At room temperature a dull surface loses about as much heat by radiation as by convection in still air, each worth roughly 6 W for every square metre and every degree.
Radiation behaves like a convection coefficient of 4εσT³. For a black surface at 20 °C that is 4 × 0.97 × 5.67×10⁻⁸ × 293³, about 5.7 W/(m²K), and natural convection in still air is 3 to 8. A bare person in a 20 °C room loses about 150 W by each route.
A shiny surface (aluminium foil, emissivity 0.05) radiates almost nothing, and any air movement multiplies the convection: a 10 m/s wind makes it about 40. For hot things radiation takes over because it grows with the fourth power of the temperature: a bare pipe at 150 °C radiates about 1.4 times as much as it convects.
Estimate with the rule, then check it against the calculator that models it properly.
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