Why does my spare room smell like damp cardboard every time I open the door?

The issue you're describing is almost certainly a lack of **return air path**. In Florida's humidity, your AC works by both cooling the air and pulling moisture out of it, but it can't do that if the air inside a room is trapped. When you close that door, you’re essentially sealing a pressurized box. The supply vent keeps pushing air in, but without a return vent in that specific room, the air has nowhere to go. The pressure builds up, the airflow from the vent slows to a crawl, and the air just sits there getting stagnant and damp. Since your house is from the 90s, it’s very common to have one central return in a hallway rather than individual returns in every bedroom. If there isn't a significant gap at the bottom of that door, you've cut off the circulation. You can test this easily: hold a piece of tissue near the bottom of the door while it's almost closed and the AC is running. If it doesn't flutter, you have zero airflow.

Before buying a dehumidifier, check the **undercut** of the door. Most internal doors need at least a 1-inch to 1.5-inch gap between the bottom of the door and the flooring (carpet or tile) to allow air to escape back to the main return. If your carpet is thick, it might be choking off that gap entirely. Shaving an inch off the bottom of the door with a circular saw is a permanent, zero-cost fix that usually solves the 'musty room' syndrome in block houses.

If trimming the door feels too invasive, you could install a **transfer grille** or a jump duct, but those involve cutting into your drywall. A simpler alternative is a 'door vent' which is just a louvered insert you install directly into the lower panel of the door itself. It looks a bit like an office door, but it allows the air to move freely even when the door is shut. Either way, you need to stop thinking of the door as a seal and start thinking of it as part of your ventilation system.

I had no idea the door was actually part of the ventilation! I always thought it was just there for privacy. If I go the "undercut" route, how much of a gap is too much before it looks like I just messed up the door? My guest room has pretty thin carpet, so I'm worried a huge space at the bottom might look a bit off.

Also, you mentioned a transfer grille—does that have to go in the door itself, or is that the thing in the wall? I'm not really handy with a saw, so I'm trying to figure out wich of these is the easiest for someone who doesn't know much about construction.

Spot on about the return air path. One thing people ussually overlook when measuring that undercut is the transition strip or the 'saddle' on the floor. If you've got a thick marble or wood threshold between the hallway and the bedroom, it can eat up half an inch of that gap you think you have. Even if the door looks high off the carpet, that hump underneath acts like a dam for the air. If you're worried about the gap looking too big, look into 'Z-line' grilles; they hide the line of sight while still letting the air move thru the bottom of the door panel.

I'd definately check if you've got those 'hollow core' doors. If you decide to trim the bottom, don't go too crazy or you'll literally cut the bottom rail right out and be left with a cardboard sandwich. If you need a massive gap, sometimes it's easier to just swap it for a louvered door entirely. They're great for air circulation and honestly don't look bad for a guest room or laundry closet. Plus, it saves you the headache of trying to get a perfectly straight cut with a circular saw if you aren't handy with tools.

I'm not totally sold on the airflow theory being the only culprit here. Have you ruled out the door material itself absorbing moisture? If it's one of those cheap 90s MDF or pressed wood doors and it hasn't been sealed on the top or bottom edges, it coud be acting like a giant sponge for the Florida humidity. Before you go hacking an inch off the bottom, maybe check if the door feels 'heavy' or if the edges look swollen. If the wood itself is holding the stinking damp, no amount of return air is going to fix that funk.

Quick update — I tried leaving the door ajar for a few days like suggested, and the smell totally vanished, so it’s defintely a return air issue. However, I checked my door and it’s one of those hollow core ones. If I try to undercut it to improve the airflow while it's closed, how do I avoid ruining the bottom rail?