2 x 4" with a 1" space in-between each board. The wall here is actually a double wall of slats with one side offset from the other side so you can't see through it, but some light filters through.

2 x 4"

Do you mind sharing the size of the slats on the reverse side? You mention they are offset for privacy, are they also 2x4 or smaller? Thanks!

Both sides are 2x4" boards with a 1" gap.

Thank you!