Is a vapor barrier of value in this 40′ x16′ barn ??? Built on a concrete slab( no insulation under it). The walls are insulated R19. The doors, two, are inexpensive overhead garage doors. The windows(few) are single thickness glass. I plan to heat it ONLY when I am working for a hour or more(woodworking and crafts). It will be heated with a small oil burner – hot air. Also a contractor told me to use plastic sheeting instead of Tyvek(or equivilent) over the sheathing and under the siding. For this building will the plastic work?? Thanks in advance.
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Are the insulated walls at R-19 fibergass batts?
If so, is there sheetrock covering it?
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yes the walls are r19 and there will be sheetrock over them after I resolve the vapor barrier/tyvek issue. the ceiling will be R19 but may be covered(only) with plastic on the shop side - as the attic trusses will be covered with 5/8 osb to create the attic floor so I will want to avoid the extra weight of a sheet rock ceiling.
I wouldn't want the plastic sheeting on the outside of the fiberglass batts since the idea is to always place the vapor barrier towards the heated area of the structure between the batts and the drywall, the vapor barrier preventing migration of moisture into the fiberglass batts. Plastic on the outside would contain the moisture in the batts, decreasing r value, ruining the insulation and causing moisture problems within the stud bays as rot and mold, especially if plastic is placed on the outside of the sheathing and also between the batts and drywall creating a double vapor barrier with nowhere for any moisture that might enter in the bays to vent to. With tyvex on the outside walls moisture supposedly can escape thru it's tiny pores but not enter in and act as a wind barrier helping prevent heat loss. That's a basic summary.
I'm just a DIYer and there are builders who frequent here that are more qualified to answer any more technical questions regarding proper placement of your vapor barrier since amount of heating done and the climate you are in can effect correct procedural details.
Hopefully someone else will enter in here to help fill in the blanks. Let the thunder crack and the waves roar.
We're going on.
Rez did a pretty good job on the basics here. The only case where you might ever possibly want the plastic over the sheathing is in a hot hunid climate such as the Gulf Coast for a building where you would be doing far more airconditioning than heating. In that case, the migration of moisture would be from the outside towards the cooler, drier air inside. If you did place plastic there, you would then want a rainscreen style wall to let the siding breathe and the condensation run out at the bottom. Pretty expensive for a barn/shop and you've only mentioned heating so I won't adress it any further. I am actually wondering how professional your builder is for such a recomendation. The possibility of causing mold in most cases like that is one to stay far away from.
Whether to put it on the inside is even negotiable. Kraft face batts will take care of a vapour bar on the inside if installed right. Also, as you describe it, there wil be little moisture generated in this building to worry about unless you will be drying green lumber or spraying latex or water based formulas.
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