Should I bring a heavy raincoat?

No. The humidity is so high that you will sweat more inside a heavy raincoat than you would get wet from the rain. A lightweight, breathable rain shell or a cheap poncho bought locally is better.

How do I stop my clothes from molding?

Keep your suitcase or backpack off the floor. Use silica gel packets in your bag and rotate your clothes daily. If you stay in a hotel, keep the air conditioning running at a moderate temp to dehumidify the room while you are out.

Packing for Monsoon Season in Southeast Asia

Prioritize quick-drying synthetic fabrics and high-quality waterproof gear over heavy raincoats. You need to keep your electronics in dedicated dry bags and accept that you will eventually get wet regardless of your gear.

Choose the right base layer. Avoid cotton at all costs; it takes days to dry in 90% humidity. Pack moisture-wicking synthetic shirts and shorts that dry in under two hours.
Use waterproof dry bags for electronics. Don't trust 'water-resistant' backpacks. Buy a 10L or 20L roll-top dry bag to house your camera, phone, and passport while moving between transport hubs.
Upgrade your footwear. Bring one pair of waterproof sandals with aggressive tread (like Chacos or Keens) rather than hiking boots, which will become heavy, soggy sponges if they get soaked.
Manage your damp laundry. Bring a dedicated 'wet' dry bag for your used clothes to prevent mold from spreading to your dry gear, as clothes will rarely air-dry fully during the monsoon.

Should I bring a heavy raincoat?: No. The humidity is so high that you will sweat more inside a heavy raincoat than you would get wet from the rain. A lightweight, breathable rain shell or a cheap poncho bought locally is better.
How do I stop my clothes from molding?: Keep your suitcase or backpack off the floor. Use silica gel packets in your bag and rotate your clothes daily. If you stay in a hotel, keep the air conditioning running at a moderate temp to dehumidify the room while you are out.