Accurate　evaluation　of　the　moisture　effect　on　the　thermal　performance　of　building　envelopes　is　essential　to　facilitate　energy-saving　building　design　in　hot　and　humid　climate　areas.　However,　in　extremely　hot　and　humid　climate　areas,　the　coupled　effect　between　the　heat　and　moisture　transfer　is　mostly　neglected　and　a　method　to　evaluate　the　moisture　effect　on　the　thermal　performance　has　never　been　proposed.　Thus,　in　this　paper,　a　model　is　developed　to　simulate　the　coupled　heat　and　moisture　transfer　inside　an　exterior　wall　subjected　to　extremely　hot　and　humid　climate.　This　model　is　validated　using　three　benchmark　tests　and　shows　high　accuracy.　Based　on　the　model,　the　effect　of　the　moisture　on　the　thermal　performance　of　exterior　walls　in　the　tropical　islands　is　firstly　investigated　quantitatively.　The　results　show　that　moisture　can　substantially　affect　the　thermal　performance　of　an　exterior　wall.　Neglecting　the　moisture　effect,　the　total　load　is　underestimated　by　6.04-7.33%.　Considering　the　moisture　effect,　the　decrement　factor　is　improved　from　26.27-38.28　to　79.40-114.41　and　the　time　lag　is　prolonged　by　71-79　min.　Finally,　to　clarify　the　spatiotemporal　dynamic　pattern　of　the　moisture　effect,　a　comparison　between　the　four　tropical　islands　and　the　continental　city　is　performed.