Vapor retarded paint instead of poly?

[191185]

I have an attached 2 car garage in Minnesota. Rockwool insulation is already in the walls..

Can I use plywood as the walls, and use "vapor retarder paint" instead of the 6mil poly as a vapor barrier ?

I am still undecided weather I'm going to use sheetrock or plywood, but was leaning toward plywood and painting it white...

 

[Rbreddin]

Is there already a vapor retarder in place? One thing you don’t want to do is encapsulate the insulation between vapor barriers.


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[191185]

No, I just finished the insulation earlier this week.

I used rockwool which is unfaced. So just bare insulation right now..

 

[rerod]

Don't use poly. Use something that will diffuse moisture..

 

[191185]

Don't use poly. Use something that will diffuse moisture..

Is the 6mm sheet of plastic poly or something else?

 

[scottydosnntkno]

Minnesota is mostly zone 6, which requires a true vapor barrier (continuous poly) due to the extreme cold temps. I do not believe paint counts as a vapor barrier, only a vapor retarder ( zone 3-5).

A true vapor barrier is poly, caulked and foamed to the backs of all electrical penetrations. Faced insulation does not meet code as a vapor barrier, only as a vapor retardee

 

[191185]

Minnesota is mostly zone 6, which requires a true vapor barrier (continuous poly) due to the extreme cold temps. I do not believe paint counts as a vapor barrier, only a vapor retarder ( zone 3-5).

A true vapor barrier is poly, caulked and foamed to the backs of all electrical penetrations. Faced insulation does not meet code as a vapor barrier, only as a vapor retardee

Thanks, I did foam behind electrical boxes, and have the 6mil poly..

What exactly do I buy for the caulk, does it have to be acoustic?

 

[rerod]

Thanks, I did foam behind electrical boxes, and have the 6mil poly..

What exactly do I buy for the caulk, does it have to be acoustic?

My home has poly, but I wish it didn't.

If you use poly under the drywall, and then you or someone else puts another vapor barrier on the outside, condensation will rot out your wall in the summer.

 

[MSPoirier]

My understanding for this is if you already have a sort or barrier to the outside then ideally you want to use an unfaced insulation with no barrier and allow the system to "breath" to the inside so covering with a material like plywood or sheetrock and using an air permeable paint layer like latex?

- Sam

New England Region

 

[191185]

I can not believe 50% say don't use one, 50% say I have to use one ... Jesus Christ ..


I am even more confused now then ever...

The house was built in 2003 using normal building codes...

so is there some super wrap on the outside of my garage, behind the siding that would make me NOT want to use a 6mm poly vapor barrier ?

 

[1953mercury]

That's why internet info is worth exactly what you pay for it. 6 mil is pretty standard vapor barrier, and should be used, but for peace of mind, just check with your local building department. Mike

 

[Jlbc212]

4 mil or 6 mil poly - it will nor allow moisture from the inside to pass through. You will not have rot as some others have wrote. I've been building and remodeling in cold NE for years. it works!!! - no rot!!!

 

[MSPoirier]

so is there some super wrap on the outside of my garage, behind the siding that would make me NOT want to use a 6mm poly vapor barrier ?

The only way to know for sure is to check, if it's a more traditional construction of wood framing and there is Tyvek or similar or even nothing at all there, you would be fine with poly I would say. This way it can still breathe to the outside if need be.

If you have something like I have with 1" xps foam under the siding you would not want poly because theoretically it would trap moisture in between the exterior air barrier (the foam) and the poly.

I am far from an expert and only gather this information from the research I've done to work on my own garage. My advice could be worth exactly what you paid for it but I'm sure they will not be shy if I'm wrong! [emoji2369]



- Sam

New England Region

 

[stm317]

This topic gets tricky because it's climate dependent, and there's a lot of confusion about the differences between a vapor barrier and a vapor retarder.

A vapor retarder like Tyvek/Housewrap/etc is semi permeable. It will keep bulk moisture from passing through, but does allow some vapor to pass through. It's typically used on the exterior of a wall to prevent water that might get under the siding from contacting the wood sheathing, but still be permeable enough so that any moisture that may get into the wall cavity can work it's way to the outside.

A vapor barrier (plastic sheeting, poly, foam insulation of a certain thickness, etc) is impermeable and no liquid or gaseous vapor will pass through it. In order to be a 100% vapor barrier envelope, all seams and penetrations must be properly sealed with appropriate materials.

The rule of thumb is that you want a single, true vapor barrier on the side of the wall that is warm most frequently. In MN, that's the inside. That should keep any condensation that forms inside your building from entering the wall cavity and causing mold/decay. It will also keep any moisture vapor that may enter your wall from the exterior from entering your garage (and the semi permeable wrap on the exterior of the wall should let it dry to the outside).

I also think that having more than one vapor barrier is just as bad as having no vapor barrier, so make sure that you know what you're working with. Multiple vapor barriers will trap moisture between them and lead to really bad long term effects for anything that happens to be between the two.

