Hello Everyone. I've been lurking for quite a while absorbing as much information as possible. I'm trying to get my ducks in a row prior to installing some 3/4 solid flooring, probably Jatoba. I have one specific question regarding crawlspace moisture.

Generally my crawlspace is very dry. It is covered with black poly and a layer of gravel. A couple of times a year, after heavy rains, water seeps in over a small area. The light colored gravel makes it easy to spot as the gravel darkens when wet. The actual moisture is very little, running my hand through the gravel to the poly reveals no standing water but my hand is wet. Downspouts are led about 25ft away from the house to drain and my yard generally slopes away from the foundation.

So, my question is, how much moisture is too much? Sealing off the vents and using a dehumidifier seems too drastic. I thought about using a large fan to stimulate airflow when needed. Or, does an occasional slight moisture increase even matter as long as the joists and bottom of the subfloor don't increase in moisture content? I don't have hard numbers yet but say I get a moisture reading from the joists every day for a week after moisture intrusion and there is no increase in moisture content. Does that mean I don't have anyhting to worry about?

On a related note, how much of a difference in moisture content should I expect, under ideal conditions, between joists/bottom of subfloor and the top of the subfloor measured inside?

Bingo! Also, this is what the vapor diffuser on TOP of your subfloor and underneath the wood flooring is for. There shouldn't be greater than a 2% variance between subfloor MC and the flooring MC, as measured by a moisture meter. On average, subfloors should read 10% MC.

Thanks Gary, that's what I was hoping. I'll take some MC readings tonight and monitor them for a while as the crawlspace dries out. Hopefully there won't be much of a difference indicating the occasional increase in relative humidity won't affect the MC of the subfloor.