Every surface above absolute zero radiates. The rate is $P = \varepsilon \sigma A T^{4}$ — P equals epsilon sigma A T to the fourth. $\varepsilon$ (epsilon) is the emissivity, a bare number from 0 to 1: 1 is a perfect black body, oxidised steel and most paints run 0.8–0.95, and polished aluminium foil drops near 0.05, which is exactly why it is wrapped around hot pipes. $\sigma$ (sigma) is the Stefan–Boltzmann constant, $5.67 \times 10^{-8}\ \mathrm{W/(m^2 \cdot K^4)}$ — a constant of nature, not something you look up per material. $A$ is the radiating area and $T$ is the absolute surface temperature in kelvin.

Absolute, always. A fourth power has no idea where you chose to put your zero: double the kelvin and you get sixteen times the radiation, but double the Celsius reading and you get nonsense. Convert first — $T(\mathrm{K}) = t(\mathrm{^\circ C}) + 273$ — and convert BEFORE the power, never after.

A surface in a room is also being radiated at. The net traffic is $\dot{Q} = \varepsilon \sigma A (T_1^{4} - T_2^{4})$, where subscript 1 is the surface you are standing at and subscript 2 is the surroundings looking back. Note what that is NOT: $T_1^{4} - T_2^{4}$ is not $(T_1 - T_2)^{4}$, and the two are worlds apart.

For modest temperature differences the radiant term can be linearised into a coefficient and simply added to the convective one: $h_t = h_c + \varepsilon \sigma (T_s + T_{sur})(T_s^{2} + T_{sur}^{2})$, with $h_t$ the combined coefficient, $h_c$ the convective one, $T_s$ the surface and $T_{sur}$ the surroundings, both absolute. On a warm lagged pipe in still air the radiant half is often the LARGER of the two — the reason a survey done by convection alone always reads low.