in my mind, help with evap = increasing evap to allow more limewater additions.
This is not the thinking of most reefkeepers.
Obviously, covering the aquarium and/or sump will decrease evap and help retain warmth.
Now look at your home's humidity.
In wintertime we want to ADD moisture to our dry home.
In summertime it's the opposite.. you want to dry it out and keep it cool... usually accomplished with AC.
So it's probably best to have a removable cover for a sump.
Cover it when necessary.
And choose what's more important to you- keeping the home humidity at a comfortable level or retaining aquarium warmth.