Install a vapor barrier in a crawlspace by completely covering the dirt floor in thick plastic sheeting, insulating the interior walls and rim joists, and capping the top of the foundation wall with thick plastic sheeting and galvanized steel flashing. Moisture levels vary in different locations, and some homeowners only need to install plastic floor sheeting to effectively control moisture, while others may require all aspects of the moisture barrier.
Secure the plastic sheeting to the crawlspace floor using landscaping stakes. The stakes keep the sheeting firmly attached to the floor while also providing small holes in the barrier. The holes are too small to affect the barrier's performance, and they can provide much-needed drainage following leaky pipes or heavy rain.
Use a moisture-resistant variety of rigid insulation on the interior foundation walls of the crawlspace. Rigid fiberglass and expanded polystyrene insulation panels, while inexpensive, are unsuitable for this application, as they readily transfer moisture, and their performance suffers considerably when wet. Polyisocyanurate insulation panels are much more expensive than the other two types, but they last much longer and provide an effective moisture barrier.
Capping the foundation wall not only seals out moisture, it can also prevent termites from entering a home through the floor joists. However, this is not a foolproof termite barrier, and the destructive insects can sometimes find ways to penetrate or avoid the plastic and flashing.