They come in all sizes and colors and provide moisture protection from the earth's floor from your subfloors, insulation and HVAC units. This is a must for any home. We have economic 6 mil poly solutions to heavy duty 10 mil applications, whatever your needs are we will have the solution.
First we would remove old rotten plastic poly sheeting, if there is an old ineffective one layed. We then lay out the floor. There are many support beams down there that require precise cuts and seeming with poly tape. It's sometimes a timely process avoiding dangerous spiders and snakes in the tight quarters of a crawl space.