Is Vapor Barrier Needed in Crawl Space?

rschlegel asks,

I recently bought a home built in 1991. It is a fine home that sits on a hill. I discovered that there is no vapor barrier on the ground in the crawl space. I do not notice any resulting problems, but the first floor feels cold to my bare feet even though there is insulation under the floor.

Should I add a vapor barrier, and if so, why?

Our Answer

Vapor barriers reduce humidity in the crawlspace as they greatly reduce the amount of moisture that can evaporate off the soil floor of the crawlspace. This reduces the risk of mold and decay framing on the floor framing, and further improves the effectiveness of your insulation which works best at lower humidity levels.

That said, its not likely this will make your floor feel warm to bare feet! For that you’d need to improve the make sure the floor joist have as much insulation as they can handle (2×12 floor joist should have 12 inches of insulation, for example). Also, crawlspaces traditionally have foundation vents that can let cold air in, which helps dry the space out. Those can be closed for the 3 or 4 coldest months (maybe November or December to February). Just make sure they are opened for the rest of the year.