Replies

1. 
   Riversong | Dec 24, 2009 03:17pm | #1

   First, you have to locate yourself - what climate zone.

   Second, are you saying that you're framing cellulose-filled walls on the interior surface of the SIPS or are insulating interior partitions?

   Third, cellulose manufacturers strongly discourage the use of poly vapor barriers and some won't honor their warrantee if it's used.

   Fourth, vapor barriers are not needed except, perhaps, in extremely cold climates. An air barrier is required, and SIPS (if properly sealed at joints) make an excellent air barrier. Taped drywall is also an excellent air barrier, as long as all penetrations are sealed.

2. GBA Editor
   Martin Holladay | Dec 24, 2009 03:34pm | #2

   Bill,
   Failures with SIP walls and roofs — most notably in Juneau, Alaska — were traced to air barrier problems, not vapor diffusion problems. If SIP seams are poorly sealed, moisture-laden air can escape through the seams, leading to condensation on the exterior asphalt felt and eventually to the rot of the exterior OSB.

   You should be very diligent when you seal your SIP seams against air leakage. In damp climates like Juneau, redundant sealing methods are recommended (for example, spray foam in the seams plus interior tape covering the seams).

   As Robert noted, interior poly is not required, as long as air barrier details are impeccable.

3. 
   CMKAVALA | Dec 25, 2009 07:18am | #3

   According to SIPA:
   Air barriers or vapor barriers are not required in SIP buildings because properly sealed SIPs create a code compliant air barrier with a permeability rating of less than 1.0 perm. In addition, the foam core of SIPs is solid and continuous throughout the wall, eliminating the convection and condensation issues that can occur in cavity walls

   If you use steel SIPs they already have an integral vapor barrier with the skin

4. 
   Riversong | Dec 25, 2009 02:19pm | #4

   CMKAVALA, be wary of using trade associations as definitive sources of reliable information.

   The statement: "In addition, the foam core of SIPs is solid and continuous throughout the wall" is, of course, not true. The foam cores are discontinuous at every panel edge unless panel joints are perfectly sealed during installation. Additionally, there are various spline systems used to connect panels, some of them creating thermal bridges through the foam (as well as thermal bridges at inset framing around openings).

5. GBA Editor
   Martin Holladay | Dec 26, 2009 06:02am | #5

   C.M. Kavala,
   Regardless of anything SIPA may have written, an air barrier is certainly necessary in a SIP house, as it is in any house. As I pointed out earlier, air barrier defects were responsible for massive SIP home failures in Juneau, Alaska.

   In case there is any misunderstanding: an air barrier is not a single product like Tyvek. An air barrier is a collection of materials that form a separation between outdoor air and indoor air. In a SIP house, the following materials are probably part of the air barrier: the concrete foundation, the sill seal, the rim-joist area, the gaskets under the wall panels, the gaskets and foam used between the panels, the SIPs themselves, the details where walls meet roofs, the windows, the doors, and the details around the perimeters of windows and doors.

   Defects in the air barrier matter! Don't think you can build a SIP house without defining your air barrier and building it carefully.