Crawlspace/Vapor Barrier

I inspected a crawlspace today. Soaking wet underneath the house. No vapor barrier. The cinder block support posts were leaning and not in contact with the floor rafters. I think I understand the purpose of a vapor barrier. Can someone refresh my memory. And when do you call for a vapor barrier to be installed in a crawlspace.

P.S. The soil was soaking wet due to a drain line disconnected.

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Hey Larry, Good info, Thanks, Matt.

I never recommend a vapor barrier because I’m not a specialist. I report the absence or presence of a vapor barrier and the moisture conditions and recommend that they consult with a crawl space specialist.

For example: “Standing water and wet soil were observed in the crawl space. The exact entry point of the water could not be determined. Moisture can facilitate the growth of a variety of molds that can promote unhealthy conditions and excess humidity can cause decay to wood structures over time. Additionally, chronically moist conditions can cause foundation settlement or deterioration over time. We therefore recommend that you consult a grading or drainage contractor or crawl space specialist to correct this adverse condition. Observation of the crawl space during a period of heavy and prolonged rain is also recommended.”

You don’t want to see a vapor barrier on the cold side of the insulation.

Here’s an excellent basic source on Crawl Space Moisture Control at

A vapor retarder is required on the floor of the crawl space for the purpose of inhibiting the ntrusion of moisture from the soil into the crawl space. Often one sees the insulation in the ceiling of the crawl space installed upside down, so that the vapor retarder isn’t up against the subfloor. (assuming the crawl space is located in a cold climate)

Maybe they are required for new construction. But would you “require” one in an older home, say a home that been fine for 100 years, with dry soil and with no adverse signs of water intrusion or excess moisture in the wood?

I would not.

Here is what we RECOMMEND when we come across this issue.

Crawl spaces can add considerable moisture to a house. It is recommended that a vapor barrier of 4 or 6 mil polyethylene be laid over the earth in the crawl space area with a minimum of joints (overlap joints a minimum of 24 inches) is generally recommended. To be effective, vapor barriers must be continuous. Installation of paper or foil-faced insulation between the floor joists will also retard infiltration of moisture into the house. The vapor barrier on the insulation should be placed against the heated side or the sub flooring. If you use single-faced insulation, the exposed insulation should face the crawl space (fuzzy side down). Insulation with a vapor barrier facing on both sides is a good option for insulating a crawl space or basement.

Hope this helps


Practically speaking, I agree…“If it ain’t broke, don’t fix it”…but technically speaking, there should be a vapor retarder, and it can do no harm to install one. I’m an architect, not an inspector, but if I were, and I came upon a 100-year-old house with no vapor retarder and no problems, I guess I’d simply explain modern practice and leave the option to the client, also stating that none of the problems commonly associated with crawl spaces having no vapor retarder are evident.

Good link on the subject Larry.

Marcel :slight_smile: :smiley:

Glad it’s helpful.:wink: