Double layered vapor barrier

Hey all - I am in the Midlands of SC so you know the climate. I just inspected a home that looks like they installed a new vapor barrier right over the old one which still looked very much intact. (top layer was clear, bottom was black).

I learned way back in Sex-Ed that two layers isn’t necessarily better, and in fact, could actually be worse…

Does the same go for vapor barriers? The reason I ask is there was a whole bunch of moisture in between the two layers and my little brain tells me that since that water has nowhere to go, it will sit there and mold.

I think mold needs two things to grow. Food and moisture. Polyethelyne provides no food for mold.
I just did this project on Wednesday actually. I’m working on projects to make my house pretty. So I covered the old 3-mil vapor barrier that the builder installed in our neighborhood. lots of gaps, no taped seams.
I think it looks better now, even if the difference is mostly cosmetic. I have not had any moisture issues in my crawl space, but a clean crawlspace looks nicer. I plan to sell this house soon.

crawl space after 1 crawl space after 2 crawl space before 1 crawl space before 2

For you eagle eyed inspectors out there, you may see a few things that I plan to fix or already fixed.

  1. Sagging duct from the gas pack is getting a new strap
  2. Efflorescence on the left wall was from before I bought the house. I disconnected the irrigation system zone that was watering a flower bed along that wall and planted low maintenance plants in that area.
  3. If you spot anything else, let me know and I’ll fix and post pictures for you :wink:

I see two glowing green eyes in the far right back corner.


“Crawlspace”!?!? Are they all that tall in your area?

Looks great though, nice and clean down there. I had a vapor barrier installed in a repair addendum before I got into this profession and when I saw the bill to install a vapor barrier my mind was blown.

No, I wish. I could throw away my knee pads if every crawl space were so easy.

Bert in the first photograph it looks like there may be a 3 inch PVC drain line. I don’t think it has a support every 4 feet.


Don’t forget the Copperhead skins on that shelf… :wink:


I’m on it, Martin. Thanks. Going to home depot :wink:


You the man! Nice looking space.


Looks like a great get away space for me when the grandkids visit… :crazy_face:


You may need a mini fridge down there Thomas.


Mini and Microwave. Sit in one of the canoes with my iPad, I’m good for a while.


It depends on how well the second vapor barrier is installed. Are all the seams and pillars taped? Is the plastic run up the wall 12” and sealed to the wall?

In my experience if you have two layers of plastic and the top isn’t properly sealed water can find a way in between the two sheets and then you’ll get like a swamp water smell.

It looks great, Bert! :smile:

But, really, the squatter should find someplace else while your selling the place. :flushed:


I have yet to come across one here installed that cleanly, this one was clean by Midlands standards but just nailed down and overlapped, no tape

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The original “vapor barrier” was just 3 mil and had many gaps where the 4" deep gravel bed was visible. the gravel is about 3/4" size and the 3 mil was full of holes from being walked on so much. So it wasn’t water tight by any stretch.
Though I did this project for mainly cosmetic reasons, I think it will actually help reduce humidity a little bit in this crawlspace, which hasn’t been a problem yet anyway.

If I installed two new 6mil layers I wonder if I would run into any problems like what you describe?

Hey Brian,

Unlike Jimmy-Hats,
Doubling up on vapor barrier is fine.
I see it often in my area, in the same orientation as what you saw, old clear covered up with new black.

Good luck.

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Vapor Retarder or Barrier? Use. Prevent moisture from entering the building assembly from the exterior or interior, and to enable the building assembly to dry to the exterior or interior or both if the assembly gets wet.

Floor, wall, ceiling?

Only need 1.

Condensate is a red flag.

I got a few support straps installed around the crawl space (Good Eye, Martin.)
Treated for algae on the north facing roof
and fixed a screen this morning.
I’m on a roll. :sweat_smile:


2 kayaks in the crawlspace is a dead giveaway for previous water intrusion. Lol :canoe: