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You would not want to use ventilation channels in combination with space foam under any conditions. Moisture would condense on your sheathing and rot out your roof.
Thanks all for the responses so far... I must say, these two conflicting schools of thought have my head spinning! The end result of this space will be a fully climate controlled living space with it's own HVAC system, as such, it is my thinking that negative impacts on the roof from continued exposure to high heat would not be an issue. ???I have talked to one professional so far here at the house. This guy was here a year ago and he has changed his tune since then. Last year it was seal it up with no vents. This year, he admits, after talking to a bunch of roofers, he is now saying vent it. He is suggesting vent chutes... Peter J, Is there a reason why you suggest building a vent, as opposed to just using these plastic vent chutes???I'm sure I will have more questions once I process some of this. I will be asking a specialized forum, as was suggested, but very much welcome any continued conversation.Thanks Jason
blues Q&A Forums Registered User Posted: 5/16/2009 2:17 PM ESTReplyShingling, cathedral ceiling, spray foam and venting.I can't seem to get the same answer out of two peeps on this question. I want to reshingle my old roof which will turn out to be a cathedral ceiling. I'm worried about the shingles being able to breath after I spray foam between the rafters and want to know the right method to proceed with venting, or if I need to vent at all if I can provide an air tight situation. I plan to use 2 lb closed cell foam and the roof sheathing is actually 1" boards, the shingles will be a 30 - 40 year asphalt shingle product. The actual rafters are also rough 2x4's so I should get 4"s of foam up there. The only thing I need to extend through the roof will be an abs plumbing stack. I'm also worried about the spacing that is present between these boards and if I need to be concerned with foam pushing up the shingles. This house is in Toronto, where we can suffer severe snow in the winter and blistering heat in the summer.Mason Moderator Posted: 5/16/2009 4:40 PM ESTReplyRE: Shingling, cathedral ceiling, spray foam and venting.Venting the cathedral ceiling or attic space reduces the benefits of the sprayfoam. Research and field history has demonstrated that installing foam to the underside of the roof deck does not add significantly to the temperature of the shingles or lead to premature deterioration. However, a few shingle manufacturers have warranties that do not allow unvented attics and foam installed to the underside of the roof deck without ventilation.Find a shingle manufacturer (such as GAF/ELK or Certainteed) that allows the use of this very beneficial approach to insulating attics and cathedral ceilings.blues Q&A Forums Registered User Posted: 5/16/2009 6:10 PM ESTReplyRE: Shingling, cathedral ceiling, spray foam and venting.Hi Mason, thank you for the reply, the only other thing that might concern me now is whether the foam, may push up the shingles from between the boards. If this could be a problem might their be a simple fix without sheathing the whole roof with plywood.RogerM Q&A Forums Registered User Posted: 5/17/2009 2:50 PM ESTReplyRE: Shingling, cathedral ceiling, spray foam and venting.You are correct to be concerned. SPF can expand out crevasses and warp the shingles. Forget about any shingle warranty if this happens. How much space is between the boards?foamdude Q&A Forums Registered User Posted: 5/19/2009 6:34 AM ESTReplyRE: Shingling, cathedral ceiling, spray foam and venting.i have done this application in the past,,with no untoward results,,,i would prefer not to do this app this way,,and i inform the customer that it is a compromised app...informed decisions rule!!obviously the foam will bond with whatever is bridging the gaps in the planking,,roof felt,,shingles,,,whatever,,,i have not seen it warp shingles with even as much as a 1" gap...foam will follow the "path of least resistance" in its rise,,,and if you spray the way you should,,an 1" pass or so your chance of pushing these shingles up are reduced,,,if you spray directly to them you are hosed on your warranty,,,no doubt about it,,,life is full of choices,,,some right,,some wrong,,but this is an existing old shingle roof so there is no warranty to worry about here,,and you mention you plan to re-shingle the roof,,,why not strip and sheet it and shingle and then foam,,as it should be done??? or,,,put a piece of tape across the gap and spray away if your freaked out about spraying into the gap..or stuff em with filerglass,,,makes a great block out,,,or this could be a perfect application for a froth pak,, like they try to sell for sidewalls,,you know the swelling foam that adds r value but does little to expand into the cracks and creaveses to create a true air barrier,,,caulk the gaps,,,neccessity is the mother of invention,,,hope this helps,,
Can I assume you mean spray foam and not space foam or you've worded this incorrectly?Closed cell polyurethane foam will act as a vapor retarder from the interior and when done correctly will essentially isolate the channel from the interior. The relative humidity inside the vent channel would be the same as exterior. Coupled with a convective airflow from eve to peak there is no disparate temperature or humidity that would promote condensation of water vapor on roof sheathing.There certainly have been cases of rot where closed cell foam has be installed tight to underside of sheathing. Under the right conditions a breach in the vapor retarder (such as a crack in foam) allows interior humidity to condense on underside of roof deck and effectively traps it there.
