Replies

1. 
   Piffin | Mar 26, 2006 06:28am | #1

   try to find a Corbond installler. closed cellpoly is also a moisture barrier.

   I believe, but might be wrong, that the icy is only rated about r 3.5 so that would only provide about R-20
   Corbond has twice that value

    

    

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2. 
   Mongo | Mar 26, 2006 07:27pm | #2

   Trying to do a thick "all-at-once" application can result in high temps and eventual shrinkage off the framing members. With the shrinkage, there goes your tightness and air barrier.

   Try spraying several thinner appications next time.

3. 
   Dinosaur | Mar 26, 2006 10:54pm | #3

   In this climate, we need to pay close attention to roof insulation details because the roof snowpack can and often does exceed 4 to 5 feet. As a result of what I see doing re-mods and repairs, I am not convinced a non-vented roof-deck is a viable way to keep the roof cold enough to prevent ice-damming and/or condensation problems.

   But cathedral ceilings are a PITA to vent, too true. What I'd suggest is a combination of techniques to get you where you want to be.

   1. Use the icynene spray foam on the underside of the primary roof deck, as originally planned, and do as suggested in laying it on in several thin coats instead of all at once.
   2. Add exterior rigid foam insulation to the roof deck. Something like Atlas Roofing's polyiso panels, which is available in 3" thickness and gives a better R-value and is more 'green' than XPS panels.
   3. Build a crib on the roof deck to serve the dual function of holding the polyiso panels in place and furnishing a nailing surface for strapping (for metal roofing), skip-sheathing (for shakes), or a secondary plywood roof deck (for asphalt shingles). The top edges of the 2x crib must stand proud of the surface of the polyiso panels by ½-¾".
   4. Design the rakes and fascias to allow fully vented soffits giving air-flow to the gap above the polyiso panels, thus keeping the underside of the 'finish' roof cold everywhere. See the drawing for a detail of one way to do this.

   View Image

   Also note that any roof penetrations, especially plumbing vents, are problem areas which always leak some heat through to the roof deck. Pay particular attention to insulating these areas. One idea which works well, if it is consistent with the architectural style of the house, is to build a louvered cupola over the vents so that no snow lies on the roof surrounding those penetrations.

    

   Dinosaur

   How now, Mighty Sauron, that thou art not brought
   low by this? For thine evil pales before that which
   foolish men call Justice....
   

   

1. 
   andy_engel | Mar 28, 2006 09:23pm | #5

   Dino, with 4 to 5 ft. of snow on the roof, how does a ridge vent do anything?

   Another point that many forget is that not all ice dams relate to heat leaking from the house. Ever noticed a rock formation with ice hanging from it? Sometimes the configuration (valleys and shed roofs up against a reflective wall) and orientation (often east or west, which get short daily warming cycles followed by long cooling cycles) combine to cause melt and ice damming that has nothing whatever to do with insulation or venting.Andy

   "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

   "Get off your dead #### and on your dying feet." Mom

1. 
   Dinosaur | Mar 29, 2006 05:08am | #6

   Dino, with 4 to 5 ft. of snow on the roof, how does a ridge vent do anything?

   Andy, I never use ridge vents, for precisely that reason. They are useless up here. I laugh when I see people installing them. All that happens is that you wind up with an ice-lined tunnel melted into the snow over the vent by whatever warm air leaks out of the building through the ridge vent. If you're real lucky, you might get a couple of cf/h out the gable ends....

   What I was suggesting to the OP was essentially the same thing Snowmon did: build a vented cold roof on top. The drawing I posted was a rake-edge detail, showing how to vent between the two roof decks along the rake. I do that by using separate, cascaded rake boards on each roof deck, and a custom 2"-wide vented soffit hidden between them. The same must be done for the fascias. This usually provides sufficient ventilation to keep that upper roof deck cold, but you can put in wind-powered turbines ('Charley Nobles') or a simple 4" 'exhaust' pipe vent near the ridge as well if necessary.

   Another point that many forget is that not all ice dams relate to heat leaking from the house.

   And that's an excellent point a lot of people forget. Sun-caused ice dams can be a real pain unless you build the roof system so as to be able to ignore them (ie: using Blueskin or some other membrane).Dinosaur

   How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....

1. 
   andy_engel | Mar 29, 2006 05:24am | #7

   Indeed my friend, you are smarter than the wood.

   Now, I'm on my third glass of a most excellent Zinfandel (Rosenblum 2004 Appellations Series, San Francisco Bay, perhaps the best I've ever had), and you need to know that the preceeding was a compliment.Andy

   "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

   "Get off your dead #### and on your dying feet." Mom

1. 
   Dinosaur | Mar 30, 2006 01:24am | #8

   Me? 'Smarter than the wood'...?

   I like to think of the relationship I have with wood as a partnership, not a competition.

   Now, assuming you finished that bottle of Zinfandel without interruption, don't forget to ventilate your own 'roof' as needed....Dinosaur

   How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....

2. 
   andy_engel | Mar 30, 2006 04:46am | #9

   You mean long enough for the building inspector's tail lights to go 'round the corner?

   BTW, that was an extraordinary wine. Fruity, bright, like a mouthful of cherries. Yum.Andy

   "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

   "Get off your dead #### and on your dying feet." Mom

4. 
   Snowmon | Mar 28, 2006 08:52pm | #4

   "He also tried "icynene" on it's own and had trouble with ice dams, so obviously the 5.5 inches wasn't enough to prevent air exchange/heat loss."

   This sounds improbable, unless there were air leaks-  perhaps pot lights and such.

   I you must vent for shingle life (studies suggest otherwise) consider building a vented "cold roof" on top of the sheathing.

   -The poster formerly known as csnow
5. 
   ericicf | Apr 01, 2006 07:38pm | #10

   Sometimes re-inventing the wheel is not worth the effort , expense or remedial if something goes wrong.

   Re-consider the non-vented approach,as there are some intelligent ,economical, alternatives to foaming in place.

   Not meaning to put down the foam guys as there are many useful areas for foams like Icynene and Polyurethane.