# Plastic Vapor Barriers



## Nealtw (Jun 22, 2017)

Depending on where you are to whether you should have. 
It is just an air barrier. It stops warm moist air from the house from getting thru to the cold surface of the outside wall where that water will condense and cause trouble like mold.


----------



## APA (Jul 13, 2018)

Nealtw said:


> Depending on where you are to whether you should have.
> It is just an air barrier. It stops warm moist air from the house from getting thru to the cold surface of the outside wall where that water will condense and cause trouble like mold.


I know, I just have been reading about the changes of thinking in construction. The next house I build is going to be a log cabin. Tired of all the details of air sealing.


----------



## APA (Jul 13, 2018)

APA said:


> I know, I just have been reading about the changes of thinking in construction. The next house I build is going to be a log cabin. Tired of all the details of air sealing.


I live right on the border between Zone 4 and 5. I can have arctic winters some years and mild winters others.


----------



## Nealtw (Jun 22, 2017)

APA said:


> I live right on the border between Zone 4 and 5. I can have arctic winters some years and mild winters others.


If you read all that is wrong with VP, you will find they are talking about systems of the problem and blaming the VP when it was a poor installation of the VP or the insulation.


----------



## XSleeper (Sep 23, 2007)

Purely opinion here, based on 30 yrs experience in the trade. We are in zone 5 and I have found lots of mold behind poly during remodels. I'm sure some of it may relate to air infiltration but in general I think vapor barriers in climates like ours are a bad idea. Vapor retarders, yes. Vapor barriers, no. Except for the ground, everything needs to be able to dry to one side or the other. It can be 100F here in summer and -25F in winter. Summer dewpoints can be in the low 80's. Interior vapor barriers will be problematic in the summer under those conditions. And if they aren't air sealed, they are a problem in the winter too.

IMO air sealing and R-value are FAR more important than vapor. It's the air exchange that causes most moisture problems.


----------



## Nealtw (Jun 22, 2017)

XSleeper said:


> Purely opinion here, based on 30 yrs experience in the trade. We are in zone 5 and I have found lots of mold behind poly during remodels. I'm sure some of it may relate to air infiltration but in general I think vapor barriers in climates like ours are a bad idea. Vapor retarders, yes. Vapor barriers, no. Except for the ground, everything needs to be able to dry to one side or the other. It can be 100F here in summer and -25F in winter. Summer dewpoints can be in the low 80's. Interior vapor barriers will be problematic in the summer under those conditions. And if they aren't air sealed, they are a problem in the winter too.
> 
> IMO air sealing and R-value are FAR more important than vapor. It's the air exchange that causes most moisture problems.


In that experience, did you find a problem when electrical boxes are wrapped and sealed. VB is overlapped and taped? 
There is no one out here in a house built in the last 25 years that has to climb in the attic and seal the ceiling. And that is because it was done before the drywall went up.


----------



## user_12345a (Nov 23, 2014)

Vapour barriers are just fine provided there's drying potential to the cold side and they're properly installed. They can be problematic when the exterior has something that prevents drying like insul-brick.

They used to use tar paper on the exterior which doesn't do that well. Today's tyvec/house wrap allows the wall assembly to dry.

You can try to stop air movement with drywall, trim, etc but it will never be as tight.

It's essential to stop warm air from moving through the wall assembly when batts are used and there's no thermal break.

If there is a good thermal break which keeps the exterior surface warm enough, different story.


----------



## XSleeper (Sep 23, 2007)

No, a lot of the problems that I'm thinking of had nothing to do with outlets, and were more to do with either the vapor barrier not being sealed to the bottom plate... the bottom plate leaking exterior air under the bottom plate, or the bottom plate shrinking and not contacting the sheathing fully. Or the interior vapor barrier was so cold from air conditioning that the warm exterior air was condensing on the exterior side of the vapor barrier, causing mold in the wet insulation and framing.


----------



## user_12345a (Nov 23, 2014)

indoor a/c setpoint should never be below the outdoor dewpoint or BAD things happen especially when there's a vapor barrier on the interior side!

People set them low partially due to being oversized. 68-72 feels bloody freezing if the humidity is properly controlled.


----------



## Nealtw (Jun 22, 2017)

XSleeper said:


> No, a lot of the problems that I'm thinking of had nothing to do with outlets, and were more to do with either the vapor barrier not being sealed to the bottom plate... the bottom plate leaking exterior air under the bottom plate, or the bottom plate shrinking and not contacting the sheathing fully. Or the interior vapor barrier was so cold from air conditioning that the warm exterior air was condensing on the exterior side of the vapor barrier, causing mold in the wet insulation and framing.


