OK, prefinished solid oak 3 1/4" plank has been in our new construction for 3 months now, still in boxes. We have had the boxes open for two weeks now. I took moisture meter readings throughout the house (first and second floor) yesterday. Worst subfloor (3/4" plywood) reading (on first floor, over ventilated crawl space) was 13% or so. Best reading (on second floor) was 9% or so. House has been fully enclosed and at 55 degrees temperature for 3 months now.

All hadwood tested had moisture content less than 7% (lowest reading for my meter). I will use laminated rosin paper between the two.

My question is how critical is the difference in moisture content between the two? I know 4% difference is recommended. It doesn't look like the hardwood is soaking up any moisture from the air, even in our damp western Washington state air. And our subflooring doesn't seem to want to dry out anymore.

Finally, at these low readings, should I be using a meter that goes down below 7%?

The soil in your crawl space is damp. Cover the ground with polyethylene as recommended by NOFMA.
https://www.nofma.org/Portals/0/Publica ... Floors.pdf
Read pg.4 "Installation over a crawl space". 1st paragraph.

Vapor varrier is in place, and the moisture content of the subflooring throughout the house is below 14%.

Let me ask the question a different way. Is there anything I can, or should, do to increase the moisture of the hardwood, to match that of the subflooring?

Stick the underneath side of the subfloor from the crawl space, see what that reads. Are you going to be living in the house at 55? Cooler air holds more moisture than warmer air will.

IE.... if your house is at 55 degrees and the humidity level is... say 50%, you are almost at the dew point inside. I think you need to get the house acclimated to your lived in conditions for a while.

13% MC for a subfloor is a little damp, unless the year round relative humidity is 80 to 100%. Typically, it should be about 10% with readings from 8 to 12% and the average being 10. If the first floor is the worst (as it is exposed to the elements) and the second floor is better, 9%, then that is what the first floor should be, on the average. Turn up the heat to 68 degrees.