Ohhh! Ok. I had missed that part. I don't know if it's damp under my house. It's open so air can go out, but it's also humid in the air around the house. It didn't feel very damp when I was under there, but that was compared to the outside area where it was like a sauna. And the condensate line does drip under there-- but on the opposite end of the house from where vinyl plank will be installed. I want to put a moisture barrier down underneath the planks just in case though. It's going to be vinyl rather than laminate.
I didn't realize that the moisture could make laminate expand that much.
You would want to have a 6 mil or heavier black plastic vapor barrier underneath the entire home but still, you would want to have the required number of vents of the proper size to allow air movement under the home.
If you seal up the vents figuring it's going to help insulate your house, every single bit of the moisture that evaporates in the ground under your house will have to go straight up through your floor. You do not want that.
The dampness will of course be in the flooring structure, such as your joists . You don't want mold, mildew, or mushrooms growing on the under structure of your home. You've got to let it breathe.
If you can afford to go off the deep end, you can seal up the foundation walls and the dirt and create a virtual hermetically sealed environment and stop 100% of the moisture from coming up through your floor. OK, 98.873%
I worked in a home back of the '80s where the contractor designed his own heating system. It was forced air and he had limited ductwork that simply pointed air in different directions underneath the house towards their preferred destination. A few of the ducts were 3 to 6 ft long and some were 20 ft long to get air from the heater location closer to where he wanted the heat to go
There was literally a total seal in the crawl space. I mean sealed up
and insulated.
It's sort of made the entire crawl space the duct. He was told by a local heating company that his design would not work. Well, ummmm....... It did. He had some other oddities such as small fans above each door casing in the bedrooms and bathroom that could move air from room to room. Interesting fella.