NaOH Normality Slowly Changing
I would be more worried about carbon dioxide (rather than water) being absorbed into the NaOH solution. A 1 molar NaOH solution is not going to be appreciably hygroscopic. But it will absorb CO2: two moles of hydroxide yield one mole of carbonate ion, which, depending on what you are titrating, may act as only deprotonate one mole of acid per mole base - reducing the effective normality of the NaOH solution. You could try protecting any air vents with a drying tube filled with sodium or calcium hydroxide, or use a regulated supply of chemically pure nitrogen as a blanket.