Thermal Bridging

occurs when the insulation value of a wall, window, roof, or slab is interrupted by a linear element such as a window sill, a point thermal bridge such as a screw that penetrates the insulation, or a geometric condition such as a corner

Passive House architects take care to minimize thermal bridges in order to avoid cold spots that could promote mold and to reduce heat loss through the building enclosure.

Building enclosure heat loss (and summer heat gain)
calculate by summing:

the area x U value of each component of the building envelope (i) +

the length and psi value of each linear thermal bridge (k) +

the chi value of each point thermal bridge (j)

equation source: Passipedia

Calculations

if a catalog value for a thermal bridge isn't available, you can use software such as THERM or Flixo