Siman 277

If one wrote the parchment and comes to place it so that it may dry – he should not turn the writing downward, even though he intends that dust should not rise upon the writing; nevertheless, it is a way of disgrace. Rather, the face of the writing should be upward, and he should spread a cloth over it. And it is proven in Eruvin (Eruvin 58a) that when it is impossible to spread a cloth over the writing – it is better to turn the writing downward and the back of the writing upward. For if he leaves it thus without turning, there is a greater disgrace (Shach and Taz). For it is a great disgrace to leave it completely open, without a cover and without turning.

Therefore, one must be careful not to leave a book open and depart, even with other books, and even with printed Gemaras and poskim, for it is a disgrace. Rather, he should either close them, meaning they should not be open, or cover them with a cloth. Aside from this, it is detrimental to memory (Shach, see there). And such is the custom to be careful in this. And it seems to me specifically when one goes out to the street. But one who enters from room to room, or even to the courtyard or the street, not for a lengthy period of time – there is no concern in this (and so wrote the Ya'avetz, see there).

A sefer Torah that has begun to decay, and it is necessary to spread it out so that the wind may blow upon it, and they need to spread it out completely and the wind must blow specifically upon the letters, and it is impossible to flip it and impossible to cover it, because the primary repair is that the wind should pass over the letters—it is permitted, since he does this for its need and for its honor (Pitchei Teshuva in the name of Chavat Da'at). And such is the custom with sefarim, when they remain for a long time and they fear that they will become moldy—they spread them out open in the courtyard facing the wind. And on the contrary, it is a mitzvah to do so, and this is not a disgrace but rather an honor, and this is their restoration.