This past January, 2022, we had a particularly long cold snap. I suspect that the cold gradually migrated in through the upper walls and resulted in some condensation developing. You see this sort of thing regularly on windows, and the solution is the same: you want some air movement to dry up the condensation. However, the shelves in the back of our closet are packed with clothes, so there is no air movement, and the moisture likely remained, which led to mold.

We had a similar issue about 5 years ago, but then it was just on the side wall of a closet that was an exterior wall. The solution I came up with then was to build a small pair of spacers out of thin scrap wood. In many ways they resemble a small pallet. These are placed along the wall and ensure that clothes do not touch the walls, keeping a gap open for air movement.

One other solution, is to use plastic (or other) bins and totes on your upper closet shelves. These will keep your clothing or items protected inside a bin, and usually they will leave more of a gap around their sides which gives that all important air gap.

And of course, another solution is to purge your closets so there is not so much stuff packed in there!!

I pulled out a bunch of scrap wood from my stash. I only need thin and small pieces for this, so it's definitely the time to work through your scraps.

For the slats I ripped a bunch of pieces to be about 1/8" (3mm) thick. The widths varied, and did not matter, but most were about 1" (25mm) wide.

For the uprights I ripped my stock into 3/8" by 3/4" pieces, and 12" long. (9.5mm x 19mm x 300mm long).

I am making spacers for three closets, all about 40-45" wide (1-1.1m), so I needed a lot of thin strips!

I want to maximize the air movement, so I used the router table to carve two gaps along the bottom of the uprights. I first started with a fairly deep 1/4" cut, but that was too hard (or my router bits were too dull/cheep). I really don't need a large gap, just something to allow air to move so I cut back to a smaller gap.

Here I am showing the air gap I cut on the router table on two of the uprights.

I used my stationary belt sander to smooth the edges and corners of all the wood strips. I don't want any sweaters or items getting caugh on these spacers!

I then positioned the uprights on my workbench and tacked the slats in place. Since these were quite long, I used 5 uprights on each, and I spaced the slats about 3/8" (9mm) apart

Here my three mold preventing air gap spacer gizmo things are ready to be installed.

They were sized to fit along the back wall with about 1/4" (6mm) to spare. These are just held against the back of the closet shelf by friction, and by the pile of sweaters in front of them.

I made mine out of scrapwood, but it would be a VERY good idea to make these out of (or at least partially out of ) aromatic cedar. I can't take the credit for that, I got the idea from a comment on my video --- AFTER I had built them.