A. The important question is the reason for the discomfort. The Talmud says that a person who is in a state of distress in the sukkah does not have to stay there and may leave (Sukkah 26a).
But what about the verse in the Torah (Lev. 23:42) that says, “You shall dwell in the sukkah”?
The sages understand this verse as saying that you must dwell in the sukkah as you would dwell in your house. In the house you are entitled to make yourself as comfortable as possible, e.g. by using a heater in cold weather, and in the sukkah you are also entitled to take necessary steps to ensure your basic comfort.
I have been in a sukkah that had central heating and another with air conditioning. So if you can ease any discomfort you should do so.
However, since the greenery that covers the sukkah cannot be so thick that it stops the rain from getting in, in time of heavy rain no-one needs to sit there and suffer.
It is of course possible to have a waterproof roof but once that is on, one is not fulfilling the mitzvah.
The question has been asked, what about a highly nervy or grumpy individual who is bound to complain of discomfort the moment they come into the sukkah?
Such a person is also likely to complain inside the house, and it actually might calm them down to have a L’Chayyim! in the sukkah and enjoy the simchah of the festival.