Vapor/Moisture Barrier in Shop

[EasyE]

Guys I have a 60X80 metal building that is insulated with vinyl backed fiberglass insulation. I am building out a 1400SF living area in the front. 2 walls are exterior walls that are OSB and felt papered then bricked on the exterior. (These walls will be spray foamed on inside) The walls exterior (shared) to the shop are 2X6. I am planning to use fiberglass batt insulation here. Before I insulate would it be wise to tyvek the exterior of the walls and use plastic vapor barrier on the inside? I am planning to use 5/8" fire rock on the outside of the walls (shared w/ shop will not get wet).

The building is in south louisiana so we do not have a cold climate. Also currently the building is not a conditioned space and the bay doors are open much of the time. I later have plans to add 10 tons of air to the shop.

Thanks

 

[NUTTSGT]

Are these the same walls getting spray foamed ?

 

[EasyE]

No, the spray foam walls are exterior to the outdoors, w/ brick veneer.

These walls are common with the interior of the shop.

 

[EasyE]

Any thoughts?

 

[stm317]

A very general rule is that you want to have a vapor barrier on the side of the insulation that is warmer. So in cold climates where you're trying to keep heat in the building, the vapor barrier goes on the inside of the insulation. In a warm climate, I'd probably put it on the outside of the insulation.
However, I'm not sure how well that rule applies when building/insulating a structure within a structure.

 

[EasyE]

Im tempted to just wrap the outside with Tyvek to slow the airflow, but I dont know how this will work with sheet rock directly over it.

 

[Marctrees]

Easy - You really need to Google "Vapor barrier hot humid building envelope" and you will learn stm317 is correct.

Cold climates ( ballpark 50% of 48 state US) need vapor barrier to inside.. Hot/Humid (ballpark 15% of US)need vapor barrier to outside.. in between (ballpark 35% of US)is still being debated by the Researchers in that field.

Your existing plastic needs to be permeable, not impermeable as it currently is, unless you got perforated face, which I doubt cause most are uneducated about it.

It can still be fine if razor knife slashed all over before covering.

Study and you will see.

Read tech stuff at like ORNL, not some **** on Houzz or whatever dumb sites.

Quick search result example - https://buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

Marc

 

[ckyle29]

A very general rule is that you want to have a vapor barrier on the side of the insulation that is warmer. So in cold climates where you're trying to keep heat in the building, the vapor barrier goes on the inside of the insulation. In a warm climate, I'd probably put it on the outside of the insulation.
However, I'm not sure how well that rule applies when building/insulating a structure within a structure.

This. On all your outside walls, you want the vapor barrier on the outside of the insulation. I'm just north of Dallas, so I'm still warm more months than cold, so the barrier went on the outside of the insulation in my building.

On my interior dividing wall, which is also 2x6 and batt insulated like you are planning to do, I did not put any vapor barrier, but only because my living/shop areas were both climate controlled from the start. If it is going to be a while before you A/C the shop and you have the shop doors open a lot, you are going to have a lot of warm, moist air up against that interior wall, which is being cooled on the living side, and hence you will get condensation pulled into the insulation of the interior wall. In that case, I would err on the side of caution and put a barrier on the warm (shop) side of the interior wall just to be safe.

 

[EasyE]

Ok thanks, so Ill use a house wrap right on the studs.

Do you foresee any issue with installing 5/8 fire rock over the top of this? I will not have any sheathing underneath as I do not need much shear strength in these walls. (They are steel strapped)

 

[ckyle29]

Ok thanks, so Ill use a house wrap right on the studs.

Do you foresee any issue with installing 5/8 fire rock over the top of this? I will not have any sheathing underneath as I do not need much shear strength in these walls. (They are steel strapped)

If by "house wrap" you mean Tyvek, don't use it. Use a vapor barrier. Per the Dupont website, Tyvek is not a vapor barrier, it allows condensed vapor to pass through it. It is more of an air blocker but also allows water vapor (but not bulk water) to pass from inside of walls out. And no, you should not have any problems putting up the barrier and then the sheetrock directly over it without sheathing.

 

[Marctrees]

He really needs to study as I said above to understand what is going on and what to do.

Once one understands the concepts, he can understand what needs to be done.

Marc

 

[mobetta]

I would purchase an actual "smart" VB, not just poly.

install on the warm side of the wall.

 

[EasyE]

If by "house wrap" you mean Tyvek, don't use it. Use a vapor barrier. Per the Dupont website, Tyvek is not a vapor barrier, it allows condensed vapor to pass through it. It is more of an air blocker but also allows water vapor (but not bulk water) to pass from inside of walls out. And no, you should not have any problems putting up the barrier and then the sheetrock directly over it without sheathing.

I thought Tyvek only let moisture vapor penetrate from one direction and was impermeable from the other way?

So some of yall are telling me I need to use visqueen on one side and other are telling me to use tyvek? The warm side of the wall is the shop side so I understand that Im just confused on what to use now?

 

[stm317]

Tyvek is a moisture barrier, meaning it keeps liquid water from passing through, but it is not a true vapor barrier. It's semi permeable. That's why it's used on the outer walls of a building. It can shed any water that gets behind the facade of your structure, and prevent the water from damaging anything within your wall. But because it's semi permeable, it will still allow humidity inside the wall structure to pass through to the outside avoiding mold issues inside the wall.

 

[EasyE]

Alright so you think wrapping the outside in visqueen is a better idea?
And then use either cellulose or fiberglass insulation without a backing?

 

[Marctrees]

Easy - Have you done reading of anything suggested in this thread?

You really should study to confirm what to do.

Yet one more easy to read example -


abovehttps://www.energyvanguard.com/blog/52970/Vapor-Retarder-Vapor-Barrier-Perms-What-the-Heck

 

[ard]

Are you really going to go with a consensus here for such a critical building design question????

 

[EasyE]

Easy - Have you done reading of anything suggested in this thread?

You really should study to confirm what to do.

Yet one more easy to read example -


abovehttps://www.energyvanguard.com/blog/52970/Vapor-Retarder-Vapor-Barrier-Perms-What-the-Heck

I read the link, and this doesn't really answer my scenario because the wall Im talking about will not see liquid moisture and the make up is not outlined in any of the examples.

Ive talked with a few builders and I have gotten several responses.

1 - Do nothing, insulate as is, paper backed insulation on inside and cover with sheet rock.
2 - Tyvek the outside and sheet rock over it.
3 - Visqeen the outside and use insulation without a backer on the inside then rock over it.

Appears no one is really sure.

 

[EasyE]

Any more advise out there?