Replies

1. 
   Riversong | Sep 08, 2010 06:20pm | #1

   Where are you located? What will be the R-value of the cellulose and the air-krete layers? Is the air-krete sprayed on the ceiling and the cellulose on the outside of that layer? What type of roofing, inderlayment and roof sheathing? Will there be soffit and ridge vents or other roof vents?

2. 
   chrisjohnston2112 | Sep 09, 2010 12:13pm | #2

   I am located in southern Illinois - mixed humid climate like Louisville, Kentucky. The attic will have 3.5-inches of air-krete to the bottom of the truss chords - according to the contractor an R value of 13.65, and then will have blown-in cellulose to get to R40. Roof is asphalt shingles on top of OSB sheathing. Two foot soffit vents around entire roof, with ridge venting.

3. GBA Editor
   Martin Holladay | Sep 09, 2010 12:51pm | #3

   Chris,
   You've got a ventilated unconditioned attic with soffit vents. Since it's a hipped roof, I imagine that you don't have much of a ridge, so that might be a problem. With a hipped roof, it's hard to get much ridge venting.

   In any case, attic ventilation is overrated. (If you have an air-sealed ceiling and plenty of insulation, attic ventilation is fairly unimportant.) But as long as you've gone to the trouble of installing soffit vents, you should certainly make sure that there are clear air channels from your soffit to your attic. That usually requires insulation dams above your perimeter top plates, and vent chutes to create an air space below your roof sheathing.

4. 
   Riversong | Sep 09, 2010 07:14pm | #4

   attic ventilation is overrated

   This is one of several issues about which Martin and I are diametrically opposed. And I believe I have most of the building science community on my side of the fence.

   The Experts on Roof Venting

   Bill Rose, ASHRAE, Illinois Building Research Council:
   Airtight ceilings are a more reliable way to ensure a dry attic than venting, but in practice most houses fall into a middle ground where venting balances moisture input.

   Don Fugler, research director, Canada Mortgage and Housing Corporation
   Houses with ceilings tight enough to meet Canada’s strict R-2000 standard “probably could get by without roof venting.” But most Canadian houses, even new ones, don’t have such perfect ceilings.

   Ned Nisson, author, The Superinsulated Home Book, editor, Energy Design Update:
   "I hesitate recommending [unvented roofs] to clients unless I’m absolutely assured of impeccable quality control. In my opinion, roof ventilation is cheap insurance against expensive callback problems. Why gamble?"

   Wayne Tobiasson, research engineer U.S. Army Corps of Engineers, Cold Regions Research Center:
   “Because of the monumental problem of ice damming, there is no question in my mind that the ventilated roof is an order of magnitude better in cold regions.”

   Anton TenWolde & William B. Rose, members, ASHRAE:
   We recommend venting of attics in cold and mixed climates. However, if there are strong reasons why effective attic vents are undesirable, unvented attics can perform well in cold and mixed climates if measures are taken to control indoor humidity, to minimize heat sources in the attic, and to minimize air leakage into the attic from below, or vice versa. The necessity and effectiveness of vents in cathedral ceilings in cold and mixed climates is still a contested issue. Unvented cathedral ceilings can perform satisfactorily in cold and mixed climates if the cavity is properly insulated, measures are taken to control indoor humidity and minimize air leakage into the roof cavity, and a vapor retarder is installed in the ceiling.

   Paul Fisette, director of Building Materials Technology and Management at the University of Massachusetts, Amherst:
   "There are many ways to treat the symptoms [of ice damming], but proper air sealing, insulation, and attic venting are the best ways to eliminate the problem."

   Joe Lstiburek, Building Science Corporation:
   "Vented attic/roof designs have the advantage of a long, proven historical track-record. However, they work best with airtight ceiling/attic interfaces and where ductwork and air handlers are not located within attic spaces. The increase in the use of complex roof shapes and cathedral ceilings has resulted in problems with vented roofs."

   "In extreme snow regions it is necessary to add a vented air space between the roof cladding (shingles) and the rigid insulation to avoid ice damming. The vented air space is needed to flush heat away trapped by the insulating value of relatively thick snow."

   Journal of ASTM International, Volume 6, Issue 4 (April 2009)
   Peter E. Nelson P.E., Senior Principal,Simpson Gumpertz and Heger, Inc., Jason S. Der Ananian P.E., Senior Staff I-Building Technology,Simpson Gumpertz and Heger, Inc.:

   All of the unvented roof assemblies are intolerant of incidental water leakage and the moisture-sensitive layers (i.e., sheathing and gypsum wallboard (for open-cell polyurethane insulation)) exceed the threshold for decay. In hot, humid climates, the most durable roof assemblies are the vented, open-cell polyurethane systems with shorter drying time of the interior gypsum wallboard when compared to the unvented roof assembly; both the sheathing and gypsum wallboard dry out within 2-1/2 months. In an unvented assembly, the drying time for sheathing is similar but the drying time for gypsum wallboard increases to 6.5 months on average. Alternatively, unvented permeable shingled roofs are a viable option in hot, humid climates, although they are slightly less durable. The least tolerant roof assemblies in either climate are the unvented closed-cell polyurethane roof assembly due to trapped moisture and slow drying of the roof sheathing (up to 12 months in Miami, FL and 27 months in Boston, MA).