Vapor barrier question for those who know

[rrrracer]

OK, after searching six pages of threads, I still couldn't come up with an answer to my particular situation so here goes...

I am finishing out the interior of my pole barn. South/Central Indiana, so temp ranges from 0-100 degrees, all sorts of weather and quite humid in the summertime.

The interior is framed out 2x6s, with Kraft-faced R19 fiberglass. Exterior is just sheet metal, no wrap. The roof has that thin bubble wrap insulation lining it, no vents. Ceiling is a drop-style with 1.5" foam boards. It has a big slider door that will be marginally insulated (as well as we can do it).

You can see pics of it here: http://www.garagejournal.com/forum/showthread.php?t=285656

Heat in the winter will be via wood burning stove; I have a window A/C unit for the summer time, although I don't plan on using it much (will be working with the doors open).

Do I want to use a vapor barrier between the Kraft insulation and the drywall I am planning on putting up or not? I was leaning towards not using one, but a friend stopped by yesterday and recommended it (4 mil minimum).

Thanks in advance!

 

[JACDes]

yes you want vapor barrier

 

[Beemer533]

Just make sure, if you are doing a poly vapor barrier, that you use un-faced fiberglass..

 

[sb7979]

The kraft facing on the insulation is a vapor barrier. No need for anything else. Make sure the vapor barrier is always on the conditioned side (heated or cooled) of the insulation, so facing inwards to the building.

 

[rrrracer]

Thanks sb7979, that's exactly what I was thinking. His comment was that I could pick up some R value from a vapor barrier, but I am concerned that it would trap moisture in the walls since the fiberglass is right against the metal exterior and it is far from water proof.

 

[PAToyota]

His comment was that I could pick up some R value from a vapor barrier

Nope. Absolutely no R-value from a vapor barrier.

Arguing semantics, you could sort of say that an air barrier improves your R-value by eliminating drafts, but that is a stretch.

 

[kbs2244]

The whole idea behind a vapor barrier is to keep vapor from condensing in the wall as it follows the heat through to wall of a well sealed building.
Your building does not fit the idea of well sealed.
You have heat leaks at the doors and any moisture will follow it out at that point.
You do not need any vapor barrier unless you plan to really tighten up the building.

 

[tfinniii]

Sealed edges of drywall with a good paint is all the vapor barrier you should need as wood stove is drying out air. Also if no inside air migrates to insulation you won't have a problem and as the bldg.isn't producing moist air like a house. The main thing is avoiding air infiltration.

 

[Kaizen]

Nope. Absolutely no R-value from a vapor barrier.

Arguing semantics, you could sort of say that an air barrier improves your R-value by eliminating drafts, but that is a stretch.

I'd say improves it definitely. not sure I'd call it r value but sealing it is a huge difference. did a wall in my kitchen where last winter it was horrible and freezing. redid my siding so put up tyvek and taped the seams. just amazing reduction on the air flow

 

[PAToyota]

As I said, it won’t give you any R-value. Vapor barrier will slow down some drafts and air barrier will stop them, but they don’t provide any insulating value.

 

[rrrracer]

Thanks everyone for the replies. Yeah, the place is definitely not sealed up tight as a drum and that isn't going to change, so I will skip the plastic and focus on brute force heating with the wood stove ☺ OK, drywall here we come!

 

[tcianci]

The purpose of a vapor barrier is to ensure the performance of the fiberglass insulation. Moisture migrating through a wall cavity will, under the right weather conditions, reach the dew point, where it will condense in the wall. Wet fiberglass insulation has a greatly diminished R value (which it will regain when it dries out). Although the fiberglass is supplied with a vapor retarder (kraft paper) a typical interior paint job on drywall provides significant vapor retardation.

Adding a plastic sheet over the kraft vapor retarder is a big no-no. It amounts to a space in the wall assembly where any condensed moisture has no place to escape.

I have had my head handed to me on a plate around here for my opinions about building too tightly but as a remodeler, I take apart as much stuff as I put together. It has been my observation that all of the damage I see from rabid "energy conservation" far outweighs energy savings. It's kind of like buying a car that delivers 70 MPG but is all rotted out and destroyed by the time it's 3 years old. You save a lot of gas, but you buy a lot of cars.