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Should I use plastic (mil 4 or 6) between drywall and ceiling joist as a vapor barrier for unconditioned attic in Saint Louis, MO?

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I want to use blown cellulose or fiberglass and needed to know if I need to lay a sheet of plastic inbetween drywall and celing joist.  I thought about using kraft faced batts since it has its own vapor barrier on it but thought that the blown in stuff was more efficient in covering all of the cracks and openings.  I did a little research and found info about heating degree days and if you are over 8000 then you should put plastic in but those are in type of numbers are in mid to north of Wisconsin.  I live in Saint Louis which is around 5500 hdd's.  Should I just put the drywall up and just blow over it?
asked in Attic Area by heatwave77 (120 points)
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1 Answer

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Hello heatwave77,

St. Louis, MO is located in climate zone #4. According to IRC (International Residential Code) vapor retarders are not required or may be prohibited in some areas of the warm climate zones 1, 2, 3, and 4 (prohibited or not may depend on requirements in your particular jurisdiction) and NAIMA (North American Insulation Manufacturers Association) which recommends Class II or III vapor retarder materials in those climate zones.

Class II vapor retarders are low permeability type; more than 0.1 perms and less or equal to 1.0 perms.

  • 0.002 inch Polyethylene Sheet - 0.16 perms
  • Vapor Retarder Latex Paint, 0.0031 inch thick - 0.45 perms
  • Insulation Facing, Foil Kraft Laminate - 0.5 perms
  • 1/4 inch Plywood (douglas fir, exterior glue) - 0.7 perms
  • Insulation Facing, Kraft – 1.0 perms

Class III vapor retarders are medium permeability type; more than 1.0 perms and less or equal to 10 perms.

  • Typical Latex Paint — 0.002 inch thickness - 5.5 to 8.6 perms
  • 4.4 lb/100ft2 Asphalt Saturated Sheathing Paper - 3.3 perms
  • 1/4 inch Plywood (douglas fir, interior glue) - 1.9 perms

According to the above recommendation neither of the two Polyethylene Sheets that you’re asking about belongs to those categories. They are both Class I which are very low permeability vapor retarders.

You should probably call your local building department and ask if they require any vapor retarders to be installed under the attics floor insulation. You may already have one in place if your interior ceiling drywall surface has been covered with latex paint.
Let me know if you have any other questions, just click “comment” below.

answered by darekrudy (21,730 points)