The Pack-Out Problem

The first week is good. Somewhere around week six, you notice you've stopped noticing them, which sounds like a compliment but isn't. By month three you're back where you started, and the insoles still look fine. No cracks, no tears, no obvious wear. So you assume the problem is you.

It isn't you. It's the foam.

Most cushioning foam, the kind in the insole you just replaced, is made of closed cells: millions of sealed gas bubbles trapped inside a polymer structure. Sealed cells are cheap to produce, easy to mold, and feel great in the store, because nobody has stood on them yet.

Every step compresses those bubbles. A fresh pair springs back fast enough that you don't notice the lag. But sealed gas doesn't move, it just gets squeezed, and over thousands of compressions the cell walls start to rupture and fold in on themselves. The bubbles that once held air permanently deflate.

The foam is still there. It just isn't doing anything anymore.

Materials engineers call this pack-out. It isn't a defect and it isn't bad luck. It's the normal, expected end state of closed-cell foam under repeated load. Every closed-cell insole reaches it. The only question is how many weeks it takes.

Open-cell foam is built from a connected network of pores instead of sealed pockets. When you load it, air is pushed out through the connected structure rather than trapped and crushed. When the load lifts, air moves back in and the structure returns to shape. It doesn't fatigue the same way, because there's nothing sealed inside it to rupture.

Three reasons, and none of them are about your feet. It's cheaper to produce. It's easier to mold into a consistent shape at scale. And it feels best on the exact day you're standing in a store deciding whether to buy it. Put those three together and you get the default that most of the category still ships.

Pull the insole out of your shoe and press your thumb into the arch for two seconds. Let go. If it springs back within a second or two, you're still in open-cell territory. If it holds a visible dent, or comes back slowly and only partway, that foam has packed out, whatever the outside looks like.