Where to use vapor barrier

[rinker1]

I am currenty insulating our 2 story garage that will be heated downstairs just when I'm working in there and upstairs will be heated probably less than thatI have a guy tellingme not to use vapor barrier in the ceiling of the garage if I use it in the walls, he says moisture has to have a way out. Is this true?

 

[Doug B]

... a guy tellingme not to use vapor barrier in the ceiling of the garage if I use it in the walls, he says moisture has to have a way out. Is this true?

I have heard this before,and I don't understand this reasoning. If moisture needs to get out,then why use any vapor barrier at all? You need a vapor barrier all the way around.In my garage I used unfaced batts in the wall with 4 mil poly , and paper faced batts in the ceiling. Any moisture issues in well insulated space needs to be dealt with by fan or heat exchanger.

 

[brownbagg]

the vapor will hit the barrier because heat rise and then it will sit at the barrier, the dry wall will rot out on the ceiling due to moisture. that why bathrooms always have ceiling problems

 

[Costner]

the vapor will hit the barrier because heat rise and then it will sit at the barrier, the dry wall will rot out on the ceiling due to moisture. that why bathrooms always have ceiling problems

The fact is, assuming you apply a coat of primer and good paint, you are creating a vapor barrier (or at the very least a vapor retarder), so if moisture is a concern with a true VB, it will be an issue regardless.

Bathrooms don't always have ceiling problems, and if you take the time to install a proper exhaust fan you should never have any issues. However the amount of water vapor and steam generated from taking a shower day in and day out (along with a few other family members) is exponentially larger than what would occur in a garage space.

I have heard the argument about not installing a VB on the ceiling before, and to some degree it make sense since the attic is generally vented and therefore any vapor that does get into the attic has an easy path to dry and escape. The walls don't have this benefit because they aren't vented, so the goal should be to prevent water vapor from entering the wall cavity in the first place.

However, if you don't install a VB there is always a chance that water vapor will migrate through the attic insulation until it hits the cold side of the insulation at which point it will condense and stay exactly where it is. With time this condensation will continue to build and it could result in moisture damage to insulation and the ceiling, or worst case it could lead to mold.

Personally I'd use a VB on the ceiling as well. If you can prevent the vapor from getting into the attic you eliminate a potential mold growth area, and if you notice condensation on the ceiling to the point moisture is a concern, you can always install a proper exhaust fan... but honestly in a garage I don't see how you should have that issue as the opening and closing of the overhead doors will allow for routine air exchanges (which in turn will carry any water vapor outside).

 

[JacobZ06]

We are having an RV carport converted to an insulated heated shop. The builder has specified vapor barrier in wall and ceiling, as per the BC building code. Also moisture escapes the attic by way of roof vents. I believe the code here requires roof vents equivalent in area to 1/300 of the roof area.

 

[StingRay]

Use vapor barrier where ever you insulate and on the warm side. If you are using foam the foam is typically it's own barrier. In Florida, Arizona or Nevada you might actually put the vapor barrier to the outside as with A/C the inside is typically cooler than the outside. In most of Northern North America the vapor barrier should be on the inside. If you leave the vapor barrier off of the ceiling then that moisture will condense out in your attic insulation and your attic. Even a small air leak will show quite a lot of frost up in the attic here when it gets 40 below. In the days when vapor barriers were not typically used houses were not well sealed or insulated and air and moisture exited the building over a large surface area. The cold zone was typically the living space side of the wall itself. You'll also notice on older houses with wood siding that the paint is peeling around the kitchens and bathrooms on the outside due to moisture finding it's way out. Vapor barrier on one part and not the other will all funnel it all through the un protected area and cause a big problem. There is alot of debate about the need for vapor barriers and even attic venting. The validity of a debate hinges on what climatic zone you are in. Ohio is probably a place where if you are heating you'd want a vapor barrier. I'd be willing to bet the codes there require it. In a living space with all of it sealed up like a zip lock bag the need for an air to air exchanger comes in. In a garage you typically accomplish this with normal opening of doors. If you don't then you may need an air to air exchanger or a dehumidifier to bring the humidity down.

 

[brownbagg]

The fact is, assuming you apply a coat of primer and good paint, you are creating a vapor barrier (or at the very least a vapor retarder), so if moisture is a concern with a true VB, it will be an issue regardless.

no because the paint on the bottom side so it seals the drywall. with vapor barrier behind the drywall, the moisture will condense on the unprotected side of drywall.

 

[Costner]

no because the paint on the bottom side so it seals the drywall. with vapor barrier behind the drywall, the moisture will condense on the unprotected side of drywall.

Either way you are still on the warm side of the structure, so the vapor should never condense. If vapor can get through the drywall and hit the VB, it can get back out. That is sort of the point.

You just don't want that vapor hitting the insulation, because as soon as it migrates through the insulation and reaches the point (temperature) where it can condense...gravity will keep it there. In the summer this isn't a huge issue because the heat in the attic won't allow it to condense and it would evaporate anything that did, but in the winter it could lead to moisture issues.

Why chance it?

 

[tcianci]

Hey Costner! Maybe you could be the Watervaporworld guy! The argument for not putting the barrier on the ceiling is weak... I'm with you on this one for sure! The purpose of a vapor barrier or vapor retarder is to protect the INSULATION from excessive moisture that could condense and ruin the effectiveness of the insulation. That's why it comes on the insulation...it is there to ensure the proper function of the insulation product. The insulation manufacturers don't put it there because they are worried about moisture in your building or you attic or wall cavities...they know the insulation is dead in the water (literally) without it.
There should be a way to sentence people to a remedial high school level science and physics class. There woud be a lot less confusion about this subject.
Personally I wish I had paid more attention when it came to spelling.

 

[e-tek]

1. VAPOUR BARRIER
2. Exhaust FAN!! It will exhaust heat and mositure from your vehicles and temp changes.Run it constantly or put it on a humidistat.
3. Possibility of Carbon Monoxide in a garage necessitates you SEAL the garage from the house.