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Cement Block Garage, Insulation and Vapor Barrier Question

Cement Block Garage, Insulation and Vapor Barrier Question

Old 09-17-15, 12:30 PM
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Cement Block Garage, Insulation and Vapor Barrier Question

Hello Everyone, and thanks in advance for your time and knowledge.

I have a cement block garage, 42.5 feet x 12.5 feet.

I want to lay a 2x4 wall on the inside, with 2x4's laid flat against the wall. Insulate between the studs with 1.5 inch polyiso and then sheetrock, or maybe barnboard plywood panels.

My question is ... where does the vapor barrier go? I'd use a 6mil or whatever plastic sheet, whatever they have at the store, but does it go against the block, or over the insulation on the inside before the sheet rock?

I will heat the space in winter and cool it in summer.

Again, thanks for your help.

Robert in CT, where it is cold in the winter and hot in the summer.
Old 09-17-15, 12:34 PM
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Vapor barrier, if used, goes to the warm side of the wall. For someone heating more than cooling, that would be the inside of the wall.

If you're going to use foam, I would attach it to the block and then build your stud wall inside that, potentially with batt insulation in the stud bays and then I would not install a vapor barrier.

Hang tight for Bud, he'll be able to tell you whether 1.5" foam is sufficient thickness in your area to avoid condensation.
Old 09-17-15, 04:25 PM
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Thanks for the intro ss.
rd, I would assume putting the 2x4's on the flat is intended to minimize the loss of space, otherwise, as ss suggested there are other ways to frame the wall and insulate.

As for insulation value, 1.5" at r-7 per inch (or whatever the package says) falls short of the typical r-20 required. But that depends upon the purpose for this space and local code officials could advise.

As for condensation issues and a vapor barrier, yes on the inside with lots of attention to air sealing top, bottom, and all penetrations. Air leaks can allow moisture to reach cold surfaces and that is when you get condensation.

Old 09-17-15, 04:46 PM
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The only way I would do it is 2" of foam on the walls, Build the 2 X 4 walls with pressure treated bottom plates, wire then insulate with R-13, 6 mil. plastic then the sheet rock.
You'll end up with a moldy mess doing it the way you suggest.
Old 09-17-15, 06:46 PM
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To save space, minimize issues related to air leakage paths and condensation, you could spray the walls with 2" of closed cell to get roughly R-14 and apply an intumescent paint to the surface to provide fire protection to the foam.

I would not mount wood studs to the wall and then install a non permeable foam between them. Moisture drive from the exterior will certainly decay the studs eventually.

Moisture related issues are relevant to what you are doing inside the structure to generate and eliminate high levels of relative humidity.
Old 09-17-15, 08:00 PM
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rd, I assume this is 95% above grade, exception is where those blocks rest on their footing. If so, I would anticipate minimal moisture issues. However, others are correct in that all wood in contact with concrete will need to be pressure treated with all fasteners rated for that wood, and that can be a pain. Ever priced stainless steel drywall screws?

How finished do you want the inside to be, painted drywall, or rather rough is good enough?


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