Replies

1. 
   Robert Riversong | Dec 08, 2009 07:50pm | #1

   What codes require and what is required for a well-engineered house are sometimes two different things. And some codes make the requirement to use eaves membranes to 24" inside of the outer wall conditional upon need: "Where there is a possibility of ice forming along the eaves causing
   a backup of water..."

   A well-insulated and air-sealed thermal envelope, with a well-ventilated roof will never approach that condition - will never experience ice dams. So it's ironic that one sees fully-membraned roof decks with unvented, spray foam insulated roof assemblies. By eliminating the venting, the chance of ice damming is increased and the necessity for continuous self-adhering membrane is artificially created. Even an R-38 cathedral ceiling with 12" of snow cover on a 21° day will experience melting at the roofing plane and potential ice damming.

   The further irony is that with impermeable layers both below and above the roof sheathing, wetting of the sheathing by minor leaks will have little opportunity to dry and rot is almost assured.

   My answer to your question, "when is a continuous self-adhered membrane required on a roof?", is "Never, if the roof is properly designed". There are many ways to adequately flash valleys, though they always require an unobstructed outflow to shed snow and rain (I've seen chimneys emerging from valleys, and gable dormers so close that coverging valleys prevent snow shedding).

   I've never built an unvented roof and I've never required anything other than drip edge and 15# felt underlayment to maintain weather integrity. In valleys, I use an additional layer of felt and either wide metal flashing for opon valleys or woven or cut shingles for closed valleys.

   I believe that bituthane is one of the most over-used building materials today, and it will come back to haunt us (in many cases, it already has).

2. 
   Doug McEvers | Dec 08, 2009 10:05pm | #2

   I agree with Robert mostly but the code in MN calls for Ice and Water Shield beyond the plate line depending on roof pitch. I do like the idea of running a layer in the valleys as well. Where I agree entirely with Robert is a well insulated and properly vented attic will not ice dam. I have built this way for years and never had any buildup of ice on any roof.

3. GBA Editor
   Martin Holladay | Dec 09, 2009 05:10am | #3

   Stephanie,
   Another point: remember that Ice & Water Shield doesn't stop ice dams — it just reduces ice dam damage. To stop the ice dam from forming in the first place, you need a different strategy. The usual factors leading to ice damming are an imperfect air barrier at the ceiling, insufficient insulation, and/or the wrong insulation (fiberglass batts).

4. 
   Doug McEvers | Dec 09, 2009 10:11am | #4

   In new construction, an energy heel truss is the solution to inadequate insulation at the top plate line of exterior walls. In MN, code requires I think a minimum 7" energy heel, I would say 12" or more is better. This allows for ventilation space above the insulation if required by code.

5. 
   Robert Riversong | Dec 09, 2009 10:42am | #5

   Martin,

   You forgot to include in the "usual factors" for ice dam formation: heating ducts or other mechanicals in the attic, and insufficient or absent attic/roof ventilation.

   While there is wide disagreement today about the need for attic or cathedral ceiling ventilation, it is still advised by many experts for cold and mixed climates, including:

   "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."
   - Don Fugler, research director, Canada Mortgage and Housing Corporation

   "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?"
   - Ned Nisson, author of The Superinsulated Home Book, editor of Energy Design Update

   “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.”
   - Wayne Tobiasson, research engineer U.S. Army Corps of Engineers, Cold Regions Research Center

   "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."
   - Anton TenWolde & William B. Rose, members, ASHRAE

6. GBA Editor
   Martin Holladay | Dec 09, 2009 10:52am | #6

   Robert,
   My omission of ventilation as an important factor was deliberate. I believe -- and William Rose agrees -- that ventilation is much less important in preventing ice dams than usually assumed.

   Most roof assemblies are poorly designed and poorly built. Build the roof properly, with a proper air barrier at the ceiling and adequate amounts of insulation other than fiberglass, and the roof shouldn't be troubled with ice dams.

   Moreover, most roofs today include valleys, dormers, and a variety of intersecting roof planes. I wish they didn't, but they do. I have argued strongly in favor of simple gable roofs without dormers, but alas, few people seem to be taking my advice.

   The typical roof designs that are drawn by today's residential designers are impossible to vent.

7. 
   Robert Riversong | Dec 09, 2009 02:10pm | #7

   Martin,

   I share your frustration over unnecessarily complicated rooflines, but I stand by my insistence on venting cold-climate roofs, and so do most of the experts. Here's another:

   "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."
   - Paul Fisette, director of Building Materials Technology and Management at the University of Massachusetts, Amherst

   And, while it's true that Bill Rose has raised some questions about the ability of roof ventilation to perform all the tasks it's exptected to do, the conclusion of the Tenwolde/Rose definitive article in the ASHRAE Journal on roof venting (which I quoted before) still recommends "venting of attics in cold and mixed climates".

   Your statement "Build the roof properly, with a proper air barrier at the ceiling and adequate amounts of insulation other than fiberglass, and the roof shouldn't be troubled with ice dams" is repeated, but conditionally, by the experts I quoted. Yes, perfectly designed and built roofs might have little probability of ice damming, but few roofs are perfect. And almost all roofs will experience the potential for ice damning under the right environmental conditions.

   Hence ventilation a roof, is "cheap insurance", as Ned Nissan said.

8. 
   stephaniezed | Dec 09, 2009 04:12pm | #8

   Thanks all for the feedback thus far. Are there any believers in the referenced detail, or the similar BSC Aspen Profile of "Designs that Work?" http://www.buildingscience.com/documents/designs-that-work/information-sheet-aspen-profile/?topic=/doctypes/designs-that-work

9. 
   susan clellen | Dec 11, 2009 04:18pm | #9

   Here in Cleveland I have noticed that older (1920's) homes with terra-cotta roof tiles seem to be spared from ice dams. These roofs stay awash with cool air that circulates under the tiles. I have also noticed almost no snow melt off or gutter damage from melting snows ! Gutter companies make a killing replacing gutters in this region.