetic waves
Needs a medium?	yes	yes	no
Building example	wall/window heat loss	air movement, hydronic loops	solar gain (SHGC)
Key metric	R-value / U-value	—	SHGC

One window loses heat three ways at once

Schematic

Conduction crosses solids, convection circulates gas, and radiation crosses the glazing gap; total R and U are reciprocal.

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R-values of layers in series add. U-value is the reciprocal of total R—do not add U-values in the same way.

Related

→ Construction: Enclosure (insulation R-value per inch, vapor/air barriers) · HVAC distribution systems (convective delivery) · Code: energy code (envelope U-factor / SHGC limits).