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Old 07-05-2011, 11:45 AM   #1
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Barrier on Unfaced Insulation


I'm currently finishing off a 200 sf room on the second floor of a home with a gable roof. It is leaving me with a small 5 foot (flat) ceiling, 6 foot sloped walls, and 4 foot knee walls. I'm using faced r38 above the ceiling, and I installed R30 in the slopped walls, which I furred out to accommodate 9.5 inches. My question: I purchased R30 for the sloped walls, but couldn't find with kraft facing. I am not inclined to use poly. Can I just staple sheets of kraft paper horizontally across the front of the rafters? Any thoughts welcome. Thanks.

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Richmond, Virginia

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Old 07-05-2011, 03:27 PM   #2
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Barrier on Unfaced Insulation


The kraft facer on insulation isn't just butcher paper glued fast to fiberglass. It has a spray applied asphalt coating that is designed to certain vapor transmission limits (permeability).
Pure kraft paper would have practically no characterisitics of a vapor retarder, but there are facer sheets available that are. Make sure that what you use is designed and rated for the purpose.

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Old 07-05-2011, 03:57 PM   #3
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Barrier on Unfaced Insulation


Do you know the name of any of these products or where I might find them. If not, what other options do I have. I'm leery of using poly and trapping moisture. Can you recommend any other options for an interior vapor barrier?

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Old 07-06-2011, 12:10 AM   #4
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Barrier on Unfaced Insulation


Find your Zone below the map; http://publicecodes.citation.com/ico...001_par001.htm

Check the requirements for insulation for your Zone; http://publicecodes.citation.com/ico..._11_sec002.htm

In your Zone- 4A no vapor retarder is required: http://publicecodes.citation.com/ico...001_par003.htm

You could use a vapor barrier paint for a Class 3: http://publicecodes.citation.com/ico...001_par005.htm
But I wouldn’t because you have an asphalt roof (vapor barrier). Unless you kept the 1” air space clearance with the baffles when using fiberglass batts (Required) otherwise you will have fast snow melt there. I would install housewrap (Tyvek) on the attic side of the knee wall, insulation in plastic bags under the knee wall in cavities and h.w. up to and covering the rake ceiling insulation, taped to the baffles. Of course, always check with your local AHJ.


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Old 07-06-2011, 10:58 AM   #5
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Barrier on Unfaced Insulation


Thank you very much for your reply.

I have installed rafter vents from the soffit to the ridge vent.

Under the knee walls I was planning on adding a batt and blocking with styrofoam and sealing the perimeter with spray foam. Is this acceptable?

When you say install Tyvek on the Attic side, Do you mean fasten the house wrap to the backside of the knee wall framing with the logo facing outward. (as would be applied to the exterior sheathing). Is this designed to limit airflow into the interior? Hold insulation in place?

Lastly, you talk about house wrap covering the rake ceiling from the interior. Are you suggesting starting at the top of knee wall (where I have paper faced installation) and running a section of tyvek up to the ceiling, where I am using r38 faced batts. In other words, just installing a section of house wrap in between the two sections of faced insulation. Would this again be installed with the logo face up...this time towards the interior?

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Old 07-09-2011, 07:06 PM   #6
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Barrier on Unfaced Insulation


"Under the knee walls I was planning on adding a batt and blocking with styrofoam and sealing the perimeter with spray foam. Is this acceptable?" --- so long as the attics are connected by air channels, you need to cover the foamboard to meet minimum fire-code, 4.2.1.2: http://building.dow.com/ee/pdf/ESR-2142.pdf

"When you say install Tyvek on the Attic side, Do you mean fasten the house wrap to the backside of the knee wall framing with the logo facing outward. (as would be applied to the exterior sheathing)."----- Yes.

"Is this designed to limit airflow into the interior?" ----- Exactly, keep the moving attic air out of the fiberglass: The "biggest loser" in fiberglass insulation....

"Hold insulation in place?" --- would also help, though I would use foamboard with a covering as stated earlier. I would air-seal the drywall on the room side: http://www.buildingscience.com/docum...wall-approach/

"In other words, just installing a section of house wrap in between the two sections of faced insulation. Would this again be installed with the logo face up...this time towards the interior?' ----- yes, cover any f.g. batt edges from air. Not on the rake ceiling, only on the side walls. http://www.buildingscience.com/docum...archterm=insul

http://oikos.com/esb/51/sideattics.html

http://docs.google.com/viewer?a=v&q=...7ghnfjDsIDd8AA

http://www.homeenergy.org/archive/he...95/951111.html

Gary

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