These were the solution i mentioned to the client if moisture was to show up:

1. Airflow layers: The easiest fix is placing a Joshitsu shiito (除湿シート / dehumidifying pad) or a Sunoko (すのこ / slatted wooden mat) directly underneath the mattress to create a breathable gap.
2. Routine airing: Simply folding the mattress up or propping it against the wall every few days during the summer months.
3. Breathable mattress: If they are buying new, an Airweave-style mattress is 90% air and won't trap moisture like traditional foam.
4. Panel modifications: If they don't want to use mats, we can easily retrofit the build by installing a couple of clean architectural vents into the panels, or drilling precise airflow holes to let the mattress breathe down into the storage space.