Is the house finished other than the floor finish? Is there a fully functional and operational HVAC system?

The house is otherwise finished and weather tight. The HVAC unit has been running since August. The only work left to do is finish the floors, then install the appliances and some minor punch list work.

The humidity levels will need to be checked - frequently. What is the reading right now? And you are aware that the whole-home humidifiers will have to stay in place for the entire life of the floors? As in "forever and ever".

You will want to see 40% humidity or so (closer to 50% is optimal) and there it will have to stay (summer time may require a reduction of humidity - especially in MA).

Thankfully, wood swells faster than it shrinks. Once you can maintain the humidity between 40% - 50%, you will need to wait a few more weeks before the wood has puffed sufficiently to put down the finish.

And yes...Hickory is considered more dimensionally unstable than others. Especially in the 4" and wider planks. You can expect this type of "humidity" control to continue for as long as the wood lives in your home.

SJ - I have a cheap monitor that I got with an old humidifier that shows the current level close to 60% (it started around 25%). However, my general contractor has a hygrometer there that is showing closer to 20% and has stayed pretty consistent for the past 5 days. The only thing I can think of is that my monitor is possibly measuring humidity and his is likely measuring relative humidity. Would that make the difference?

There are two numbers showing on the digital unit. The 20 is obviously the RH. Is the 63.0 the temperature in "F"? That is quite low for "living temperatures". If that is the temperature then you will need to UP the temp to 70 F. That's much closer to everyday living temperature. And the higher the temperature, the higher the humidity will climb.

I would say the "old" hygrometer is just that = old. It is sooooo "off" that it is useless (actually it looks to be reading "Temperature" rather than Humidity). There is no way your floors would gap at 60%+ humidity. That means the 20% RH of the digi meter is correct.

What type of "whole home" humidifiers are you using? Are these temporary humidifiers that stand in a room somewhere (they are only "good" for a single room)? Or are the linked into the HVAC system (a rather expensive, permanent application that is several thousands of dollars worth of material/work)?

The "standing in a room" one's aren't going to do the trick. You need the expensive one that is an "add on" to the Heating/Cooling system of the home. If you don't, you will find those gaps you see today will get bigger and bigger and bigger until you can lose a dime or a quarter between the planks.

Thanks, SJ. I'm not yet convinced that the manual meter is broken - only because when I bring it to my apartment, it reads 30% humidity. I just think it's measuring something other than the digital meter.

I'm curious about your comment re: temperature. Everything else I've read has emphasized how the higher the temperature, the more dry the environment will become. By raising the temp, am I not simply going to dry things out further?

Re: the humidifier, it's a Space Saver by Aircare (https://www.homedepot.com/p/AIRCARE-6-Gal-Evaporative-Humidifier-for-2-300-sq-ft-826000/205997943?cm_mmc=Shopping%7cVF%7cG%7c0%7cG-VF-PLA%7c&gclid=EAIaIQobChMI78enxO3A2AIVRrbACh29CwQ5EAQYBCABEgJJ0fD_BwE&gclsrc=aw.ds&dclid=CJawmN3twNgCFQVaDAodqRYFdA). It's a stand-alone unit that is supposed to cover up to 2,300 SF. We actually put 2 of them in the house, running on high and have gone through almost 40 gallons of water since Saturday.

Sigh...OK. Back to grade 10 Science. Here we go. Warm air holds MORE moisture than cold air. That's why the Amazon basin feels so "sticky" that you are wandering around in a wet towel whereas the Arctic/Antarctic feel like you are unable to take a breath it is so dry.

Warmer air holds MORE moisture. It's that simple. Now the trick comes with artificial heating systems for indoor spaces. Most indoor heat in N. America is either forced air. Forced air is produced by lighting a flame and blowing the heat around. The fire/flame dries out the air and then it dries out wood in the house (and everything else).

So...it is true that artificially heated air is drier. But if your home is well constructed, it will be able to maintain humidity and temperature quite nicely. You will most likely have to keep pumping humid air into this space for MONTHS before your wood "puffs up" enough to close the gaps.

And then you will need to lug those stand alone things out EVERY WINTER (ahem for the next 25 years) and filling the reservoir on a daily basis. These are considered "short term" fixes. They will not be what your home needs long term (long term is anything over 3 years).

Because this is 4" wide hickory and because you live in a climate that has large humidity swings between winter and summer, I HIGHLY recommend you pay the piper and add the expensive humidifier/dehumidifier to the furnace/HVAC system.

Summers on the East Coast can be VERY humid (70+ humidity) which means you will need a way to REDUCE humidity in summer. That means you need the high-end add-on.

Please find out the make of your furnace and then contact the local dealer (preferably the people who installed the darn thing) and have them quote a price for the whole-home humidifier/dehumidifier.

As for the wood...you have weeks and weeks and weeks of waiting and refilling the reservoirs in the stand-up humidifiers before you will see a difference in the floor.