Pole Barn Vapor Barrier

[KennyG]

I need a check on my logic. I'm reworking an old pole barn. I have enough used sheets of foam insulation to put a nominal 5 inches of foam in the walls. By the time I'm done, I will also put new metal on the outside and I'm going to finish the interior with metal panels also. I had been thinking of putting a tyvek type of vapor barrier on the outside (over the purlins). If I do that, there will be a 1-1/2 inch gap between the barrier and outside metal and the foam. Should the vapor barrier go on the inside, where it can be against the foam, instead?

On the ceiling, I will also put up metal panels and then blow in cellulose over them. A friend suggested putting plastic sheeting up first before the metal to add a barrier. Does this make sense, to have a barrier below the ceiling insulation? Would it make any difference since I'm blowing in insulation?

Your thoughts appreciated.

 

[BeezFun]

I need a check on my logic. I'm reworking an old pole barn. I have enough used sheets of foam insulation to put a nominal 5 inches of foam in the walls. By the time I'm done, I will also put new metal on the outside and I'm going to finish the interior with metal panels also. I had been thinking of putting a tyvek type of vapor barrier on the outside (over the purlins). If I do that, there will be a 1-1/2 inch gap between the barrier and outside metal and the foam. Should the vapor barrier go on the inside, where it can be against the foam, instead?

The primary function of housewraps is to shed the water that gets behind the siding. So it should go on the outside to perform that function. If you also want something to act as a vapor barrier, that should go on the inside of the wall insulation since you live in a northern climate (different in a southern humid climate). I have experience with vapor barriers here in the Chicago burbs, they are a great help to reduce air infiltration and prevent moisture in the walls, however if they aren't installed properly they can malfunction with nasty consequences. For example they need to be integrated with the vapor barrier on the ceiling: the ceiling wrap should extend a short distance down the wall, then the wall wrap goes over it. The wall barrier should run under the flooring material a short distance to prevent air rushing out at the floor intersection. Typically in a home this was carpeting, I'm guessing you have a concrete floor so I'd bond it to the floor plate and then caulk the base before putting up shoe. We overlapped and used caulking along every seam and around every penetration.

The Journal of Light Home Construction published results of the performance of various housewraps including plain old felt paper, the study was done at some university. The felt paper did as well overall as any of the much more expensive materials, particularly in recovering from being water soaked. So I'd just go with felt paper if I were doing it. You might also caulk or tape the felt paper to help with air infiltration. You might think about a water screed under the siding, they have clever stuff now that's material like those little scrubby pads. It provides a continuous water drain from the top of the wall to the bottom so water can't get trapped between the siding and a purlin for example.

On the ceiling, I will also put up metal panels and then blow in cellulose over them. A friend suggested putting plastic sheeting up first before the metal to add a barrier. Does this make sense, to have a barrier below the ceiling insulation? Would it make any difference since I'm blowing in insulation?

This is a very good idea, but again the workmanship has to be good to avoid complications. If you were using drywall I might suggest using the type that has a moisture barrier built in (it has foil on the back). But since you're using metal that's not an option. Metal is obviously not permeable to moisture but has many seams and screwholes that will leak air, which will carry the moisture out. Metal also is very good at condensing any moisture out of the ceiling cavity and causing rot/mildew, so the barrier will really help with that.

 

[KennyG]

Thanks for the experience. I won't be able to actually make the wall and ceiling barriers continuous. The walls have a 2 x 10 plate at the top that the trusses sit on. I had intended to butt the metal wall paneling (with a channel) against the bottom of the plate. The ceiling panels will then fit against the top of the side of the plate, leaving it exposed. If the barriers are effectively sealed to this plate, does that work?

 

[BeezFun]

Thanks for the experience. I won't be able to actually make the wall and ceiling barriers continuous. The walls have a 2 x 10 plate at the top that the trusses sit on. I had intended to butt the metal wall paneling (with a channel) against the bottom of the plate. The ceiling panels will then fit against the top of the side of the plate, leaving it exposed. If the barriers are effectively sealed to this plate, does that work?

Yes, that works. Just run a continuous bead of caulk along the plate (or whatever) and press the barrier into it, then staple. As long as whatever you do prevents moist air from bypassing the barrier and getting into the insulation cavity, you're good. Staple right into the bead of caulk under the plastic. That will prevent air from going through if the staple pops through the plastic or tears a little. The most important thing is good workmanship and attention to detail sealing the plastic. Since you're doing it yourself that should guarantee a good job. You can also tape seams, we found that pressing it into caulking was faster and easier to move around as you stretch it out, then tack down with staples until you get the wall material up.

Make sure you get the right type of plastic, and get something thick. Some plastics are fire accellerants, make sure you buy something that says it's fire retardant. Although 4mil is plenty thick as a moisture barrier, it might be easier to work with 6mil because it doesn't tear as easily and survives more abuse while you're putting up material over it.

 

[BeezFun]

Another thing to keep in mind is that if you use a vapor barrier and water does get into the wall cavity (for example if you have a leak in the roof that runs down the wall), you can have really serious mold problems because the moisture is then trapped and can't get out, other than the small amount of transmission through the house wrap. So you need to be double dog sure you don't have any leaks or windblown rain that can find it's way into that wall cavity. We redid several homes that had trapped water and the 2x4 wall studs had turned into mushrooms- you could push your finger all the way through the 2x4 it was so soft.