No. Sort of. The usual process is to install insulation between the floor joists. If that is batt type, it can be covered with some kind of sheeting or not. If it is spray foam, the seal is much more secure.

As long as the crawl space is dry, any problems are minimal. Animals being the major possibility. A simple set of vents in the foundation are usually all that is necessary.

Damp/wet crawlspaces need to be fixed. Positive sealing from the living space plus dealing with the moisture causes is necessary.

Thank you, HandyMac. I'm wondering if toxic chemicals are used in my crawlspace (to retrofit for earthquakes), if I might be breathing them in the house.

I'm not in earthquake territory(at least for building codes). Most earthquake related construction is done with building materials. I am not aware of any chemicals in those materials that would be a hazard inside the building.

If there is such a situ8ation, I would hope the local codes would specify a remedy, such as sealing the underside of the floor joists with plastic sheeting.

If you are referring to radon gas, there are several remediation methods to reduce/prevent the gas from entering the home. Research radon gas remediation for that info.

Thanks, Handymac. They use epoxy paste for this kind of retrofitting.