I'm going to buy a house and have the attic insulated. I believe I'm going to use spray foam. There is a lot of information indicating that spray foam applied directly to the exterior sheathing of a roof does not damage the shingles.Seehttp://www.sprayfoam.com/spps/ahpg.cfm?spgid=66http://www.buildingscience.com/documents/guides-and-manuals/irc-faqs/irc-faq-conditioned-attics/?searchterm=spray%20foam%20attic%20shingleshttp://www.buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies-for-all-climates/?searchterm=spray%20foam%20attic%20shinglesNote that the rafters attached to the external sheathing should also be covered, as wood has a low R value. Additionally, in some locations, you need a fire barrier over the spray foam.
You're right, the shingle life thing is largely a myth. My greater concern is moisture control and where interior moisture will condense. The Green Building article specifies that spray foam insulation applied directly on underside of roof deck is fine IF you use an airtight drywall installation. Theory is one thing, practice in the field is quite another. Retrofitting a comprehensive airtight drywall system in an existing structure without taking the whole house to studs is going to be problematic at best. The vent channels provide a little leeway in terms of a less than perfect installation, which defines most remodels.Building from scratch one could really optimize a structure for thermal performance, but in my experience two things frequently short circuit that; The added expense to do it right and the lack of knowledge and coordination on the part of those involved...all the way from the contractor down to the subcontractor's subs. There are exceptions, of course, but by and large the actual building of residential structures is just archaic, but that's another discussion.As suggested in another post, it would be closer to ideal to build on top of existing roof, but I'm assuming that's not a possibility for OP.As evidenced in the posts herein, there is much to concern oneself with. It's not clear cut as to "best" method, which is certainly confusing. The fact is every installation is going to be a little different.
PeterJ.If you completely air seal the underside sheathing and perimeter of the attic space with enough foam (and it was installed correctly and you were bothered by the chemical), then airtight drywall would not be necessary, correct? If air can't contact the sheathing and cool, then it cannot condense out moisture. At least that how I'm understanding the building science. But as you said, it is really hard to know how to proceed. At this moment, I am trying to find an affordable way to insulate a cathedral ceiling without spray foam. Six inches of rigid polyurethane on outside would work, but for big $$$$.
Jason, I'm not sure I understand the highlighted paragraph above. The basic idea is as JP suggested; wrap the living space in air barrier and insulation, and isolate it as much as practical.As for building the chutes as opposed to installing channels. I know I could get closer to a complete "isolation" (my words) of living space from roof deck The only contact of roof deck to interior would be through rafters themselves, not true of channels, which are kinda hit or miss by their nature. I probably don't do a great job explaining it, perhaps envision a box suspended within a box for a more graphic mental picture.Another possibility would be to build the "floor" of the vents out of sheet foam. In essence replacing the OSB or plywood with foam. Extruded polystyrene boards (exp, typically pink or blue), has more r-value per inch than expanded polystyrene (eps, looks like compressed beads or Styrofoam). The downside is cost. If you can't create more space below rafters, you wanna shoot for highest r-value possible in given space.
I am many posts behind here but need to respond while I have the time...Peter J, The paragraph you are not understanding is actually an addition to the post from my wife She wanted to add a thought /question, so I let her, of course, and she pushed send... I think she was asking about the "hot roof' scenario, thinking that if the space was a conditioned area that the inside of the roof/foam would not be hotter than the outside, so reducing any 'cooking' of the shingles.But anyway, I do not believe we will be doing the 'sealed' / hot roof and will in all likelihood, follow what you and our first professional have suggested and go with some sort of vent channel. This adds the expense and job of creating better airflow behind our gutters (house with NO eaves or soffits), but I believe it is doable with a couple a couple plans brewing on that front...To my wife's credit, she had the same idea as you did, Peter J, about using foam board spaced away from the roof decking to create the air channel.So here is what we are mostly decided on: A venting channel constructed of foam board (inch thick?), then 3 to 3 & 1/2 inches of spray foam, then hat-channel the drywall off of the rafters, so the rafters can be covered by an inch or so of foam themselves...I know there will be the details to work out and cost has just jumped considerably for this stage than what I though it would be...but it sounds like a must.We have a couple more professionals coming and more research and reading to do.Thank you all so much for the insights and I will continue listeningJason
