The silvering in thermos flasks is done to avoid heat transfer by:

A thermos flask, or vacuum flask, is designed to minimize heat transfer through conduction, convection, and radiation. While the vacuum between the double walls effectively eliminates heat transfer by conduction and convection (which require a material medium), it cannot stop radiation [1]. Radiation consists of electromagnetic waves that can travel through a vacuum [1]. To address this, the inner surfaces of the flask are silvered to create a reflective, mirror-like coating. This silvering reflects infrared radiation back into the flask (if the contents are hot) or away from it (if the contents are cold), thereby significantly reducing heat loss or gain via the radiative process. Consequently, the specific function of the silvered surface is to prevent heat transfer by radiation.

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 7: Heat Transfer in Nature > Why is it more comfortable to wear white or light-coloured clothes during summers and dark-coloured clothes during winters? > p. 97