Airtight lunch boxes don't stop condensation. Here's what does.

Airtight seals won’t stop your lunch box from sweating — the water is coming from the air, not the box.

Condensation happens when warm, humid air touches a cold surface. If your lunch box just came out of the fridge and your bag is room temperature, the outside of the box will collect moisture no matter how tight the lid is. Same thing inside: warm food sealed in a cold box will fog up the lid. The seal only stops air exchange; it does nothing about temperature difference.

What actually works is managing the temp. Use an insulated lunch bag — that’s the real dry-bag here. If you’re using a regular backpack, wrap the box in a kitchen towel or an old t-shirt. That adds a buffer layer so the outside of the box doesn’t get cold enough to trigger sweating. A small freezer pack also helps because it keeps the box’s temperature stable, so you get less of that warm-air-meets-cold-surface effect.

A separate dry-bag is overkill. Those are for keeping water out when you’re submerging something in a lake or river — not for condensation. Condensation is on the surface, not a leak. Unless you’re canoeing to work, skip it and grab a towel.

Final thought: save the dry-bag for kayaking, and let the towel do the real work.