Glassed-in means shut in by glass — walls or a roof of glass around a space, so you are inside a glass structure. A glassed-in porch still feels like a porch, but rain and wind stay out. A glassed-in balcony or walkway is the same idea: not open to the air, not a solid brick room either, but wrapped in glass. It is a plain, practical adjective, used in houses, hotels, and building talk. The thing in the middle is not itself a sheet of glass; the glass is around it. Hyphenated glassed-in is the usual written form when the words sit before a noun. Though actually, in British slang glassed can mean someone was hit with a drinking glass — a completely different, violent use. That is not this word.