The term for the space beneath a “cathedral” ceiling
Solution 1:
Up in the eaves
Yes, I know what eaves are to an architect and builder, but when you are talking about the interior of a building, up in the eaves means above the roof/wall joint. A google image search of up in the eaves returned the following images -
Curated examples are a bit harder to find, but Collins Dictionary online has the following example - A wasp nest right above my desk, in the eaves of the roof. Times, Sunday Times (2014). Eaves: Collins dictionary online
And after all this, the question is probably a duplicate of this one.