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Typically we have a double walled container with a vacuum between the walls which limits or stops conduction and convection heat transfer. They are also (usually) aluminized to minimize heat transfer by radiation.

Does this have much effect? In other words, how much worse would a regular flask be without the reflective coating?

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Consider a black Body at 100°C with an ambient temperature of 0°C. The energy loss $P$ due to radiation is $$P = \sigma \times (T_1^4-T_2^4)\text{ (Stephan-Boltzman)}$$ which gives $782.6\:\mathrm{W/m^2}$. With a surface of $500\:\mathrm{cm^2}$ it turns out as $\approx 40\:\mathrm{W}$, so quite significant.

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    $\begingroup$ Please use MathJax for math typesetting. $\endgroup$
    – Danu
    Commented Nov 8, 2015 at 20:27
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    $\begingroup$ +1 A full answer would also estimate how much of this 40W was turned back by the aluminum coating. Not being critical, just noting this so that the OP can modify their question if they want to. $\endgroup$
    – Selene Routley
    Commented Nov 8, 2015 at 22:15

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