Brad,

You built as 85% of all others have so while you have constructed a wall assemby that once wetted will have trouble drying out you're in good company.  Robert's advice to check the attic ceiling for air leaks and seal them if they exist is good.  You should also make sure that your ventilation system (assuming you installed one) is operating appropriately.  An automated mechanical ventilation system should be able to keep your winter RH around 30 to 35%.  By ensuring that you maintain the lower RH you will also reduce wetting potential in your walls by limiting the amount of water vapor available.

Kind regards,
Jeff

Vermont ENERGY STAR Homes
  A service of Efficiency Vermont & Vermont Gas Systems

LEED for Homes
  A U.S. Green Building Council program

800-893-1997
802-767-3861 fax
  ----- Original Message ----- 
  From: Brad Denny 
  To: [log in to unmask] 
  Sent: Monday, December 15, 2008 4:19 PM
  Subject: Re: [VGBNTALK] damp cellulose


  Jeff,
      Thanks for this quick response.  I built a new house four years ago with a continuous PVB on the main living floor (walls, not ceiling), R-19 fibreglass insulation in the walls and R-50 in the attic, 1/2" plywood sheathing, housewrap and 1" shiplapped pine siding finished with a solid, latex stain.  I could have done a better job sealing outlets, but the house is pretty tight and there is no sign of a moisture problem affecting either the stain or the interior paint.  Should I be worried?
  Brad 
    ----- Original Message ----- 
    From: Jeff Gephart 
    To: [log in to unmask] 
    Sent: Monday, December 15, 2008 3:08 PM
    Subject: Re: damp cellulose


    Brad,
    It is essentially gluing the drywall to the building's exterior frame and sealing all penetrations.  At corners we only attach to one side of the frame so framing lumber shrinkage doesn't crack the drywall in the corners.  As a backer for the non-mechanically fastened side of the corner drywall clips are often used (Simpson Strong-Tie # DS or USP Lumber Connectors # DC1).  If you review the two pdf's at the links below written by Joe Lstiburek you will get a pretty good idea about how the airtight drywall approach works.

    Air Barriers - www.buildingscience.com/documents/reports/rr-0403-air-barriers/?topic=/doctypes/researchreport
    Drywall, Wood & Truss Uplift - www.buildingscience.com/documents/reports/rr-0107-drywall-wood-and-truss-uplift/

    Kind regards,
    Jeff Gephart

    Vermont ENERGY STAR Homes
      A service of Efficiency Vermont & Vermont Gas Systems

    LEED for Homes
      A U.S. Green Building Council program

    800-893-1997
    802-767-3861 fax
      ----- Original Message ----- 
      From: Brad Denny 
      To: [log in to unmask] 
      Sent: Monday, December 15, 2008 2:36 PM
      Subject: Re: [VGBNTALK] damp cellulose


      I am late tuning in to this interesting discussion.  How is "air-tight drywall" different from conventional drywall?
      Brad Denny
        ----- Original Message ----- 
        From: Robert Riversong 
        To: [log in to unmask] 
        Sent: Friday, December 12, 2008 11:04 AM
        Subject: Re: damp cellulose


              I believe that a plastic vapor barrier is an invitation to problems in almost any thermal envelope. It was promulgated by code when most houses were insulated with fiberglass and there was little attention to (even recognition of) the need for air-tightness.

              In addition to preventing a thermal assembly from breathing (diffusion drying to the inside), it also prevents a wonderfully hygroscopic material like cellulose from performing as a moisture buffer to stabilize indoor RH, and likely creates a static charge which draws negative ions out of the living environment.

              Now that it's been proven that, with reasonable indoor RH levels, diffusion contributes as little as 1% of the total moisture load in a thermal envelope during the heating season, and that stopping air movement is the key to preventing indoor-generated moisture problems in the structure, the air-tight drywall approach solves all the problems without creating more.

              Unfortunately, particularly for the sustainable building community, sprayed foam is being touted as a solution to moisture problems, when it (much like plastic VBs) creates its own set of negative consequences, since closed-cell foam has no moisture storage (buffering) ablity and open-cell foam can trap moisture and cause wood rot and mold. This, of course, in addition to the non-renewable resource depletion, embodied energy and carbon contribution issues.

              I have calculated that, compared to a 2000 SF reasonably well-sealed code-standard fiberglass-insulated house (0.5 ACH), a urethane sprayed house (with insulated gable walls and roof assembly rather than ceiling), even accounting for an increase in air-tightness (min. 0.35 ACH), will have an additional embodied energy cost that would require 23 years of operation to pay back.

              The same fiberglass-insulated house with 1" of exterior XPS and 0.35 ACH would have an additional embodied energy payback of less than 1 year, in large part because it results in much greater energy savings than the typical sprayed urethane, mostly by eliminating thermal bridging.

              Open-cell sprayed Icynene has almost the same embodied energy liability as the fiberglass, but it would result in a less energy-efficient house.

              A similar house with 2x8 framing 24 oc (instead of 2x6 16 oc for the others), which uses no more total wood, and made very tight with the air-tight-drywall system (0.25 ACH, which is adequate in a non-toxic breathable house), would have 41% of the insulation embodied energy of the fiberglass (less than 5% of the urethane) and use 39% less heating energy. 

              --- On Fri, 12/12/08, Tim Yandow <[log in to unmask]> wrote:

I have found that using a vapor barrier with wet spray is
an invitation to disaster though. The walls need to breathe.
Tim Yandow