How to control the temperature inside a fume hood/ fume cupboard?

If you are doing a wet reaction, solid CO2[-28C], or liquid N2[-193C] or suitable combinations .

If the reaction generates moisture, the hood has to be suitably modified. You must keep enough of cooling agent to finish the reaction, & an appr. temp. rise in the expt has to be computed.

Very difficult if a current of air is actually running through it;

have to add heat to compensate for heat outgoing.

Depends upon what you're doing.