crawlspace vapor barrier

Is there any time that’s acceptable to attach a vapor barrier to the underside of the insulation that is in the ceiling of the crawlspace? Saw this on a modular home the other day and it got me wondering if this can be done or the contractors who did it had no idea how to install a barrier. I know that they should be on the floor but is there any time it’s acceptable this way?


I have seen it installed that way. The contractors use it to hold the insulation in place. I would have to agree not many know the correct way of installing it. BTW the correct way could also be not installing a vapour barrier.

Here is some info.

thank you Mario. Now that I think back, it did look like landscape type plastic covering the insulation. This, in essence, would not be a vapor barrier then as moisture would naturally pass through it. Correct?

Depends on the climate. In cold climates, the vapor retarder should be on the winter warm side of the assembly, be it floor, wall, or ceiling., or in this case, up against the subfloor. In warm climates, the vapor retarder should still be on the winter warm side, which may be the opposite side of the assembly. Insulation in crawl spaces, in cold climates, is frequently erroneously installed upside-down.

If what you saw was indeed landscape fabric, it may have been intended to allow ventilation, but it’s possible that it might act as a partial vapor retarder, and not have enough free area to allow sufficient passage of air. It might, in fact, trap some moisture within the insulation. That would reduce the insulating value of the insulation, and possibly lead to mold and structural decay issues. It would depend on the porosity of the particular fabric.

Is there a vapour barrier on the underside of the floor? If there isn’t, I think this installation is wrong. You didn’t say how old the home is so it would be important to ascertain if there is a vapour barrier was ever installed or just placed on the ceiling of the crawlspace?

the home was built in 1999. I was unable to tell if the fiberglass insulatuion was faced or unfaced at the subfloor (and with some of the bugs down there, I certainly wasn’t blindly sticking my hand up there to find out) The material that held it in place was a black plastic, much like the kind that you put down in landscaping to prevent weed growth.

Scott… Unless it was a replaced material, it sounds like the typical vapor barrior used on mfg… homes. Black plastic with cloth like webbing…

The common thing that you’ll find, this is torn open and not resealed, to fix plumbing leaks etc.


Thanks for the reply. So, I guess that it’s ok to use this type of material above the crawlspace floor. I didn’t notice any cloth like material, only plastic. What is the typical mil for this type of plastic? Is it a vapor barrier to just a water repellant that will allow moisture to pass through?

If it’s the original material it is OK.
As far a on the crawl space floor/ dirt? I’ve been under 1000s of mfg. homes in AZ and have never seen any barrier installed on the ground…under a mfg. home.
The way to tell if is it continuous/ all one piece on on top of the metal framing on each 1/2 of home.The factory installed material is a very good barrier, in fact it will hold 100s of gallons of water from a water leak from the plumbing in the subflooring before it tears and leaks into the crawl space.

FYI… I teach a Mfg. Home Insp course for AZ Sun Teck… If you send me your address I’ll send you some of the material from that course.
A good site to visit try Foremost Insurance… or something like that]
On that site they have a good blue print showing how the Mfg. home is built along with construction info.

If the house was in Arizona, then probably what you saw was indeed a vapor retarder, and it is probably in the right location, the winter warm side of the assembly. It depends where in Arizona, of course.

If the fabric or film can hold hundreds of galllons of water, it certainly isn’t meant for ventilation.

Mobile homes in my area all have plastic sheeting installed at the underside of the floor framing. This is not a vapor barrier. It has small perforations that allow the vapor to pass through.