Converting garage to shop - vapor barrier on walls

[Rob Music]

I am in the process of converting a detached unheated garage to a workshop. I plan on adding heat later. I have a question about finishing the walls. It has cinder block walls and the first two to three feet are below grade. The interior side of the walls have been painted with a 'masonry' paint. I plan on using 2x4s laid flat against the block, pink 3/4" rigid foam insulation, and covered by 1/2" chip board. I have never noticed water or moister coming through the walls, though I am testing with plastic taped to the walls at various points.

My question is: where should the vapor barrier be? Should the barrier be between the block and the 2x4s or on the interior side of the 2x4s? Or both? Oh, I live in northern Kentucky, so it gets hot in the summer and can get cold in the winter.

Thanks in advance…Rob

[Mitchell Andrus]

Vapor barriers protect the insulation from the indoor humidity that migrates to a colder place where condensation turns it into water. So... the vapor barrier goes to the inside side of the insulation.

In the case where there's no shower, cooking, etc., and the garage will get plenty of air change, the barrier will serve to keep the wind from whistling through the walls' cracks, gaps, etc. preserving your heated air - to a point.
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[Rob Fisher]

Vapor barriers protect the insulation from the indoor humidity that migrates to a colder place where condensation turns it into water. So... the vapor barrier goes to the inside side of the insulation.

In the case where there's no shower, cooking, etc., and the garage will get plenty of air change, the barrier will serve to keep the wind from whistling through the walls' cracks, gaps, etc. preserving your heated air - to a point.
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Typically the vapor barrier goes on the warm side of the wall. In NJ this would probably be the inside. This can get a bit hairy when you heat and cool a significant portion of the year. Which side is the warm side? In the OP's case, in KY, typically the outside side of the wall would be considered the warm side. This could vary if you are in an area that has unusually colder microclimate, higher elevation for instance.

The OP did say that the wall is currently CMU, with masonry paint on the inside. How is the exterior finished? The existing masonry paint is probably acting as a vapor barrier and no othe vapor barrier should be used as water vapor could get trapped between the two and create problems later.

Also in theory vapor barrier and air barriers are two different things, in reality they are often the same physical thing but they act differently. Air barriers stop air movement and usually vapor. Vapor barriers only stop vapor not air.

Rob

[Rob Music]

Thanks for the replys.

More specific information:
- The exterior is painted stucko
- I am NOT planning on cooling the garage in the summer.
- I live in the Ohio River valley - not a cooler microclimate

More questions:
- Will the rigid foam insulation act as a vapor barrier?
- If I put a vapor barrier (plastic) on the interior side of the studs, will I have a problem with the studs being in direct contact with the painted block?

Thanks for the help!!!

[phil harold]

if you tape your seams with tyvek tape and caulk around the foam you will have a vapor barrier

But I would tweak you construction methods...

1. Glue foam to wall
2. Use 1" foam or thicker or 1" foilfaced foam
3. screw 2x4 on the flat with tapcons thru the foam into the block wall or frame the wall normal and insulate of your choice

By insulating the block wall with the foam first you will get a better r value and stop condesation in the wall cavity