 

[Dzlpete]

Nobody can give accurate info without knowing the exact composition of walls. We know you have a stud bay full of rockwool unfaced- good
You should be using a vapor barrier- 6 mil poly- good- behind plywood or sheetrock.
But, what is your sheathing outside? What is your housewrap product, specifically?
Is the stud bay the ONLY insulation? Or is the outside skinned with foam panels?
If you are using foam panels outside, and/or the zip system sheathing and tape on joints outside, instead of plywood, it will change the prescribed methods.
Foam panels, and to a certain degree, the zip system act as a moisture barrier, so the poly on the inside with foam or zip outside = trapped moisture in wall cavity.
Hopefully stud bays are deep enough- 2x6 or more that you do not need an outside supplemental layer of foam to get the r-value you need.
Explain the whole wall cross section, and the advise can be more accurate.

 

[rerod]

If you have something like I have with 1" xps foam under the siding you would not want poly because theoretically it would trap moisture in between the exterior air barrier (the foam) and the poly.
- Sam

Exactly.. That's why poly is a bad idea, because the next owner wont know it was used, and could install foam outside and end up with mold and rot inside the wall.

There are plenty of better products that will stop air flow but allow moisture to pass.

 

[191185]

Thanks everyone. I'm going to go out on a limb and say I do not have 1" xps foam under the siding, because my house was built like the other "dime a dozen" houses built in 2003.

It is a super typical split level, builder grade everything. If I have cheapo carpet, and builder grade cabinets, why would the original owner go crazy with 1" xps ?

So if I use 6 mil poly, and then use plywood, caulk the seems, and then paint the plywood. is there now a vapor barrier between the plywood and the poly ?

should I NOT caulk the plywood, and leave 1/8th " gaps for airflow ?

 

[benjamintmiller]

What kind of siding do you have? The vapor permeance of your outdoor wall assembly and the indoor conditions such as temperature and relative humidity dictate what you should do inside for vapor retardation.

The IRC has a lot of detailed information on wall assemblies, and the building science corp has the gold standard of whitepapers here. There's also a pretty good summary article here.

Let's assume your house has vinyl or aluminum siding over OSB or similar, with Tyvek wrapping the house. Looking at page 26 of the white paper, you can see the flow chart for zone 6. This wall assembly has greater than 1.0 perms, so following the chart you can see you class II vapor resistance. You can achieve this either by taping and finishing drywall and then applying a vapor resistant paint, or applying a smart vapor retarder to the wall studs. Kraft paper counts as this when used with fiberglass, or you can use a smart vapor retarder (SVR).

Here is an appropriate SVR that you could apply over your insulation. That is what I would do if I wanted to install plywood, because plywood will not be airtight.

As others have alluded to, there is a huge amount of misinformation on the Internet (including in this thread!), and you might not want to listen to contractors in your area either. I'd bet that very few of them have ever came back after 20 years to see if there's a moisture problem in a wall they put poly on. The building science guys do exactly that -- they build walls, they test them in a controlled environment, they make hypothesis, and they apply the scientific method to learn more about building science.

 

[MSPoirier]

So if I use 6 mil poly, and then use plywood, caulk the seems, and then paint the plywood. is there now a vapor barrier between the plywood and the poly ?

should I NOT caulk the plywood, and leave 1/8th " gaps for airflow ?

No, if you use poly then plywood then the poly is the only true vapor barrier as long as you stay away from epoxy or similar type paints. A decent latex should be fine now that your environment has been controlled.

I cannot speak to chalking the joints but I would imagine you would not get a very finished look to your wall with 1/8" gaps unless that is not your end goal. Maybe someone has more input on that particular question.



- Sam

New England Region

 

[Rbreddin]

What kind of siding do you have? The vapor permeance of your outdoor wall assembly and the indoor conditions such as temperature and relative humidity dictate what you should do inside for vapor retardation.

The IRC has a lot of detailed information on wall assemblies, and the building science corp has the gold standard of whitepapers here. There's also a pretty good summary article here.

Let's assume your house has vinyl or aluminum siding over OSB or similar, with Tyvek wrapping the house. Looking at page 26 of the white paper, you can see the flow chart for zone 6. This wall assembly has greater than 1.0 perms, so following the chart you can see you class II vapor resistance. You can achieve this either by taping and finishing drywall and then applying a vapor resistant paint, or applying a smart vapor retarder to the wall studs. Kraft paper counts as this when used with fiberglass, or you can use a smart vapor retarder (SVR).

Here is an appropriate SVR that you could apply over your insulation. That is what I would do if I wanted to install plywood, because plywood will not be airtight.

As others have alluded to, there is a huge amount of misinformation on the Internet (including in this thread!), and you might not want to listen to contractors in your area either. I'd bet that very few of them have ever came back after 20 years to see if there's a moisture problem in a wall they put poly on. The building science guys do exactly that -- they build walls, they test them in a controlled environment, they make hypothesis, and they apply the scientific method to learn more about building science.

THIS

The reason you don’t want to listen to mob opinion about this stuff is because as others have eluded to, vapor barriers/retarders are a location dependent science that has been evolving for decades.

The information in the post above is accurate..


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