An interior wall (such as a wall separating your living room from your kitchen) is a tlhoy'.

An exterior wall (that is, a wall which separates the inside of a building from the outside) is a reD.

For the interior side of an exterior wall, it is quite common to use tlhoy', but the phrase pa' reD, literally "room's exterior wall" (pa' "room") is also heard, referring to the wall in a room which faces outside (as opposed to the other walls in the room whose other sides are still indoors).

(Usenet message of December 07, 1998)

Use reD "exterior wall" for an object's surface that's not the top or bottom or front or back. That is, the use of reD need not be restricted to structures. If you need to be more specific, you can add in poS "left (side)" or nIH "right (side)."

(QelIS boqHarmey – Alice im Wunderland auf Klingonisch, p. 238)