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.
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.
Billy
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
Gary
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 
Glad it’s helpful.
Hi All,
I realize this post is old but I hope to get some clarification. I am in central Florida. I have a client who had a contractor install new floors and a vapor barrier. I believe the plastic vapor barrier should be on the bottom side of the joists and the insulation vapor retarder (paper) should face out - away from the living space. Please see my quick drawing of her current set up. She still has smells in the home.
IMO for FL, Insulation paper to the warm side. Vapor barrier on the crawlspace dirt floor.
Then, vapor barrier atop the subfloor under the new flooring. But this is typical for hardwood and you are showing tile?
And it is not going away until she controls the humidity in the crawl. She likely has …mold. (we can say that here 
)
I agree with Brian, but I would say NO insulation beteen the joists, as it can trap moisture.
And to add another thought, if there is mold/organic substace on the joists and sub-floor materials, she may want to consider having it treated.
And to fix it right once and for all, encapsulate and condition it.
I agree, it falls down all the time due to elevated humidity. However, it creates a dilemma for you should insulate the building envelope. For a vented crawlspace, what are the other options? There are none.
So, you must encapsulate and then you can choose to insulate the crawlspace walls or the underside of the floor structure. (typical encapsulation cost in my area for a 2,000 sq. foot home is $10-20K)
Absolutley not cheap, but I tell my clients, what’s in the crawl, is in the house.
I was blessed with a really good sensitivity to smells, and I can usually smell it as soon as I walk in the house. Found this last week, doesn’t look too bad but I smelled it as soon as I opened the front door.
**That is what I thought. The multiple vapor barriers in the wrong place could result in additional mold concerns, correct? Do you know where I could find a diagram of the proper placements? I have searched the internet and Nachi but no luck.
I’m sure about the vapor barrier but you may find some of this interesting:
There are too many variables for a generic diagram on the net.
You need to research regional area specific sources for proper information.
Begin with your local AHJ / Building Dept. for their requirements for your zone.
