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Old 10-15-2012, 05:38 PM   #1
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moisture barrier question


I want to cover a roof and siding. Here are the layers...

1) 3/4" plywood sheathing
2) moisture barrier
3) radiant barrier
4) 3/4" furring strips
5) 24 gauge sheet metal with Energy Star reflective paint

My question is whether or not I can use 6 MIL plastic as a moisture barrier? I have a bunch left over from a job. I have been told by some that I need to use the heat rated underlayment. The stuff is pretty expensive. The ranch is in the high desert. So it can get fairly hot in the summer. The structure does not require a county permit. So an inspector won't see or evaluate it.

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Old 10-15-2012, 06:28 PM   #2
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moisture barrier question


The radiant barrier, if sealed properly, will give you all the bulk moisture prevention that you need.

The roof shouldn't rely on the underlayment to be water tight at the end of the day.

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Old 10-15-2012, 07:07 PM   #3
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moisture barrier question


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Originally Posted by Windows on Wash View Post
The roof shouldn't rely on the underlayment to be water tight at the end of the day.
Good point. So should I omit the moisture barrier layer altogether. Or should I just use 6MIL plastic and not worry about it getting to hot and melting?
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Old 10-15-2012, 07:12 PM   #4
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moisture barrier question


You are in an area that is hot and dry.
Do NOT use a moisture barrier at all, let the components breath.

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Old 10-16-2012, 08:21 AM   #5
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moisture barrier question


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Originally Posted by jackwashere View Post
Good point. So should I omit the moisture barrier layer altogether. Or should I just use 6MIL plastic and not worry about it getting to hot and melting?
Do not put a 6 mil plastic on your roof regardless. Use the radiant barrier as it will serve two purposes.

You roof/attic should still be ventilated though.

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You are in an area that is hot and dry.
Do NOT use a moisture barrier at all, let the components breath.

Andy.
+1

How is the attic ventilation set up?
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Old 10-16-2012, 01:41 PM   #6
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moisture barrier question


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Originally Posted by Windows on Wash View Post
How is the attic ventilation set up?
The roof is already done. A year ago. There is no ventilation or attic. It's a small 10'x12' stand alone room with a very low pitch roof, almost flat. I can build a structure that small without any approvals from county or inspectors. If an engineer designed it no doubt it would have an attic. It's got 2x6 joists 16" oc. I filled them with R19 and R30 insulation. On top of the roof is 3/4" plywood sheathing. Then I put 6MIL plastic on top as a moisture barrier. I figured you have to have something on a roof in lieu of tar paper. Therein lies my problem. I'm wondering (see original question) if the heat will melt the plastic? On top of the plastic I have atticfoil brand radiant barrier. Then 3/4" furring strips that act as a layer of air between the radiant barrier and the metal roof. Then the metal roof with Kynar 500 Energy Star reflective white color paint.

The siding remains to be done. I want to use sheet metal siding on top of radiant barrier and furring strips. That is why I'm asking here about the implications of the 6MIL plastic under radiant barrier. The response that I'm getting is that it is not needed and it may be bad because it prevents the building from breathing.

Last edited by jackwashere; 10-16-2012 at 01:43 PM. Reason: oops
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Old 10-17-2012, 09:15 AM   #7
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moisture barrier question


Post up a picture.

It is always a good idea to allow for some venting so the structure can breath in batt style insulation.

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