Can I install a plastic sheet over the existing insulation under the crawl space of my house without causing mold to the insulation? The floor inside the house seems cold in the winter, and I would like to try to keep the air in the crawl space from affecting the heating of the living space. -Glen
I would advise against it, since applying plastic to the bottom of the floor joints in the crawl space under your house can trap moisture from the air in the insulation which can result in mold and rot.
A better approach would be to cover the ground of the crawl space with plastic to reduce the moisture, and reduce the ventilation in your crawl space during the winter. This will lower the humidity while raising the temperature of the air under your home during the winter, which in turn will make the floors above feel warmer.
While it’s important to have ventilation in your crawl space, if it’s open to the outside, you should enclose the foundation, and install vents that can be closed during the winter. Foundation vents are available with a temperature sensitive metal spring that causes the vent to close automatically when the temperature drops. If your crawl space has open latticework around it, attach landscape fabric to the back of the lattice to reduce the amount of air passing through.
Good luck with your project,