Forgive me for this post, I have been thinking about how I wanted to say this without sounding too technical. I'm going to try to simplify this a little bit.
A thermal bridge is any location in a building where conduction of heat can travel from inside to outside easily.
An Infrared scan will help visualize these bridges. This is NOT an X-Ray. The infrared camera only measures surface temperatures. This is a thermal image of a poorly installed window. So what you are seeing are the yellow/white colors are warm temperatures and the blue and black colors are cold temperatures. As you can see, there is a substantial thermal bridge around the window opening allowing the cold temperatures outside to come in. In this case the efficiency of the window doesn't matter due to the poor quality installation. There is a temperature difference of over 12 degrees inside the box. This spot will feel cold all the time!
Proper thermal bridge free installation of the window will show minimal changes in temperature, (See my Window Installation Post).
Thermal Bridge Free Construction is vital for a passive house. Eliminating cold spots also will improve comfort, eliminate the potential for condensation and boost the efficiency of the house.
Thermal Bridge analysis is done with a computer program called THERM. Once a wall assembly is designed, we can model all of the intersections of the house (Floor to Wall, Wall to Window, Wall to Attic, Basement Slab to Foundation Wall, Etc). Passive House studies have shown that maintaining surface temperatures within 6 degrees of each other is optimal for comfort (due to radiant temperature gains and losses) THERM allows us to simulate all of the assembly interior surface temperatures to ensure all of the surfaces of the house are within 6 degrees of each other. That is right... Your Basement Floor is within 6 degrees of the upstairs ceiling. The window frames are within 6 degrees of your living room floor. This is the recipe for comfort!
When you combine an air tight structure with the THERM analysis you can ensure superior comfort, superior efficiency, zero mold growth potential and longevity of structure.
Now I do need to clarify that Thermal Bridges are much more complicated than I described above. I would need to go into ψ values, boundary conditions, ISO Testing (International Organization for Standardization), positive and negative thermal bridges, and more. In fact there is a 3 day course you can take after you get certified as a Passive House Consultant just to learn how to identify and properly analyze.