I might not have said my thought articulately enough...
I think calibrating them is a GOOD idea.
I really meant that I'm not sure putting it in a box to slow temperature changes is that value added (although technically superior, and easy enough to do), as a few degrees difference is within the accuracy of the instrument.
But it sure doesn't hurt. 
The idea of a box (cardboard) isn't to necessary slow down changing room temp but to isolate it from changes do to:
I started doing this when measuring temperature rise of electronic equipment (before FLIR cameras)
And, I agree the humidity is not very sensitive to temperature, however those who actually bother to do this will likely want to be exact as possible.
Good point.
Its also a good idea when comparing several temperature measurement devices. Put them in a box somewhere with minimal air currents and let them sit for a hour or so. Not every device has the same thermal lag so slowing any temp changes makes a much better comparison.
Actually, you want a box with controlled circulated air. Every calibration chamber for environmental temp/humidity has a minimum air flow to get rid of temperature gradients. Air as a temperature medium is not very uniform. You want to stir the air in a thermos like environment with a known good thermometer. Even then, a few inches can make a half a degree difference between devices.
We were referring to the saturated salt solution in post 16, not outside air, which I think you are referring to 
Actually, you want a box with controlled circulated air. Every calibration chamber for environmental temp/humidity has a minimum air flow to get rid of temperature gradients. Air as a temperature medium is not very uniform. You want to stir the air in a thermos like environment with a known good thermometer. Even then, a few inches can make a half a degree difference between devices.
In an engineering test I would agree, but the thermometers we're discussing here do not generate any internal heat (to speak of) and a pile of thermometers at the bottom of the box will be closer in temperature than any < $1000 thermometer will be.
Keep in mind we're doing a sanity check on low cost home automation devices.
@JasonJoel Didn't mean to step on your post. I wrote mine without reading yours.
John
OK... So you're saying the charts are wrong? If not, they show very little change due to temperature in standard room conditions for the saturated salt humidity measurement.
Not saying you are wrong, I guess I'm not following what you are saying versus what the chart is saying. Sorry if I'm being dense (wouldn't be the 1st time).
So, in summary, the most accurate humidity sensor that is affordable, available, and works natively with Hubitat is the average value from two or more Sonoff temperature/humidity sensors?
A rolling average is an interesting idea. The batteries seem to last (77% after 9 months), but lifetime cost of CR2450 replacements could ding the affordability.
I'm using these. One or two out of 10 start out at 50-70% but most are OK and seem to last a decent while.
LiCB 10 Pack CR2450 Battery 3V Lithium CR 2450 https://www.amazon.com/dp/B0797QRD73/
I've had very good luck with Renta or Eveready. I buy them on line from one of the watch battery sites.
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