I ran my bathroom extractor fan up into the loft just to avoid drilling through the wall: when I lifted the insulation two winters later, it was too late
Venting a bathroom extractor fan into the loft instead of outside seems like an easier installation, but it unleashes a hidden disaster: warm, moisture-laden air condenses on cold roof timbers, insulation, and felt, causing mold growth, thermal performance loss, and structural rot that can cost thousands to repair. Building regulations explicitly forbid this practice for good reason.