宇

Top roof radical — a pictograph of a peaked house cover with side walls. The indexing radical, anchors 宇 in the dwelling family: 家 home, 安 peace, 宿 lodge, 室 room, 宫 palace. Marks the original meaning of 宇 — the eaves of a building — which broadened metaphorically into 'space, the heavens, the cosmos' (宇宙 universe = roof of space + flow of time).

Bottom 于 supplies the sound — yú shifting to yǔ with tone change. Pure phonetic role; the 'in / at' preposition meaning of 于 contributes nothing semantically. Same phonetic family: 吁 to sigh, 盂 vessel, 竽 mouth-organ. The character was crafted in classical times as a clean phono-semantic compound — roof for meaning, 于 for sound.