Building Science, summarized simply, is the study of heat, air and moisture movement through walls, floors and roofs. In Europe it's called Building Physics. Building Science experts work to make buildings more energy-efficient, healthy, and durable. They know about types of foam, and vapor retarders, and pressure differentials.
Most architectural historians don't pay much attention to Building Science. They care about buildings' narrative meanings. They like to 'read' and interpret buildings much like literary scholars like to read and interpret texts. They don't like to study heat transfer.
I prefer to read and interpret buildings too. I love to discover spaces that have rich layers of meaning, such as William Alexander's Halliburton house. I enjoy offering new ways of seeing important buildings, such as my look at the gravity-defying details in Kahn's Kimbell Art Museum (pdf). And a primary interest remains: how are social relationships constructed and reflected in modern housing? To talk about r-values and roof overhangs might seem pedestrian by comparison.
So why do I care about Building Science? I think I can explain it best by analogy: to be an architectural historian interested in Building Science is like being an art historian interested in picture frames. Imagine an art historian walking through the Louvre and examining the surrounds rather than studying and interpreting the content of the paintings. You'd wonder about that person's good judgment.
Now imagine every picture frame in the Louvre was clearly wrong in some manner --- out-of-square, or built of a material which damaged the painting, or prone to falling off the wall. In that context, you'd understand why an art historian would be curious about the history of picture frames, and why they were made that way.
To study the canon of 20th-century architecture is like walking through a Louvre full of paintings in broken frames. Take the Farnsworth house: the (immensely interesting) content is, for me at least, overshadowed by the fact that the building simply did not work as a building. Ice formed on the inside of the walls, the space overheated in summer. This is hardly an exceptional story. By 1960 James Marston Fitch noticed that "the modern architect [is] quite removed from any direct experience with climatic and geographic cause-and-effect."
Eventually you get more interested in the frame-making than the content of the picture. And pretty soon you find a few people who advocated for better picture frames, and then some people who used new methods to make picture frames correctly, and finally the very rare figures who understood that the making of the frame was in fact integral to the making of meaning within the frame. You begin to see that it's impossible to separate the picture and the frame. You want to honor the people who figured out how to do it right.
In other words, I care about Building Science because I need to care about it in order to understand and explain 20th-century architecture properly.