In a typical (leaky) compartment, there is a small overall pressure rise COMPONENT associated with the growing quantity and temperature of heated gases dominating leakage. Of course, if prexisting flammable vapors are present there will be a significant pressure surge.

As a fire develops in a typical leaky compartment, the average temperature rises, leading to a continuous rise in pressure in the top portion of the compartement and a continuous drop in pressure (below atmospheric) in the bottom of the compartment.

A hermetically sealed compartment would undergo a substantial net increase in pressure during the growth of a fire. The magnitude of the pressure would be roughly proportional to the average absolute temperature with a correction for changes in the number moles of gas in the compartment resulting from combustion, vaporization and pyrolysis. In rare cases, such as metal fires, the change in the number of moles could be negative.

Examples:

Charcoal: C (solid) + O2 (gas) --> CO2 (gas) -- No change in number of moles of gas

Carbohydrate: (approximate) CH2O (solid) + O2 (gas) --> CO2 (gas) + H2O (gas) -- Number of moles of gas increases

Magnesium: 14Mg (solid) + O2 (gas) + 4N2 (gas) --> 4Mg3N2 (solid)+ 2MgO (solid) -- Number of moles of gas decreases (to zero in an ideal reaction - yielding a vacuum)