The key passages in the base game rules are:
Flashover – Flip any Smoke marker in a space Adjacent to Fire to Fire. Repeat this step as many times as you can – there should be no Smoke Adjacent to Fire at the end of a turn. Remember: Smoke Adjacent to Fire = Fire.
Under Secondary Effects on page 6.
Any reference in the game to Adjacent spaces means those spaces that are up, down, left, or right from a space. Diagonal spaces are not Adjacent. Closed Doors and Walls prevent spaces from being Adjacent – unless the Wall segment is Destroyed. A Wall segment with two Damage Markers is Destroyed. A Wall segment with one damage marker is Damaged.
Under Adjacent Spaces on page 2.