Continuous Plastic Sheeting under Attic Insulation

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  #1  
Old 10-26-07, 01:02 PM
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Question Continuous Plastic Sheeting under Attic Insulation

I was planning on placing a continuous sheet of 6-mil plastic sheeting on the "floor" of my unfinished attic before installing R30 insulation batts.

My questions/concerns are:

Is having a continuous plastic sheet - which would mean also covering the studs - a good idea?

Or should I cut out 16" wide sheets - enough to cover only the drywall, leaving the studs uncovered?

I would also like to point out that I intend to run R19 unfaced insulation on top of and perpendicular to the R30 batts.

And since there will be gaps between the R30 where the studs are, the R19 will effectively not have a vapor barrier over studs.

May not matter. Don't know.

If you opined to go the 16" wide vs. continuous plastic sheeting route, skip the rest of the questions.

I plan to use kraft-faced R30 batts because they are cheaper than unfaced where I live.

If applying continuous plastic sheeting, do I need to remove the kraft facing of the R30 insulation batts? Or is it okay to keep it on?

Thanks.
 
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Old 10-27-07, 08:42 AM
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You would probably be better off doing the 16" between each stud. If you lay it on top, it will leave a space for the moisture to collect. If your attic is ventilated, you can do what most houses do and not use a vapor barrier, just insulate.
 
  #3  
Old 10-29-07, 05:31 AM
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Also air barrier

Originally Posted by adamplghtg View Post
You would probably be better off doing the 16" between each stud. If you lay it on top, it will leave a space for the moisture to collect. If your attic is ventilated, you can do what most houses do and not use a vapor barrier, just insulate.
Thanks, Adam.

I forgot to mention that the continuous mode of the poly was as an air barrier, which would give me the vapor retarder for "free."

So, the main question is whether it is safe to put kraft-faced insulation on top of the poly.

Thx.
 
  #4  
Old 10-29-07, 07:13 AM
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A Complete Waste of Time

Putting any poly up in your attic now is a complete waste of time at this point.

The purpose of the poly is as a vapor barrier. The poly should have been installed to the underside of the roof trusses/rafters and sealed to the poly used as a vapor barrier to your walls in the living space. You then sheetrock over the poly effectively sandwiching the poly between the sheetrock and the rafters/trusses.

You would never be able to cut the poly to a consistent width now and lay it between the trusses to have it act as an effective vapor barrier, so don't bother with this approach.

Just put the unfaced R-19 perperdicular to the faced R-30 batts that are laying between the trusses.

Make sure you don't butt any insulation up against the roof at the eaves that would block air flow from soffit vents.
 
  #5  
Old 10-29-07, 06:25 PM
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If you don't have any insulation up there(or even if you do) perhaps you should consider doing cellulose insulation instead of fiberglass. It has a higher R-value and will do a much better job with air infiltration that fiberglass. Plus it will fill up all the nooks and crannies. Its also rodent and insect resistant.
 
  #6  
Old 11-02-07, 10:49 AM
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Poly Blocks airflow, too

Thanks everyone for your feedback.

My understanding is that while the kraft facing is a (mostly) adequate vapor retarder, it does not block airflow. Poly does both ... if installed at the "right" time.

Nevertheless, I agree that it is probably not worth the effort; and given my review of the situation last weekend, I'm even less inclined to do any such thing.

Some of the attic already has blown in cellulose, albeit dirty (but dry). There is no vapor barrier under this insulation but the attic is adequately ventillated with gable/soffit vents.

And given how compactly and nicely packed the prior blown-in-cellulose is, I'm not going to disturb it if not necessary.

I might, however, put unfaced fiberglass on top or maybe blow in some more cellulose where it could use some extra R.

TIA.
 
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