Thermal Flow is the same as saying how heat moves. We control the movement of heat with insulation and we prevent uncomfortable drafts with air sealing.
Insulation slows the passage of heat through a wall, ceiling or floor assembly.
Air sealing helps to restrict and reduce leakage.
In construction...
Thermal RESISTANCE is described as R-Value
Thermal TRANSMITTANCE (CONDUCTANCE) is described as U-Value
Air tightness is measured in Air Changes per Hour or ACH.
And so:
A well-insulated (high R-Value) and air-sealed home (low ACH), with good windows and doors (low U-value), reduces the amount of energy needed to keep the home comfortable.