Pole Barn Walls - Moisture Barrier & Polystyrene Insulation

[sjvicker]

I have spent years reading threads and watching you tube videos about these topics and have dreaded this point where I ask a bunch of random strangers on the internet their opinions on barriers and insulation only to get a recommendation for every possible option with the only consensus that you shouldn't have 2 vapor barriers. That said, I'm at a decision point on my build and am curious what recommendations are out there for my specific application.

Details first:

1. I am in the PNW and we have a climate that is neither a heating or cooling climate. This shop will not be cooled and I will have a wood stove running occasionally to take the edge off in the winter (bring it from 40deg to 60deg). Most people that ask these questions seem to be from very clearly heated or cooled climates.
2. From what I can tell, moisture barrier (Tyvek) is not common here on pole buildings
3. I have trees that block the sun in the morning and with the building orientation, I wont get a sudden blast of sun every morning on my south wall that would cause a sudden temp change.
4. I am planning to install 1.5" of Polystyrene insulation between the girts and seal it with spray foam, effectively creating a vapor barrier.
5. I built a small pole barn a couple of years back and sheeted it in OSB, Tyvek then metal. I wanted to avoid as much moisture from getting to the OSB as possible so in this application it made sense to me to use a moisture barrier.

The Question:

Is there really any benefit to the Tyvek moisture barrier if I have the Polystyrene working as a vapor barrier? I can see it having a marginal improvement in that it might keep some moisture off the girts on the South facing wall but otherwise I'm considering omitting it.

Thoughts?

 

[billconner]

I would not call them or other house wraps "moisture barriers". It's to stop mass water - that might get blown through siding - from penetrating but still allow air - including moist air - to pass.

I'm assuming metal siding, but don't know. Based on no AC and some heat and the rigid foam insulation, I'd think Tyvek would be somewhat beneficial in stopping rain and some wind from penetrating, but damage and energy waste would be minimal. I do wonder if it would be easier to remove/replace siding because the spray foam won't reach sliding - a very small issue.

Like putting rebar in footings when it's not required by code, it might be better but probably won't be missed.

 

[sjvicker]

That's exactly what I was thinking and I am planning metal siding. Our weather is so mild we also dont really get driving rain either.

 

[jack stand]

House wraps are to prohibit air infiltration while while allowing water vapor to pass through.
It will act to prevent water infiltration but it's effectiveness is dependent on flashing and other traditional "mechanical" water diversions and sealing tapes and is not an intended leak prevention product.

 

[billconner]

Thread made me wonder if putting rigid foam continuous outside hurts and fastening siding through it would be better both thermally and water wise. Just tape seams, and no cutting of foam.

 

[sjvicker]

It would be a good thermal break and pretty similar to the Zip panels that come with insulation glued to them but I've never seen one done this way. I also dont have it in the budget right now so the metal has to go on first and the insulation can follow this winter.