The way home inspection standards address firewalls is somewhat convoluted. Though home inspection concerns and building code concerns regarding firewalls are not really at cross purposes, they aren’t perfectly in sync either. A firewall is an implementation designed to satisfy a twofold desire.
The motivation behind home inspection Standards of Practice regarding firewalls is not to verify that the building code has been followed so much but to determine whether the dual desire holds true. The first part of this desire is to prevent exhaust fumes, particularly carbon monoxide, originating in the garage, from entering the living areas of a residence. The second part is based on the presumption that house fires are most apt to ignite in the garage; should this occur, the wish is to isolate the fire to the garage or at least delay its spreading to the house proper long enough to give its occupants, especially children, adequate time to evacuate.
The abstract concept of a firewall, then, is something that accomplishes both parts of the protective desire. Building codes govern how the abstract idea is materialized, and years of putting the codes in practice determine whether the purported goals are achieved.
Home inspection standards essentially circumvent the building codes and go directly to the question at hand. They stipulate that the home inspector shall inspect fire separation between the house and the (attached) garage. The accepted interpretation of “fire separation” is that a fire begun in one space will spread to the other space only after a time delay of at least one to two hours. Furthermore, the time delay must hold for all possible paths, both directly through windows and doors and indirectly through ceilings and attics.
Obviously, during the course of a home inspection it is impossible to measure delay of fire penetration. Therefore, the home inspector must go one degree further removed from the abstract idea and use guides that translate the implementation of certain construction practices and materials into an expected penetration delay. What this comes down to practically for inspectors are concerns about pedestrian doors between garage and house and concerns about drywall thickness.
Home inspectors check for the pedestrian door(s) to have a solid core and to be fire rated. A fire-rated door usually has a label identifying it as such on the side of the door where the hinges are mounted. Inspection standards waive the fire-rating determination requirement when the label doesn’t exist. At any rate, the solid core and fire rating yield a satisfactory expected penetration delay.
The appropriate drywall thickness for both walls and ceilings has been empirically determined to be one-half inch. Home inspectors can’t really measure this reliably, although drywall edges are sometimes exposed in unfinished spaces and attic hatches. Here, building codes and inspection standards coincide, so that if the construction of a home has gone through the proper channels, the inspector can be reasonably confident that building inspectors have verified drywall thickness.
Codes for commercial buildings and multifamily dwellings stipulate the additional requirement that walls between units must continue through the attic to the roof. This requirement does not apply to single-family residences, even between garage and house. Usually garages have either no attic or an attic isolated from the house attic, but the author has witnessed one continuous space over both areas.