Question about closed cell foam and vapor barrier

[air cooled]

Building a pole barn this winter and I have a question.

I cut rigid closed cell foam panels to fit between the exterior wall girts and sprayed the small gaps with closed cell spray foam.

We are now going to put fiberglass insulation blankets in between the poles and i'm not sure if we should put a plastic vapor barrier up before we install the interior wall steel or consider the closed cell foam panels and exterior wall girts the vapor barrier.

So, the layers from the outside in are:
Exterior Steel -
House Wrap -
Closed Cell Foam Panels & wood wall girts -
Fiberglass insulation blanket -
(plastic sheeting vapor barrier?) -
Interior wall girts -
Interior Steel

View media item 65496(sorry, dont know how to rotate photo)

Thanks

 

[Chris705]

I would suggest not putting up an interior VB due to having a VB at the outside face. Unless you are heating with propane salamander, washing lots of vehicles with hot water in the dead of winter or putting a Jacuzzi in you garage....a lot of vapor typically isn't produced in a garage. For that reason if you did put up an interior VB you run the risk of trapping it between VB's. If you can, try to leave a space between fiberglass & metal liner panels to allow the fiberglass to "dry" inwards. I seriously doubt it will lead to a problem. Let us know if you think you'll be producing lots of water vapor....

 

[hefnerconstructionlc]

I would also suggest skipping the vapor barrier. Especially if you are tining the inside of the building you basically already have a vapor barrier. Vapor barrier is on the interior are really outdated building practices that created more problems than they prevented. Really most the time they just trapped moisture.

 

[cajunfirehawk]

Is this orientation correct?

 

[air cooled]

Thanks for rotating the photo

 

[air cooled]

I don't expect to be making much moisture in the barn.

I am leaning towards calling the closed cell foam my vapor barrier and not installing a plastic VB as you guys suggest.

Anyone else have any thoughts?

Thanks guys.

 

[tcianci]

The purpose of a vapor barrier is to retard the migration of moisture (theoretically originating from the interior of the building) into the insulated wall cavity. The reason you would want a vapor barrier is that when the exterior of the building is cold and the interior of the building is warm, there is a thermal gradient within the wall cavity. Think of it like this; the exterior of the insulation is close to the ambient outdoor temperature, the interior of the insulation is at room temperature. Somewhere in that wall cavity the temperature of the insulation is just right for water vapor to condense into liquid water. Liquid water being a great conductor of heat, kills the R value of the insulation until it dries out. This is most problematic with fiberglass, cellulose and other types insulation like Roxul.
Closed cell foam, does not have the ability to absorb moisture so there is no vapor in the insulation to condense.
In the case where closed cell foam is being topped with fiberglass, the possibility for water vapor to be in the fiberglass still exists but chances for the inside face of the closed cell foam to be cool enough to cause condensation are slim unless the layer of foam is extremely thin.
Placing a vapor barrier behind the wall finish essentially creates a cavity to trap moisture in the fiberglass. The vapor can't escape through the closed cell foam and although the layer over the fiberglass is a vapor barrier, it should really be thought of as a vapor retarder, since it's not perfect and it's typically much more vapor permeable than the closed cell foam. And, since any water vapor is likely to enter the wall cavity through a breach in the vapor barrier and not actually migrate through the vapor barrier itself, the likelihood of trapping vapor in the fiberglass goes up.
So, in this example, it looks like you wouldn't want a vapor barrier on top of the fiberglass and personally, that's the wall assembly I would use. However, a typical painted drywall finish is an excellent vapor barrier, so whether you wanted one or not, there's going to be a high degree of vapor retardation contributed by your interior finish.

 

[air cooled]

Makes sense, Thanks