Questions & Answers about Из‑за влажности в ванной мне плохо спалось.
Из‑за is a fixed compound preposition and is normally written with a hyphen. It requires the genitive case (родительный падеж): из‑за (чего?) влажности.
Влажности is the genitive singular form of влажность (feminine noun). It’s in the genitive because из‑за governs the genitive: из‑за влажности = because of the humidity.
Because в ванной describes location (being somewhere), which uses в + prepositional: в (где?) ванной = in the bathroom / in the bath area.
В ванну would be direction/motion (going into), using в + accusative: в (куда?) ванну.