Why does a wooden door swell up and stick in damp weather?

Wood absorbs moisture from the air and expands slightly in volume - it's a natural process you can't really avoid. To keep the door from sticking, you need to coat it with varnish or paint on all sides (including the edges). This will protect the material from excess moisture and significantly reduce swelling.

Silicone sealant around the perimeter of the door frame - that's what actually works when the door is already installed and it's too late to redo everything. We had an old wooden front door at home that would start sticking in the summer humidity so badly that my husband could barely open it. We sealed the gaps with sealant, and the problem disappeared - moisture stopped seeping through the cracks between the frame and the wall.

Of course, it's not magic and doesn't prevent the wood itself from swelling, but it stops the main source of moisture - direct contact with outside air. It helps even more if the door was treated with quality oil or wax during installation rather than just painted.

Do you have a lot of gaps between the door and the frame? If the door's already sticking and it's too late to redo it, try installing a higher threshold or adding weatherstripping at the bottom - that'll at least cut down on drafts and keep moisture from coming in as aggressively from below. But honestly, the most reliable way is really to treat the wood before installation, and not just with varnish but with a deep-penetrating primer that creates a dense layer and prevents the wood from absorbing moisture so actively.

Don't listen to anyone who says there's nothing you can do about it - you can actually work with a wooden door, you just need to understand that the process happens from both sides at the same time. I had an entry door in an old house, and the problem started precisely because someone had only sealed it on the outside back then - it looks nice, but the inside was bare. In spring, the door started sticking so badly that you couldn't open it without real effort, and closing it was even worse.

I had to redo it: I sanded the door down and varnished it on all four sides, including the edge cuts (the end grain absorbs moisture especially heavily). After that, the swelling problem practically disappeared - you can really feel the difference. If the door is already installed and you don't feel like redoing it, at least seal the perimeter of the frame with caulk to slow things down, but that's only a half-measure.