# Do I need a vapor barrier in my shop?



## Underdog (Oct 29, 2012)

My shop is one of those glorified stick built sheds you can buy from HD or Lowes. So the walls are framed 2×4 with exterior siding nailed on the outside. Everything is optimized for 4×8' sheets and quick construction… Overall dimensions are 16' x 20', and set on piers. The floor is 5/8" treated ply, with 3/4" oak flooring nailed over some of that roofing paper.

So if I wanted to insulate and put some paneling of some sort on the inside, do I need to first put a vapor barrier somewhere?


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## johnstoneb (Jun 14, 2012)

Yes If you use the fiberglass batts with the kraft paper backing that is your vapor barrier. You could also use plastic sheeting under the paneling over the insulation for a little better barrier.


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## Underdog (Oct 29, 2012)

Thanks.
How about the floor? How would one insulate that? The insulation would go underneath the shop, but where to put the vapor barrier since the flooring/joist combination is opposite the walls.

Actually, it already has sort of a vapor barrier with the tarpaper under the flooring, right?


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## crank49 (Apr 7, 2010)

Yes, the tar paper under the hardwood is a sort of a vapor barrier, intended to reduce the swelling and/or cupping of the hardwood. That's a nice floor for one of those type buildings; did you add it yourself?

Do you have crawl space under the floor? Can you physically get under there?
I don't know how you could insulate under the floor if not.

If you can get under the floor, at this point I would opt for rigid foam glued to the bottom of the sub-floor, between the joists. You can fit it in there with a gap on the sides and fill in with that "foam in a can" stuff.
The foam is its own vapor barrier.


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## Underdog (Oct 29, 2012)

Yeah, I used to work in a flooring plant, and after I quit, the rough mill foreman came by with about 400 sq/ft of worse than #2 grade that they were going to throw away. I glommed on to it, and borrowed a buddy's flooring nailer and a box of nails, and went to it. I never even sanded it, but it's been a good floor.

Yep, I can get under the floor it all the way around. I poured the piers myself, although I didn't provide enough of them, so the 4×4s are sagging a little. At some point, I need to put some blocks down and wedge some wood up under the sags.

So what would you use to glue the foam with? And what thickness foam would you use?

This is in Georgia, and eventually I'll install that A/C unit in the wall…


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## crank49 (Apr 7, 2010)

I'd use the 2" pink styrofoam if it was me.
In fact, I'm building a new shop right after Christmas and that is what I'm putting under the slab.
I'm in south central Tennessee. A lot like Georgia, without the nats ;^)

The foam around the edges can be the glue.
Some folks use construction adhesive as well. Just check that it's not the type that dissolves foam.


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## moonie (Jun 18, 2010)

Yes I done my hole shop floor's celling and the walls and its warmer in the new shop now.


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## RichardHillius (Oct 19, 2013)

I'm no builder but I have heard very different things from people who build in the south compared to those who build in Northern climates around vapor barriers. If they are used at all in the south they are recommended to be put on the outside of the building under the cladding.

I would call your local code department and talk to them. I have always found them helpful in the past and they should know the local best practices better than anyone.


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## canadianchips (Mar 12, 2010)

Typically the vapor barrier goes on the heat side of insulation. The principal is to keep insulation from getting wet from moisture created when change in temperature happens. 
In high humidity states or provinces use spray in foam (more money to buy insulation, no need to buy vapor barrier and labor cost.)
eg. on your walls insulation between studs then vapor barrier then drywall.
An option is to use spray foam (this has its own vapor barrier formed when sprayed in.)
Your floor you can use styro foam between floor joists or spray foam.


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## Grandpa (Jan 28, 2011)

Vapor barrier goes on the heated side of the insulation…..on the inside next to the living quarters. Many of the adhesives will melt your foam so experiment a little. There is adhesive that is listed as foam adhesive. Many lumber supply store carry it. Look for that if you can find it. The problem with spray on foam is it will not allow us to find a leak if we have one. The building will literally rot around us before we find a leak because the foam holds the water back from our view.


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## john111 (Dec 18, 2012)

Is there a real reason to insulate the floor? Would you lose that much?


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## Underdog (Oct 29, 2012)

Why wouldn't you have as much heat transfer through the floor as from the walls or ceiling? Why insulate at all if you leave a gaping hole in your insulation? Unless I'm missing something one would want to insulate the whole building if at all.


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## NinjaAssassin (Sep 5, 2013)

I would imagine there'd be less loss through the floor since hot air is lighter than cold air (i.e. it rises). That's not to say there won't be heat loss through the floor, but it shouldn't have as much impact as what you would lose through an uninsulated ceiling. Now, when summer time comes and you've got cool/cold air blowing around your shop, the difference between an insulated and uninsulated floor should be pretty significant.


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## a1Jim (Aug 9, 2008)

In my area a 6mil vapor barrier is required under wood structures . If you haven't put your walls up yet I recommend a house wrap,it's engineered to keep moisture and air infiltration out but let moisture on the inside out,it's much better than felt(tar paper) Yes you do lose a lot of heat through your floor(a surprising up to 60%)if you don't insulate.A mistake home owners typically make is to use a plastic vapor barrier on the inside over the paper side of your wall insulation,this t creates condensation on your vapor barrier and eventually mold. It sounds like you may have used 4×4s as your rim and or floor joist ,4×4s are only meant as vertical building members ,so at this point all you can do is add some additional supports,perhaps a 16"x16"x4" concrete block with a 4×4"on top placed under your horizontal members.


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## NinjaAssassin (Sep 5, 2013)

60% heat loss through the floor? That's quite surprising, indeed. I stand corrected. Thanks Jim


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## Grandpa (Jan 28, 2011)

I believe the percentage thing is getting us skewed on heat loss. If you insulate the walls and ceiling then you probably lose 60% of the heat lost through the floor. If you don't insulate the walls then a larger percentage would be lost through the walls etc. They tell us that 75% of the heat lost is through wind infiltration so I imagine all the insulation stuff is dependent on location and climate.


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## Underdog (Oct 29, 2012)

The things is already built, so placing a vapor barrier will have to be done after the fact.

And no the 4×4 are not the rim. The band and floor joists are 2×8, and they sit on 4×4 rails which rest on the piers. I put the piers too far apart though. So yes, the only thing I can do now is place some blocks between piers…

If I already have roofing tarpaper under the flooring, is that a no-no? I live in Georgia, so in the summer it's really hot and humid. I already have a mold/mildew problem since the thing is shut up so much during the day. I'd hate to exacerbate the problem by placing a barrier in the wrong place.


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## a1Jim (Aug 9, 2008)

Those are some good size joist. As far as a mosture barrior on the ground I think it can only help not hurt,it helps protect your joist even if the're PT material along with the rest of the building. Your inside moisture might indicate you need some roof and or soffit vents.


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## johnstoneb (Jun 14, 2012)

I'm guessing that there was a gravel or rock bed laid down before the piers and building were put in place. There should have been a moisture barrier laid under the rock if not, You could lay a moisture barrier over the ground and that would stop moisture from migrating up into the joists and and flooring. You will need to have good drainage around the building and away from the building so you are only dealing with vapor.
You live in a very humid area so you probably need to vent the building during the day with windows open or vent fan. 
I put my shop on a concrete slab and also in a low humidity area so only have to deal with the cold.
10F and 15% humidity this morning/


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## wormil (Nov 19, 2011)

I believe the current understanding (which is different than I was taught decades ago) is your house (or shop) acts similar to a chimney and without insulation and a barrier it will pull moisture and cold air in through the floor. So sealing off the floor and putting a vapor barrier on the ground under the building should reduce the interior humidity.


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