Yosef Karo

“ There are those who say that this small space should not be empty space but must be wall or schach (Ros"h in the name of the R"i, Rambam ch. 4) . It must be 7x7 tefachim and taller than ten tefachim (Tur) . The walls, which are taller than ten, must also be made out of materials that can be used as schach (Ra"n the end of ch.1 of Sukkah) . If all of the walls are made of materials that can be used for schach one may sleep even under the walls (commentary of Rabbeinu Yerucham 1:1) . ”

“ If it is thick, [making it seem] like [the roof of] a house, such that the stars are not visible from inside, it is valid.
סעיף ג
דרך הסיכוך להיות קל כדי שיראו ממנו הכוכבים הגדולים היתה מעובה כמין בית אע"פ שאין הכוכבים נראים מתוכה כשרה:
631:4 If the schach is a very lightly placed with much empty space but no one place has three handbreadths of empty space and as a whole, the shade exceeds the sun, it is kosher. ”