Positive-pressure ventilation systems can be combined with a heat exchanger to recover energy in the process of exchanging air with the outside, and simply exhausting basement air to the outside is not necessarily a viable solution as this can draw radon gas into a dwelling. Homes built on a crawl space may benefit from a radon collector installed under a "radon barrier, or membrane" (a sheet of plastic or laminated polyethylene film that covers the crawl space floor).