Which construction method is most vulnerable to the upward extension of fire through walls?

The balloon frame construction method is particularly susceptible to the upward extension of fire through walls due to its design and structural characteristics. In a balloon frame, the wall studs run continuously from the foundation to the roof without any horizontal fire stops at each floor level. This creates vertical spaces or "chimneys" alongside the wall cavities where heat and flames can travel unchecked from one floor to another.

In contrast, heavy timber construction features large wooden members that, while they can ignite, have significant mass and resist collapse longer than lighter materials. Platform framing includes floor systems that form a barrier at each level, thus restricting the vertical movement of fire. Ordinary construction combines materials like brick or concrete with wood framing, which also helps mitigate fire movement.

Because balloon frame structures lack these crucial fire-stopping features, they pose a heightened risk during a fire, allowing flames to spread rapidly through vertical spaces, thereby increasing the danger to occupants and first responders.