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.

Ванной is prepositional singular. It can come from:

• the noun ванная (common for bathroom), or
• the adjective ванная used as a noun (literally bath-room). Either way, in в + prepositional, it becomes в ванной.

This is an impersonal construction: мне плохо спалось.
Here, мне is dative and marks the person who experiences the state/feeling (literally: to me it slept badly). It’s a common Russian way to describe how something felt or worked out for someone, especially sleep, thinking, working, etc.

Спалось comes from the reflexive verb спаться (not спать).

• Я плохо спал = a straightforward statement: I slept badly.
• Мне плохо спалось = emphasizes the experience/conditions: I couldn’t sleep well / sleep didn’t come easily (often because of something like noise, heat, humidity, stress).

Because the sentence is impersonal—there is no grammatical subject like я. In Russian, impersonal past forms often appear in neuter singular: (мне) спалось, (мне) не работалось, etc.

It’s past tense and effectively imperfective (ongoing/overall situation rather than a single completed result). It describes the general quality of sleep over that time: it was sleeping badly / sleep wasn’t good.

Плохо is an adverb modifying спалось. It describes how the sleeping went: плохо спалось = slept badly / had poor sleep.

Word order is flexible, but it changes emphasis. For example:

• Из‑за влажности в ванной мне плохо спалось. (neutral: cause → place → result)
• Мне плохо спалось из‑за влажности в ванной. (focuses first on the result)
• В ванной из‑за влажности мне плохо спалось. (brings the location forward) All are grammatically possible; the original sounds natural and clear.