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Thread: Do I need a vapor barrier?

  1. #1

    Do I need a vapor barrier?

    My shop is one of those glorified sheds you can buy from HD or Lowes. So the walls are framed 2x4 with exterior siding nailed on the outside. Everything is optimized for 4x8' 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?
    Last edited by Jim Underwood; 12-19-2013 at 8:52 AM.
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  2. #2
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    If you use faced insulation and staple the flanges to the studs, you do not need a vapor barrier. The vapor barrier is used when you use un-faced batts (without the paper). Insulation is best installed before you move into the shop and start placing tools, cabinets, etc on the walls. A real pain to move all this stuff after the fact to install insulation. Good luck.

  3. #3
    You generally want the vapor barrier on the warm side of your insulation, then panelling over that. I've never used the faced batts, around here (Canadian prairies) the standard residential construction is unfaced batts with a separate thick plastic sheet sealed to the studs with Acoustiseal and then stapled.

  4. #4
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    Are you planning on heating or cooling the shop?

  5. #5
    I don't know exactly what I'll do about heat, but I already have an A/C unit that I'd like to install. Doesn't do much good without good insulation though... My little ceramic heater is only good enough to keep my feet warm on a cold day. Doesn't do much in that un-insulated shell when it's 30F outside.
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  6. #6
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    Quote Originally Posted by Mike Wilkins View Post
    If you use faced insulation and staple the flanges to the studs, you do not need a vapor barrier. The vapor barrier is used when you use un-faced batts (without the paper).
    This makes no sense... the paper on faced insulation is not a vapor barrier, and even if it was, a few staples along the edge is not going to seal it.
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  7. #7
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    Quote Originally Posted by Dan Hintz View Post
    This makes no sense... the paper on faced insulation is not a vapor barrier, and even if it was, a few staples along the edge is not going to seal it.
    Well, no one said nobody isn't direct around here.

  8. #8
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    Direct or not, Dan is correct. IMHO, the basic problem is the way things are advertised and the confusion over Vapor Barrier vs. Vapor Retarder. Having no vapor barrier is much better than installing one without a good understanding, particularly in a humid part of the country that can freeze.

    This will be a good start: http://www.buildingscience.com/docum...vapor-barriers

  9. #9
    Well it's not just the advertising ,the words " barrier " and " retarder" do not have enough difference between them to be useful to any one not in the know on this subject. That is most certainly by design. Many barriers are pervious ,by design. How many of us have bought the DELUXE model of something to later find out the SUPER DELUXE was real top of the line ? Virginia meets the criteria of being hot,humid but freezing aplenty in winter. I'm glad to now be better informed by the knowledgeable comments,but the common paper is always referred to around here as vapor barrier and the only caution is install "vapor barrier " to heated side. I now under stand it's not the super deluxe solution.

  10. #10
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    One solution for all parts of the country is not reality. I use the "Builders Guide to Cold Climates" by Joseph Lstiburek as my go to reference. He has books specifically for other climates as well. I do not use plastic in any of the homes I build. I came to this after seeing too much damage by mold inside walls. What works for AC does not work for heat, so if you are doing both you need a controlled permeability on both sides of the wall.

    Go to ***.buildingscience.com for some out of the box store thinking.

    I have tried something new on my new home that I have always wanted to try. I have a balanced perm on both sides of the wall with a layer of 1/2" urethane foam on the inside and the outside. Headers are insulated with 2 1/2" of poly, and even the exterior trim is insulated. The ceiling is a double layer affair with a sealed dead air space of 3/4" in between the layers, the first layer is 1/2" urethane, spacer and 5/8" drywall. The foam layer was spaced so I could seal in between the sheets with foam. The top of the walls are insulated with poly,, cut to match the pitch of the roof, and the vent baffles are 1" foil Thermax, again any gaps sealed with foam. Windows are set with foam as are doors. No wires were allowed to penetrate the plate on the eve sides where insulation is the weakest and closest to the roof. All wires and vent pipes are sealed where they go through to the attic. There is very little contact between framing members and the interior surfaces, or the outside surfaces for that matter. Wall cavitys are 5 1/2" of wet cellulose, cealing has 24" of cellulose with a bit of binder to prevent settling above the double ceiling. I will have an air to air exchanger to control air quality and humidity before I move in.

    I have set glass jars, olive jars actually, into the north and south walls so I can keep an eye on what is going on in the wall cavity as I have not seen or read of a wall system done quite this way. I heated the house to 65 last winter with natural gas and the bill on the coldest month was $72. This house is 1300 ft with 11' ceilings in the main room, 8'4" in the rest, has 25 windows and glass doors, and the basement is not done yet. I am 90 miles from Canada so it gets cold here. When I move in I will heat with wood from the property.

    Rambling a bit, but my point is don't buy the "Got to have a vapor barrier" train of thought with out looking at all of the variables. For instance if you put the vapor barier on the inside in the summer when you run the air conditioning the moisture will be collection on the back side of the vapor barrier. So how you will use the building has to be considered.

    By the way, with tools, the best place to have a vapor barrier is under the floor. Even with your built on stilts building you can work plastic under there. Some will say that it will have voids so it can not work but it works by percentage. If you can get 90% of it covered you will have 90% less vapor transfer, not at all bad.

    Larry

  11. #11
    I've been looking at insulation mfg. sites and only seeing RETARDER mentioned . I'm pretty sure THEY used to use the word BARRIER. Gonna look at old stuff,too. I'm wondering if the change is due to AC use changes. It used to be said by installers that AC was intended to bring temp down 10 to 20 degrees cooler than outside. That's gone. I still follow the old advise because any cooler than that is such a shock if I need to go back and forth frequently. My AC is never set lower than 80 degrees when outside temp is 100 or more. But I know people who stay inside a lot and always cool to 70 degrees because they want to and the equipment will do it .That makes for high inside relative humidity. Is that why this retarder term is now used?

  12. #12
    The proper term is Vapor Retarder. The word "barrier" is no longer used because it implies that no vapor or air movement can pass through a product. It changed because it was quickly discovered that most of these products were semipermeable and that making a house as air tight as possible resulted in other vapor related problems. Products used as vapor retarders are classified into 3 groups based on there permeability and assigned a Perm rating. Kraft faced insulation IS considered a Class 3 vapor retarder when installed properly, but by no means the most efficient one on its own, not solely because of the kraft facing but that fiberglass insulation does not stop the movement of air. Stopping a structure from breathing is not always a bad thing. You should consult with some local contractors or building officials to see what the practices are is in your region.
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  13. #13
    Wow. Great information here, but I'm not sure I'm not more confused than I was to start with.... That's what comes from expecting easy answers I guess.
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  14. #14
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    I went through this when trying to determine what side the vapor barrier goes on. I found it depends on where you are. I am in southern Indiana and the recommended is just using the Kraft paper ( no plastic) on the heated side. It does vary be region.

  15. #15
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    I can't help but think of how Mike Holmes likes to insulate exterior walls (a Canadian show). He prefers to spray the stud cavity with a thin layer of foam that looks like it seals pretty well and appears to be a vapor BARRIER. Then he might add additional bats to build the R value, then he might add a layer of poly on top of the studs, fully taped at the perimeter. Now that appears to be a vapor barrier. Question: it seems that code in the US requires an exterior wrap with Tyvek. Is that material semi-permeable, or is it just to keep the wind out? When I built my house in 1975, before the wrap was required, I had them sheath the outside with 1" of foam before they went with R-13 between the studs. Not sure how much it helped.

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