XPS and fiberglass and vapor barriers

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  #1  
Old 07-07-14, 08:13 PM
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XPS and fiberglass and vapor barriers

Hi,

I have a question regarding vapor barriers specifically related to kneewalls in my finished attic (1.5 story house). First, some background…The finished attic space runs down the middle of the house (there are no dormers) resulting in three unfinished attic spaces, one spanning the length of the rear of the house and two small, unconnected spaces in the front that are separated by the staircase. Although the house has no soffits, I have installed one gable vent in each of the two small attic spaces, while in the long attic space there is one gable vent and one attic fan on the opposite gable. Also, above the drywall of the sloped ceiling in the finished space, there are baffles leading to the ridge vent beneath which sits batt insulation with the paper side facing the interior. The kneewalls have R13 batt insulation with the paper facing the interior. Lastly, there are two extra intake vents leading into two of the three attic spaces from the side and rear porch overhangs. Basically, I installed intake vents wherever possible.

My question is in regard to XPS. I bought several 1” R5 Pactiv boards (green with no foil) at Lowe’s to install on the back of the kneewalls, behind the batt insulation, in order to increase the R value somewhat. If I loosely space the 24” x 96” the boards (I had to cut them in two to get them into the attic) by placing them a few inches apart at the seams, will I avoid the sorts of problems that can occur if two vapor barriers are installed, the two “barriers” being the paper on the kneewall’s batt insulation and the vapor retardant XPS? If not, any suggestions? Thanks!

Oh, I forgot to mention that my concern is the hot Virginia summers and difficulties keeping the attic cool, not anything winter related. Although I have central air, I rather not continuously cool hot attic air if at all possible.

Mark
 
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  #2  
Old 07-08-14, 04:17 AM
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The key to avoiding moisture issues is air sealing, thus those gaps don't sound good. I'll add a link on vapor diffusion retarders and one on moisture and air sealing.

Be sure you meet all codes with this conversion, especially an emergency egress.

And another link on kneewalls.

Vapor Barriers or Vapor Diffusion Retarders | Department of Energy
Air Leaks Waste Energy and Rot Houses | GreenBuildingAdvisor.com
http://www.finehomebuilding.com/pdf/021230088.pdf

Bud
 
  #3  
Old 07-09-14, 07:38 AM
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ok thanks for your help and the info
 
  #4  
Old 07-12-14, 06:35 PM
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Just wondering...on a kneewall, does the code prohibit one from using a thicker batt size, say 5 1/2" thick, in 2x4 stud bays if the backside of the studs are extended out either by 1) adding 2" of wood to the backside of the studs along their entire length or 2) installing small blocks every foot or so where the metal insulation supports would be. If this practice is acceptable, which approach is typically used? Thanks.
 
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