Shiny coatings and radiation

Question: How does the shiny outside coating on the jug reduce heat loss?

Answer:

Heat energy loss from an object depends on three things:
(a) the difference in temperature between the surface and the surroundings
(b) the area of the surface
(c) the nature of the surface (whether it is rough, shiny, black, white etc.)

You wanted to know about the last one.

Rough matt black surfaces are the best radiators of heat energy at a given temperature.

Shiny surfaces do not radiate well and so they lose much less heat energy than a black jug at the same temperature. Therefore the shiny coating on the outside of your jug helps to keep the jug warm. (the quantity to look up in advanced books is emissivity and see how it relates to the Stefan-Boltzmann Law).