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Old 05-09-2011, 07:39 AM   #1
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Vapor barrier over garage


Hi all,

We have built a detached garage with a loft over it. The garage and loft will be heated, but the garage part will be a lot colder. I am going to insulate the ceiling of the garage with bats between the floor trusses - they are 16" high open web type. The loft floor is 3/4" OSB.

Can I put 6 mil vapor barrier on top of the OSB, and then install a floating floor? Would it be better to paint the OSB with one of those vapor barrier paints?

I would have liked to use spray foam between the trusses instead, but it is too expensive for me.

Thanks!

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Old 05-09-2011, 03:30 PM   #2
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Vapor barrier over garage


You'd put the varor barrier under the OSB, not on top.
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Old 05-09-2011, 03:55 PM   #3
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Vapor barrier over garage


The OSB has a perm value about the same as asphalt-coated paper-faced insulation and v.b. paint. The floor/ceiling will have drywall to stop air from reaching the insulation. The insulation will not have a gap to the floor. The "biggest loser" in fiberglass insulation.... Use ADA on the ceiling below; http://www.buildingscience.com/docum...wall-approach/

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Old 05-09-2011, 03:59 PM   #4
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Vapor barrier over garage


The OSB is already attached to the joists. I had asked about putting vapor barrier under it, but was told that the OSB should be glued directly to the joists.

I'm not sure how I could put it underneath now.
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Old 05-09-2011, 04:05 PM   #5
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Vapor barrier over garage


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The OSB is already attached to the joists. I had asked about putting vapor barrier under it, but was told that the OSB should be glued directly to the joists.

I'm not sure how I could put it underneath now.
Use insulation in the ceiling below with an integrated vapor barrier.
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Old 05-09-2011, 04:23 PM   #6
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Vapor barrier over garage


Thanks!
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Old 05-10-2011, 01:05 AM   #7
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Vapor barrier over garage


I really don't think you want a true vapor BARRIER in that location. A vapor RETARDER, like Gary mentioned, and an air barrier is very likely the preferred method of sealing things up.
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Old 05-10-2011, 07:23 AM   #8
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Vapor barrier over garage


Thanks!

After reading some of the information about fiberglass, I am wondering if it even makes sense to used batts in this location. I am not sure what other options I would have.
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Old 05-10-2011, 11:10 AM   #9
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Vapor barrier over garage


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Thanks!

After reading some of the information about fiberglass, I am wondering if it even makes sense to used batts in this location. I am not sure what other options I would have.
You have an option of using a closed cell spray foam.
http://www.sprayfoam.com/spps/ahpg.cfm?spgid=6
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Old 05-10-2011, 11:45 AM   #10
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You have an option of using a closed cell spray foam.
http://www.sprayfoam.com/spps/ahpg.cfm?spgid=6
Thanks for the suggestion, but spray foam is too expensive for my application. The best quote I have to spray 4 inches thick is a little over $5000.
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Old 05-10-2011, 11:51 AM   #11
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Vapor barrier over garage


FG is OK, but it is the worst batt insulation on the market. I see no reason why you could not dense pack cellulose through mesh or holes, though that would be pretty thick (my walls will be 18" of dpc, over 30" in the ceiling. "Pretty thick" depends on where you are.) If that does not appeal to you, there are cellulose, cotton, rock wool, and wool batts, too, all of which are better than fg.
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Old 05-10-2011, 11:54 AM   #12
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You could also use an extruded polysterene or XPS and double it up, that would give you an about R20 value.
Gaps would have to be selaed with a can type spray foam.
No need for the Vapor retardant with XPS, deduct that from the total per SF cost.
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Old 05-10-2011, 01:17 PM   #13
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Thanks for all the suggestions. I will look into cellulose and the other alternatives.

If I was to use XPS sheets, can those be attached to the bottom of the joists, as long as the rim board is well sealed and insulated? I would lose 4" height, but the ceiling would still be over 12 feet.

Thanks again!
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Old 05-10-2011, 01:30 PM   #14
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Vapor barrier over garage


I wouldn’t recommend installing XPS exposed to the bottom of the joists that would be a code violation.

XPS foam its flammable and bi product fumes released are toxic, this specific insulation needs to be enclosed with a fire rated covering such is 5/8 drywall.
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Old 05-10-2011, 01:49 PM   #15
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I believe I need to put 5/8" drywall there in any case to meet the code here, no matter what kind of insulation I use.

For the XPS I meant two layers against the bottom of the joists and then the drywall below it - it just seemed a lot easier than cutting all those strips and foaming. There's probably good reasons not to do it.

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