Vapor barrier under building?

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Forum topic by JoeinGa posted 07-19-2014 05:05 PM 1744 views 0 times favorited 14 replies Add to Favorites Watch
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7736 posts in 2036 days

07-19-2014 05:05 PM

My new shop building is a “double-wide”. It used to be a portable school classroom. I actually WANTED them to drop it straight on the ground, with NO concrete blocks under it so it would sit pretty much flat on the ground. Unfortunately, the guys who delivered it had no way to be able to do that. They had to have SOME clearance in order to be able to use the hydraulic jacks when lowering it, and still be able to get their jacks out. So we took the tongues and axles off, and they got it as low as possible but it still sits approximately 20” off the ground on concrete blocks.

Prior to delivery I sprayed the site with 2 heavy doses of RoundUp so there’s not even a glimmer of anything green under there. I am also sealing up the bottom all the way to the ground by using roofing tin as a “skirt”.

My question is this …. Will it be beneficial if I crawl under there and put vapor barrier (plastic sheeting) underneath? If I dont, will grass and weeds eventually grow back in, even there will be NO sunlight to ever get under there?

And even though I will skirt it all the way around and all the way to the ground, I do understand that it will NOT be “waterproof” from rain runoff going under there. Do you think I’ll get dampness/moisture in the shop if I DONT put the vapor barrier down? Or will I be ok without it?

These pics will give an idea of how it sits on the ground.



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14 replies so far

View oldretiredjim's profile


206 posts in 2414 days

#1 posted 07-19-2014 05:13 PM

I don’t think I would worry about grass and weeds. Critters love to live under these things. Need to encourage a big black snake to take up residency.

View AnonymousRequest's profile


861 posts in 1578 days

#2 posted 07-19-2014 05:15 PM

I personally wouldn’t worry about a vapor barrier. I pop in a couple vents, maybe two on each side to ventilate.

View Buckethead's profile


3194 posts in 1897 days

#3 posted 07-19-2014 05:23 PM

Agree with Freddy. No vapor barrier, ensure ventilation. The only vapor barrier I would want would be beneath the floor sheathing, and above the joists. If it doesn’t already exist, I wouldn’t worry about it.

-- Support woodworking hand models. Buy me a sawstop.

View JoeinGa's profile


7736 posts in 2036 days

#4 posted 07-19-2014 05:50 PM

Ah Ha, I hadn’t even thought about vents. Looking underneath, I do see some insulation up between the floor joists. Appears to be fiberglass batting and has some kind of black cloth holding it up.

The shirting will completely close it off underneath, so I’m not too worried about “critters” getting under it (especially the neighbors damn cat!). And as to snakes… NO THANKS! Yeah, yeah, I know all about “good snakes versus bad snakes” and I dont like EITHER KIND!

So as to the vents… 2 on each side on all four sides? Or just at two opposite ends for air flow?

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View crank49's profile


4032 posts in 3000 days

#5 posted 07-19-2014 06:02 PM

You want to ensure the bottom is sealed from critters.
I “sealed” mine with Tyvek stapled to the bottom of the joists because it retards moisture but still breathes enough to prevent sweating and won’t promote mold growth.

You do need vents in the underpinning. I had to put 4 vents in my 16’ x 24’ building to meet my local code.
I put two in each of two corners; like NW and SE corners.

View Handtooler's profile


1579 posts in 2161 days

#6 posted 07-19-2014 07:53 PM

Hear, hear on preventing mold and mildew! Vents are a good thing. If you plan to work often during the winter months even though you are south, you want that insulation to be effective. Also check above the ceiling.

-- Russell Pitner Hixson, TN 37343

View papadan's profile


3584 posts in 3397 days

#7 posted 07-19-2014 08:52 PM

When installing the underpinning dig a 4” trench around the foundation and put your metal in the trench and bury it. This will help greatly on keeping out mice, snakes and other critters.

View tyvekboy's profile


1766 posts in 3042 days

#8 posted 07-19-2014 10:07 PM

Tyvek? Someone mention Tyvek? Yes. I think Tyvek stapled to the bottom of joists wouldn’t hurt. Great vapor barrier. Wait. Didn’t you say that there is insulation under the floor joists? I would just check to see if there is a vapor barrier on the other side of the fiberglass. If there is, i wouldn’t put the Tyvek under the floor joists. It would just trap the moisture … I think I would leave it off.

-- Tyvekboy -- Marietta, GA ………….. one can never be too organized

View crank49's profile


4032 posts in 3000 days

#9 posted 07-20-2014 04:08 AM

Tyvek is not quite a vapor barrier and that is why I suggested it.
Think about the main product Tyvek is used for. “House Wrap”.
It is designed to go outside the sheathing. That is always on the outside of the insulation where the vapor barrier is on the inside.
Tyvek will actually shed liquid water but allow air and water vapor to pass through with a very slight pressure..
Wonderful stuff.

View Crank50's profile


173 posts in 1605 days

#10 posted 07-21-2014 08:18 PM

I agree

View JoeinGa's profile


7736 posts in 2036 days

#11 posted 07-21-2014 08:33 PM

Thanks for the ideas guys. I decided to forego the vapor barrier and just skirt it in. I shot another treatment of RoundUp under there and also threw a bunch of Amdro, some mole repellant and also some mothballs under it too. I’m going to pick up some vents this week to put in, and I’ll be posting updated pics to my new shop blogs.

I didn’t “bury” the skirting, but I did sort of slide it side-to-side as I was putting it on so it cut it’s way into the dirt and it’s probably 1/2” to 1” into the ground. Sealed up pretty well. I’ve got to close up the corners better and I’m going to stop by a mobile home rebuilders place near my work and buy a few feet of aluminum sheet angle and slip them under the tin roofing to seal the critters out at the 4 corners.

I did crawl back under there and there IS insulation up between the joists with the black fabric stapled up holding it in. When they were setting up the “double wide” I did look up into the “ceiling” and there’s pretty good fiberglass batting up there to. Plus there’s vents at the ends of each half.

Once again, I appreciate all the input.

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View Ocelot's profile


1981 posts in 2667 days

#12 posted 07-21-2014 09:32 PM

In the shade beneath the building, nothing will grow.

I would put black plastic down on the ground and weigh it down with bricks – and definitely would put vents in the underpinning. You are asking for a serious moisture problem otherwise. It will rot your floor. I’ve seen it. We have a trailer like that here where I work. Initially, it had vinyl underpinning with no vents. Within a couple of years started to get soft spots in the floor.


View Redoak49's profile


3289 posts in 2017 days

#13 posted 07-21-2014 09:43 PM

I would have used the Roundup that is good for 6 months. The regular stuff kills what you have now but will allow new weeds to grow in about 7 days.

Black plastic also sounds like a good idea.

View JoeinGa's profile


7736 posts in 2036 days

#14 posted 07-21-2014 09:59 PM

Definitely going with the vents. And I bought the RoundUp that’s supposed to kill ANYTHING for a year.

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