What's too humid for inside the containers? Just for fun, I put my Govee sensor inside some of my shoeboxes with vermiculite. Humidity is ranging from 80 to 96 % depending on the box. Outside the box, in the bulkhead it's often in the 40-50 percent range, but wide swings there. I'm in New England and historically have struggled with tubers drying out, but never rot. Last year though, half of mine rotted (they were the later half though, so I think they needed extra drying time because of a monsoon that was sweeping through.)
Too humid?
Humidity is 84% here in Oregon right now. Healthy tubers love it. Since it is so cold in the storage area , they do not do much rotting now. That is why they recommend low 40s if possible for storage as they do not rot much in cool temps. Yes,, tubers that were over watered by mother nature need to be dried well and since fungus spores are in ground water, they may have been exposed. When in doubt ,we use cinnamon and on the most important ones we may use some even if they look good. In cold areas of the US people seem to worry lot about tubers drying out too much. I would much rather have a somewhat shriveled dahlia tuber than one that turned to rotten mush. Rotten is generally fatal and shriveling not nearly as much. 
Watermelon color is popular right now.
5 days later
We have had over 6 inches of rain this last week or so. Humidity may be a bit high. On a weather subject, our premier weatherman Matt Zaffino, noted that on December 4th the record for the high temperature for the day was broken by BOTH the low temp for the day(58F) and the high temp for the day(68F). In other words,, it was record breaking all day long! I did notice one undivided clump started to sprout but all the rest have remained dormant. Since I posted the picture above of one of the good hopefuls, I will post the picture of the one that started to sprout and it is a good tuber maker.
Our creek is up to its "fun" stage of watching it rise. Any more heavy rain could take it past the fun stage. This is up 3 or 4' above where it was a few days ago. The source of this little creek has to be somewhere up in the mountains behind us. After it leaves here it wanders around in the marshes along the Columbia River before joining it.
Ideal humidity % largely depends on your temps. If you've got it in the low/mid 40s, high humidity is great. If you've got it in the low 60s, that'll lead to rot. Remember that warmer air holds more moisture than colder air. So when you have high relative humidity and it's 10°, you still have really dry air.
I moved here in 1989 and in about 1995 or so it got down to 14F. In the last 10 years or so it probably has not been much below 25F or so. So far this year, I believe the lowest has been 28F. I no longer have to cover up the tubers in the garage with old blankets but I still do so, just in case. Humidity must be rather high when it is not raining. As I write this it is 93% and 48F.
If I can keep it above 50% in my storage space in the dead of weather, that's an accomplishment. Right now, it's still in the mid 60s%. I Saran wrap packs of 5 tubers rolling them each in their own layer of wrap them the whole pack gets three more wraps. Even with that, without fail, the outermost tuber is still the most prone to some shriveling. I also wrap my stacks of trays with some plastic and put a bucket of water between the stacks.
In the PNW too humid can be problem. Bill Mishler ran a de-humidifier in his storage building. I believe it had a dirt floor and got rather damp in there.
- Edited
Honnat Even with that, without fail, the outermost tuber is still the most prone to some shriveling
That's interesting, I never would have guessed they would shrivel with that treatment. I think dry climates present the most challenges for storing tubers. In dry Montana my father would run a humidifier in a cool basement room for tuber storage. I remember he monitored the humidity and temp. closely.
Some thoughts on storage. My pot tubers are stored in their pots for the winter. They are harvested after the Fall rains start and the pots are rather wet. I believe that very wet and very cold is not a problem. As temps warm, my pots get drier and since they are drier, they continue to store well when temps increase in the Spring. They do dry to the level where they do not sprout much without adding water. I wonder it this could be translated to tuber storage. Proposal: Place a tuber in the Fall into a pot of wet potting soil. Store it where it is about 40 degrees and do not cover. The soil should be kept wet(very damp at least) during the cold months. When it starts to warm up in your storage area to about 50 degrees, make sure the pots are only slightly damp. Water them to somewhat damp when you want them to start growing and the temps need to be increased to 70F or so to get them to grow. It may well be that a pot tuber has hair roots in the pot and tuber placed into pot would not have hair roots and that may make difference. I doubt it.
Teddahlia I think your proposal would go as you describe. You have the storage thing figured out pretty well. 
Whenever I get tubers from you they are eyed out and have short sprouts. This is quite a feat of timing and I don't think most customers realize how difficult it is to reliably time new growth from tubers.
I am lucky enough here to have the option to store my tubers in the soil and only dig them in the spring when the soil warms up. The soil stays cold enough so that the tubers stay dormant until spring and we only get moderate rains so the moisture is perfect for storage. I think these conditions are similar to those you proposed.
Teddahlia I’m glad you reminded me to check on my pot tubers. This is my first year storing them, and it was unusually dry before I tucked them away. The regular tubers are in boxes of vermiculite, stacked inside some Igloo coolers that help regulate the temp and humidity swings. When it gets too warm or too dry, I can tuck a frozen cold pack into the cooler, wrapped in a rag.
Everything is in the unheated garage. So far, so good. A few turned moldy or shriveled, but I think I was able to make better choices when dividing them this year.