You guys don't get to play with the black goop down there. 
How To Install Vapor Barrier - YouTube


----------



## Nealtw (Jun 22, 2017)

Here you can see what happens when inside air gets passed the insulation on the right.
Other spots had poor fitting insulation but did not have an air leak and the insulation stays clean.
We can see the the steel post to the left of the window was exposed to the outside air and it is nothing like the one on the right.


----------



## SPS-1 (Oct 21, 2008)

If you are on the border between zones 4 and 5, I would suggest doing as code/recommended in zone 4. Winters are getting warmer, and that cabin is going to be there for maybe a hundred years.

But in a cold climate, the simple physics is that when warm, moist interior air hits a cold surface, it WILL condense to liquid water. In my home, I would not want warm moist interior air to have the ability to contact the cold outside sheathing.

Up here, putting the vapor barrier on the outside seems to becoming more popular. In this system, they put closed cell foam panels (which are vapor impermiable) on the outside of the house. This has the advantage that it minimizes the thermal bridging through the studs. But then, the exterior insulation has to be thick enough that the interior surface of the foam board will not get cold enough to have condensation when warm, moist interior air contacts it. In my new place, I have (from outside to inside) 1" polyiso boards on the exterior of the studs, (no sheathing), then 2" polyurethane closed cell sprayfoam, then 3.5" of fiberglass batts. 

And of course never have two vapor barriers. Walls (and ceilings) need to have the ability to dry either to the inside or outside.


----------



## AllanJ (Nov 24, 2007)

The problem with those poly' plastic sheet vapor barriers put up on the insides of the studs just before putting up the drywall is that the stud bays did not get hermetically sealed from the inside space. So over the course of the winter moisture got inside and condensed on the sheathing, as described previously. Depending on the frequency and length of time polar vortexes hit the region, the layer of ice could persist for some time possibly weeks, before evaporating or subliming to the outside world.


----------



## user_12345a (Nov 23, 2014)

With a proper exterior barrier - tyvec, moisture from outside shouldn't be getting in.


----------



## Nealtw (Jun 22, 2017)

any moisture in cold air would have little reason to condense inside a cold wall. 
Any moisture condensing in the wall in the winter is coming from the inside. Problem with air barrier.
If a wall is sealed on both sides, why does it have to breath or dry to one side? 
It is often argued that drywall is good enough to seal the wall. 
An attached garage wall is drywalled on both sides one warm side one cold side and sealed both side. That creates what problem 
The garage ceiling below a bedroom is insulated and drywalled there by sealed both sides, one side warm one side cold.
What problems are caused by that.


----------



## SPS-1 (Oct 21, 2008)

user_12345a said:


> With a proper exterior barrier - tyvec, moisture from outside shouldn't be getting in.


No.

Cold outside air contains little moisture. The moisture is in the warm conditioned air.

Also, Tyvek is not a vapor barrier. It specifically is designed to be vapor permeable. However, it does keep liquid water from entering.


----------



## maxw72 (Jun 8, 2020)

Nealtw said:


> Here you can see what happens when inside air gets passed the insulation on the right.
> Other spots had poor fitting insulation but did not have an air leak and the insulation stays clean.
> We can see the the steel post to the left of the window was exposed to the outside air and it is nothing like the one on the right.
> View attachment 634488


Noob question here Neal, but why is the insulation black? Is that mold?


----------



## Nealtw (Jun 22, 2017)

maxw72 said:


> Noob question here Neal, but why is the insulation black? Is that mold?


Yes, warm wet air from the house gets to the outside wall and the cold side of the insulation. That air also has dirt in it so mold will grow on that dirty insulation and the outside sheeting.


----------



## user_12345a (Nov 23, 2014)

SPS-1 said:


> No.
> 
> Cold outside air contains little moisture. The moisture is in the warm conditioned air.
> 
> Also, Tyvek is not a vapor barrier. It specifically is designed to be vapor permeable. However, it does keep liquid water from entering.


I never suggested tyvek is a vapor barrier!

What it is: An air barrier and a water barrier for any rain/snow that penetrates the siding or brick - *it's a drainage plain*.You can get water from outside.

When the interior vapor barrier is done correctly, there should not be significant moisture/condensation in the wall from inside. There shouldn't be ice forming. The small amount that gets in needs to get out - hence needing drying potential.


----------

