I live about 1 mile from Lake Erie in Monroe, Michigan. We heat our house with a wood stove and we don't use a humidifier in the winter. Nor do we air condition the house in the summer. Winters are externally dry and summers are very humid here.

I have already installed two floors 2 1/4" plank oak floors (one nailed and one stapled) without any problems (both floors are 3 years old). However, they were installed during the summer. I get seasonal gaps in the winter, which always close up during the summer months. The floors have 30 pound felt for underlayment and I have 8 mil visqueen in the crawlspace.

I'm ready a start 500sqft of 4" wide, hand scrapped hickory, stapled to 3/4" plywood, over a crawlspace. I'm concerned since this is a different species, wider plank, and the humidity is very low right now.

Any thoughts would be greatly appreciated.

you are going to have to space the floor to allow for the summer expansion. you really need good moisture readings to know how much. My guess is about every two feet your going to need a spacer row. Wide plank hickory will move probably 5 times what your 21/4 oak does. If you install it tight at late winter when its at its driest point and the wood is way dry it will do more than cup come summer it will heave and buckle up off the floor. Myself would let it sit in a back bedroom until summer. Hickory is one of the biggest movers ive seen many good looking installs go haywire with wide hickory that was installed too dry in late winter months.

I was thinking about buying a humidifier and letting the wood acclimate with the humidifier on for 7 days. Then use a washer row every 3rd row.

The living room is down to the bare sub-floor right now. I don't know if I can wait until summer. I myself have no problem waiting, however my wife would not be very pleased and the next 3 months would be a royal pain.

Id say your looking at least a 5 point swing when summer comes.
5 x .00411 = .02055 x 4 = .08 inches . So if this is where you are at that floor will grow about almost an inch in a ten foot span when summer humidity peaks.

I like that idea. Your going to need to do research on what the median rh level is and even then like Kevin says it's going to move, but not as much as without the humidification. Plus you need a meter to check the moisture content as mentioned before, without a meter your just guessing as to the amount of moisture gain in the wood. Then maybe you could reduce the amount of spacers in the floor.