壁面 and 外壁: a wall’s surface and an exterior wall
Compare 壁面 used for posting a map or cleaning a wall surface with 外壁 for repainting or damage to an exterior wall. Follow the naming criteria of surface and exterior location.
The wall surface named 壁面
壁面 (へきめん) means wall surface. 「駅の壁面に大きな案内図が掲示されている。」 means “A large guide map is posted on the station wall.” The map is posted on a wall surface; this sentence alone does not locate it specifically indoors.
「作業員は建物の壁面を洗浄した。」 means “The worker cleaned the exterior wall surface of the building.” This translation describes an exterior wall surface. 壁面 itself names the surface of a wall.
The exterior wall named 外壁
外壁 (がいへき) means exterior wall. 「古い家の外壁を白く塗り直した。」 means “We repainted the old house's exterior walls white.” The outside walls of the old house were repainted.
「強い風で建物の外壁が一部壊れた。」 means “The strong wind damaged part of the building's exterior wall.” The damage concerns part of the exterior wall, rather than a statement that the whole building collapsed.
Surface and exterior location are different criteria
壁面 focuses on a wall’s surface, while 外壁 focuses on the wall on the outside of a building. An exterior wall also has a surface, so the targets can overlap.
Do not restrict 壁面 to indoor walls and treat the terms as indoor and outdoor opposites. Follow the surface carrying the map, the surface cleaned, or the wall repainted in each sentence.
Are 壁面 and 外壁 always indoor and outdoor opposites?
Show the answer and why
No. 壁面 names a wall’s surface, and 外壁 names an exterior wall. An exterior wall also has a wall surface, so the targets can overlap; 壁面 is not restricted to indoors.