Product review (5 stars): Accurate Normally Open humidistat
Accurate as measured from multiple hygrometers I have placed around the house  (http://www.amazon.com/La-Crosse-Technology-302-604S-Thermometer/dp/B004QM97CU/ref=sr_1_7?ie=UTF8&qid=1417638858&sr=8-7&keywords=hygrometer+and+temperature) and my Samsung Note 4 hygrometer.

~5% RH deadband.  Switch is Normally Open, Closes when humidity < setpoint, opens when humidity > setpoint +5.  I.E.  Knob points at 40%, contact closes at 40%, opens at 45%

Replaced a Carrier humidistat that stopped working and measured  the humidity in the air handler return duct.  Installed this upstairs on top of my thermostat using two spare thermostat wires.

This humdistat mounts flush and comes with a gasket to seal it against the mounting surface.  However it does have a hole(~0.25" x ~0.25") in the top above the knob and I was afraid that the negative pressure in the return duct would pull air in and the humidity would be measuring my basement humidity.  Either way I'd rather have control of it upstairs.

Note that I wouldn't recommend moving your humidistat unless you are comfortable re-wiring things and can do it not only up to code but not have it look jury rigged...no one wants to buy a house that things jury rigged.