The Insulator is an expert on this. I certainly am not but still have my $.02. Having thought of it, he's dead right about the house not being too tight. In other words, if the house was too tight, how would air/moisture be drawn into, and thru, the house? The house is probably NOT tight, hence, as fans and such draw air out of the house, it's probably replaced by the moist, dense air from the crawlspace. Hence, sealing off the crawlspace from the rest of the house may work as well as incorporating it into the rest of the house. The latter I'm not a big fan of but maybe he can tell you what to do. You may need to give more details but if the need be, he'll ask for it.
He's also dead right about warm air from the outside cooling and condensing in the crawlspace. At my house, I have a 3' poured crawlspace that's 15' x 30'. It has 2 screened vents. It's about 2.5' below ground. I live in SE PA. I have no problems w/this situation causing any moisture problems. The vents are opened all year 'round. As much as I'd like to dig up the addition to insulate it, I'm leaving well enough alone. I'm sure The Insulator can offer options that might mitigate the problem, maybe not 100% solve it. I have have moisture in my crawlspace from warm air moving to cool air but I'm not seeing it, yet. The addition is 25 years old and original. I'm sure I'd seen any problems by now.
My best to ya and hope this helps.
PS: If there's more I can offer, e-mail me directly ...