Crawlspace encapsulation keeps outside moist air out of the crawlspace. This is good but you also want to keep crawlspace bad air out of the house.
When you encapsulate a crawlspace you need to understand a few safety rules and the principals of thermodynamics.
The safety rules deal with health concerns. The crawlspaces air usually contains mold spores. If the crawlspace air is allowed to enter the house, the house will contain the mold spores. Mold spores are very dangerous to the residents.
The principals of thermodynamics comes into play when trying to keep the crawlspace air out of the house. The principals are simple; cold air moves towards warmer air and warm air rises. The crawlspace air is cooler than the house air at least in the summer. Therefore, the crawlspace air is constantly moving into the house through air spaces around the heating ducts, pipes and cracks.
One way to keep the crawlspace bad air out of the house is to install an exhaust fan in the crawlspaces' foundation wall. This keeps a negative air pressure in the crawlspace by forcing air out of the foundation wall. The negative air pressure will also cause air to more from the house down into the crawlspace. This is helpful since the house air is clean/conditioned. The exhaust fans force the bad air out of the crawlspace and this air is replaced by the house's good air. The crawlspace fans are also required by the National Building Code Section R408 for either vented or unvented crawlspaces.