Is it permissible for women to tie their hair in a bun or a knot due to the summer heat or to avoid its visibility from under the hijaab?

Will Salaah be valid if done so?

If the hair can be tied in such a way that it does not become a bun on top of the head then this must be done, otherwise it has to be tied at the back of the neck.

This does not invalidate the Salaah but Salaah is an act of obedience so if someone can obey Allah in performing Salaah then they should obey him in removing the camel hump hairstyle or something similar to it, so as not to be guilty of "selective obedience."

And Allah knows best.