Vapor Barrier Question

[ToolJunkie]

I have an unusual situation and I'm wondering what I should do regarding a vapor barrier.

I have a steel building in the midwest. The building is a steel building with R-13 insulation sandwiched between the steel framing and the outside tin. The plastic coating of the insulation is visible on the inside of the building. Obviously, where the insulation is pinched at the horizontal steel frames, the R-value is considerably less than R13.

I'm building a workshop along the back wall of the building with a loft apartment above. The walls will be of 2x4 construction and I plan on insulating them with R-13. Although the building itself will be heated just enough to prevent freezing (say 40 degrees F), the shop and apartment will be heated to a warmer temperature.

The back wall of the shop/apartment will not be touching the insulation on the back wall of the building. There will be a gap of between 2" and 5 1/2" between the back of the studs and the building insulation vapor barrier. I was going to use R13 Kraft-faced in the shop/apartment walls with the Kraft face on the inside of the shop/apartment.

My buddy (whose family used to sell insulation, although I would by no means consider him an expert) said that if I did that, I would create a double vapor barrier because of the Kraft face, and the plastic face of the building insulation. He said I could get moisture/mold problems in my studded wall and suggested using no VB at all.

I've already got the R-13 Kraft-face on site, and now I'm not sure what to do. There is a 2-5.5" gap, that may allow the use of the Kraft, but I don't think that there would be a lot of airflow back there, so I wasn't sure. I worry about having a heated shop/apartment without any vapor barrier at all.

[Shophound]

Will you also be cooling the apartment and shop, with the larger building remaining uncooled? Do you live in a humid climate?

[doc havoc]

Your buddy is correct. You do not need a poly vapor barrier if you use kraft faced glass. The trick is to make sure you overlap your seams and repair any tears in the paper before you cover it with dry wall.

[Carnak]

the kraft is not a complete barrier like the plastic

the only problem I see is critters and crap getting in the gap

I would have tar papered the back side of the apartment studs myself

[Shophound]

Here's where I was going earlier...if you keep the shop/apartment considerably cooler (for summer a/c purposes) than the surrounding building, which if in a humid climate and not air conditioned will load up with moisture, there is the possibility that moisture in the larger building could possibly condense on the insulation side of the drywall/kraft paper. I saw possibly, for several things would need to occur; a) if the shop/apartment is kept quite cool, say 70 -72 degree range, and the dew point of the air within the large building goes above 70 - 72, condensation could occur, but it would require the exterior side of the drywall enclosing your shop/apartment to be that cold. With the structural members of the metal building skin itself not insulated (as you mention in the OP), and with likely little air movement between the gap of your stud wall and the plastic face of the building insulation, the drywall probably won't sweat. However, it's possible, and something to consider. The unanswered question is will you have any form of cooling/dehumidifying for the large metal building during warm weather (and if you live in a humid climate).

For heating, the greatest risk of condensation would be via vapor diffusion through the drywall into the insulated stud cavities. The kraft paper would slow this diffusion down significantly. If the back of your shop/apartment walls is not sheathed, and there's a gap between the fiberglass insulation in the stud bays and the plastic vapor barrier of the metal building skin, there may be enough air circulation to prevent a buildup of vapor, leading to condensation. The exposed metal structural components could possibly be points of condensation, but the kraft paper facing the interior of your shop/apartment should take care of that, IMO.

Your apartment will likely create more moisture internally than the shop will, unless you plan for a lot of activity within the shop that generates moisture (such as spray painting with water based materials, etc.).

[ToolJunkie]

Thanks for the replies. I'm not planning on installing air conditioning for the time being. I'll have to see how hot it gets in the shop/apartment in the summer time. If I can get by with some open windows and a fan or two, I'd probably stick with that.

There will be no dehumidifier in the big building, but there possibly will be in the shop (woodworking shop) and maybe the apartment if I feel it needs it.

A poly vapor barrier was never in the plan. The question was whether or not the Kraft barrier would create problems.

[12stones]

Are you going to sheath the exterior of the apartment walls? Fiberglass insulation in a wall cavity requires air blocks on all sides or it doesn't insulate very well. If you are installing sheathing on the exterior of the walls and there is a small air gap between the sheathing and the large building walls I would build the apartment walls just like any other house with the kraft faced insulation in the walls, kraft face to the inside.

[ToolJunkie]

12Stones,

Sorry for the late reply. I'm on vacation right now and my Internet access is sparse at times.

I have not sheathed the back side of the apartment walls, mainly because the back of many of the studs were notched an inch or so to accommodate the bracing in the steel building -- so it would have been virtually impossible to use anything rigid as sheathing. The best that I could do at this point is maybe roll out some tar paper or something behind the wall and get in there with a stapler, but that would be very difficult to do.

I didn't think that there'd be much for air movement back there, which is why I figured that the open insulation would be ok -- but also the reason that I'm concerned about moisture .

[12stones]

I would try to get something back there even if it's not continuous. I would not use tar paper as that would be a vapor barrier you need something breathable like Tyvek. Some gaps or holes won't be a big problem you are not trying to keep water out but the more you can contain the air in the stud cavity's the better the insulation will work. Insulation works by holding air pockets still. In a vertical wall as the air in those pockets warms up it will want to migrate upwards and if the whole back side is open they will escape and you will end up with much less effective R value than you put into the wall.