Do I need a vapor barrier in my shop?

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Forum topic by Underdog posted 12-19-2013 01:49 PM 2124 views 2 times favorited 19 replies Add to Favorites Watch
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1130 posts in 2087 days

12-19-2013 01:49 PM

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?

19 replies so far

View johnstoneb's profile


2967 posts in 2224 days

#1 posted 12-19-2013 01:55 PM

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.

-- Bruce, Boise, ID

View Underdog's profile


1130 posts in 2087 days

#2 posted 12-19-2013 02:23 PM

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?

View crank49's profile


4032 posts in 3023 days

#3 posted 12-19-2013 02:50 PM

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.

View Underdog's profile


1130 posts in 2087 days

#4 posted 12-19-2013 06:09 PM

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…

View crank49's profile


4032 posts in 3023 days

#5 posted 12-19-2013 06:22 PM

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.

View David Dean's profile

David Dean

608 posts in 2951 days

#6 posted 12-19-2013 06:34 PM

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

View Richard H's profile

Richard H

489 posts in 1732 days

#7 posted 12-19-2013 06:35 PM

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.

View canadianchips's profile


2602 posts in 3049 days

#8 posted 12-19-2013 06:39 PM

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.

-- "My mission in life - make everyone smile !"

View Grandpa's profile


3260 posts in 2727 days

#9 posted 12-20-2013 03:34 AM

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.

View john111's profile


73 posts in 2037 days

#10 posted 12-20-2013 05:43 AM

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

-- john111

View Underdog's profile


1130 posts in 2087 days

#11 posted 12-20-2013 12:58 PM

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.

View NinjaAssassin's profile


640 posts in 1776 days

#12 posted 12-20-2013 01:01 PM

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.

-- Billy

View a1Jim's profile


117160 posts in 3629 days

#13 posted 12-20-2013 01:43 PM

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.

-- wood crafting & woodworking classes

View NinjaAssassin's profile


640 posts in 1776 days

#14 posted 12-20-2013 01:56 PM

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

-- Billy

View Grandpa's profile


3260 posts in 2727 days

#15 posted 12-20-2013 03:37 PM

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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