The short answer: your house gets sun all day, and the roof and walls turn that radiation into heat that enters by conduction and radiation. The useful answer is below.
Your house heats up because a surface exposed to the sun, almost always the roof, absorbs radiation and transmits it indoors. In Panama this is made worse by humidity, which keeps the body from cooling off, and by increasingly hot nights.
A conventional roof exceeds 65 °C at the surface. That's the main source. See evidence
Without a ceiling, the roof radiates straight inward. Without eaves, windows let the sun in.
High humidity stops sweat from evaporating, so the same temperature feels more stifling.
Gallardía's thermal diagnosis identifies the actual critical surfaces before you spend a dollar.
Every important claim links to its evidence. Tap to see the source, its context and its limits: DOE Heat balance MiAmbiente. Full bibliography in the evidence library.