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Old 02-28-2017, 12:22 PM   #1
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Faced batts or unfaced with plastic sheeting


Probably a simple question...maybe a dumb one.

I'm looking to insulate/drywall my garage and from what I'm thinking I have two choices. Wasn't sure if code allows for it though. I can do the standard faced batts and drywall over that, or I could go with unfaced (which is much cheaper), but a huge roll of thick plastic sheeting and put that up then drywall. Any suggestions? If I go the plastic route, is there a specific thickness I need to get?
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Old 02-28-2017, 12:38 PM   #2
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Re: Faced batts or unfaced with plastic sheeting


Do you even even require a vapor barrier in this case for an unconditioned space?

What does the exterior wall consist of?

https://energycode.pnl.gov/EnergyCod...?state=Indiana
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Old 02-28-2017, 12:42 PM   #3
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Re: Faced batts or unfaced with plastic sheeting


In this case, I do as I have a lot of cast iron tools (that are having rust issues right now), and I plan on storing/drying hardwood lumber for woodworking so I need to keep humidity pretty even. May even put a dehumidifier in.

Exterior is wood siding panels similar to this: http://www.homedepot.com/p/Plytanium...3699/100000016
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Old 02-28-2017, 01:46 PM   #4
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Re: Faced batts or unfaced with plastic sheeting


If the drywall is installed properly and air tight (see Gary's ADA link: https://buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach)

Air movement carries the vast majority of moisture removal from the space and if the drywall is tight, you can maintain humitidy in the garage with no issue assuming the doors are tight.

How much are you going to be pumping in there? Depending on what the what the exterior cladding consists of, you may want to consider the plastic.

The reason I ask about the wall construction is if the exterior wall has a class I vapor retarder in its construction, we worry about putting plastic on the interior and creating a vapor trap.
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Old 02-28-2017, 02:02 PM   #5
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Re: Faced batts or unfaced with plastic sheeting


Lot of good information, thanks!

Not sure what you mean by 'pumping' exactly...are you talking about how much moisture will I be generating?

As far as I can tell, the exterior wall is just simple wood construction without any sort of vapor retarder. I've cut a hole in it in the past and the cross section simply looks like plywood.
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Old 02-28-2017, 02:11 PM   #6
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Re: Faced batts or unfaced with plastic sheeting


Whats the cladding...i.e. siding, brick, stucco, etc.?

Pumping was referencing your heating the space and keep in mind, if you store a car in there, they let off quite a bit if they are wet or have snow/ice on them.
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Old 02-28-2017, 02:22 PM   #7
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Re: Faced batts or unfaced with plastic sheeting


Cladding is pretty much just this stuff screwed (nailed?) directly to the studs, nothing on the inside, so I can see the backside of the wood siding from within the garage.

http://redirect.viglink.com/?format=...99%2F100000016

Probably won't do much heating, maybe an electic space heater if I need to get in there during the winter, but it'll be only while I'm working. Cars are only stored there during storms to protect from hail, otherwise they're outside.
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Old 02-28-2017, 03:10 PM   #8
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Re: Faced batts or unfaced with plastic sheeting


An unconditioned garage will be a difficult place to keep metal (high mass) items from rusting. Normal air exchange through a building is much higher than expected and overnight it will cool the contents. Then in the morning the air infiltration switches to warmer air with more moisture and those cool contents start to sweat. Even a concrete floor can become wet when there is no leak in the roof.

If the wood you will be storing starts out dry then it will contribute little. But here in the cold north many people learn the hard way about storing green firewood in their garage. All of that moisture goes right to any cold surface, walls and windows.

As for your vapor barrier question, as Windows said not required, but not bad as long as the outside isn't covered with one already.

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Old 03-01-2017, 07:45 PM   #9
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Re: Faced batts or unfaced with plastic sheeting


Your T1-11 structural siding is rated "Exterior" or Exposure 1" both have multiple layers of waterproof glue and act as a vapor barrier. Do not add plastic (Type 1) vapor barrier to the inside or you will have a double vapor barrier with insulation in the middle. IF ever a moisture/water problem, the wood would mold/rot for you. A paper-faced FG insulation is fine, with perms similar to latex paint. BTW, never use an oil based paint on T1-11, only an acrylic latex primer and finish coat, as per manufacturer.

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Old 03-02-2017, 07:37 AM   #10
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Re: Faced batts or unfaced with plastic sheeting


Good to know, thanks everyone!
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