If your garage is 45 degrees and you have a bonus room heated to 72 degrees above the garage the vapor retarder faces the upstairs room.
Putting a vapor barrior in your garage attic.
Plastic vapor barriers should only be installed in vented attics in climates with more than 8 000 heating degree days.
Holes in the vapor barrier that allow humid air through may allow a lot more water vapor into an assembly than the vapor barrier is stopping.
This acts as a barrier to keep heated moist air from rising up into the attic during cold weather.
Such a location works both as a vapor barrier as well as an air barrier and eliminates the need for an external air barrier not a easy application on a multi storried building as well as any special electrical pans and their sealing.
After the insulation is in place you will want to add a vapor retarder sometimes called a vapor barrier if you need one.
Your first job after the studs are in place is to fill those walls with insulation.
Not every wall does.
Over time this trapped moisture can rot the wood framing in your attic or cause mold and mildew to appear.
The insulation fiberglass or cellulose is then placed on top of the drywall with no vapor barrier above or below.
The best approach for a vented attic in a cold climate is installing a layer of drywall with a good coat of latex paint the paint creates a semi permeable vapor barrier.
The inner surface in colder climates and the outer surface in hot humid climates.
A vapor barrier reduces the movement of water vapor by diffusion.
I install my vapor barrier in the middle of the wall system between the two walls.
You can forego the plastic and use a vapor retarder kraft faced insulation or latex ceiling paint in all other climates except hot humid or hot dry climates.
When a vapor barrier is on the side of a wall where the dry air is i e outside in winter or inside in summer moisture problems can occur.
The classic answer is that the vapor barrier goes on the warm in winter side of the assembly.
If you are not sure if you need insulation in your walls follow along with this flow chart.
Adding a second vapor barrier could cause condensation to become trapped in the insulation between the two vapor barriers.
In hot humid climates attics should not be vented and vapor retarders should not be installed on the interior of assemblies.
First of all a garage is not a signicant source of vapor as it is not occupied that much and you don t cook or bath in in it.
In existing spaces oil based paints or vapor barrier latex paints offer an effective moisture barrier.
Vapor barriers are usually best installed on the side of the wall that experiences the hotter temperature and moister conditions.
