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I understand the need to ventilate and attic… but what about a cathedral ceiling. If there are no knee walls, and no attic area at the peak, is there any reason to attempt to provide ventilation between the insulation and the roof? Would installing a poly vapor barrier suffice for the moiture problem.. and not worry about the heat from the roof??
Or… should I attempt to put soffit vents in, vent props along the whole length of the rafters and a ridge vent at the peak?
Thanks
Chris
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Well I am wondering if there is a need for ventilation under the roof at all in a cathedral ceiling. Or can I fill the whole space between the rafters with insulation?? I guess the question is related to the shingles... will enough moisture work its way up past the vapor barrier to be a problem and it there a concern with the heat buildup with no air flow under the plywood.
I could manage to get air flow under the roof by using soffit vents and ridge vents and keep the space open with the styrofoam vent props.. but is that all all necessary??
Chris
*An the lamb gets led to slaughter.Gabe
*OK....I KNOW this is a controversial topic here. Gabe, Fred, and whoever... tell me YOUR opinion anyway. Yes, yes, I have read the debate here... to vent or not to vent.. etc... But was I mistaken in thinking it only applied to attic spaces? I didn't mean to get it started.. but I "thought" there was a convention for cathedral ceilings..... No???????Chris
* Chris,
Joseph Fusco View Image
*Hi Chris,A lot has to do with location and roofing type, color and material, lifestyle and expectations.In some areas of our country, white shingles are not used because the temperature doesn't get hot enough to weld the shingles after installation and all shingles are tarred prior to installation.So I can assure you that ventilation in these areas simply to reduce the ability for heat to build up under the shingles would be a waste of time.Typically in the southern regions the heat build up does effect the longevity of the shingles.Most of us are somewhere in between.Your next question regarding the poly is another regional thing. In Canada we have higher standards for poly than in the US, so I have never seen poly come apart between the walls, even after 20 years so I can't comment on deteriation problems.Here poly, when properly installed, will make an excellent line of defence against vapour related problems. Even with the installation of poly we still use accoustical caulking to seal around outlets and opennings.Poly by itself is not thermal resistant, it only helps to prevent the warm air from escaping your living area. Therefore it, by itself will not negate the need for proper insulation and any ventilation requirements.The answer to your last questions, are discussed at lenght in a flood of comments about ventilation and insulation. Depending on where you are, depending on the materials and relevant warrantees that you intend to use on the roof, depending on your expectations, will determine your personal requirements regrding this subject.While there are many arguments favouring that ventilation is beneficial, there are none that can prove that it is detrimental to the shingles.Hope this helps,Gabe
*Thanks...I enjoy this site, and I have learned a bit from the give and take. (I am savvy enough to pick what is good, when it makes sense, so a different opinion is welcome)I did some additional research at Certainteed's site and for their Bird shingles they will NOT warranty them if there is no ventilation, so that has answered that question. Whatever benefit I may have gained from that last 1/2 to 1 inch of insulation I will trade for the comfort of knowing that the folks making the shingles think the ventilation is worth it.Thanks again.. and I will be back.....Chris
*Joe. (Did you receive my recent e-mail in answer to your wondering why I hadn't e-mailed you anything?)Meanwhile, back at the ranch, Chris is left wondering why he bothered asking.GeneL.
* Gene,
Joseph Fusco View Image
*Wait, before all the grouches leave -- what of SIPs forming a cathedral ceiling? Surely this is impossible to vent?And Chris, it turns out that Elk will extend their shingle warranty to certain nonvented roof decks -- all SIPs.
*Chris,The main reason I wouldn't go the poly/fg/vent chute route is that I believe you get better thermal performance in both hot and cold climates with other systems such as SIP (structural insulated panels) or a dense-packed cellulose system. Fiberglass is not very good at keeping things cool in summer heat. I think you wind up with a more foolproof air/moisture barrier with these other systems as well.If these benefits are not great enough to offset the loss of the shingle warranty then go with the status quo.Steve
*AndrewNot only would a SIP's roof probably be "impossible to vent" but they would be officiallyi allowedto be unvented by changes to the Ontario Building Code. Roof cavities packed tight (no air space above) with an impermeable insulation (foam) need not be vented!!Surely the shingle manufacturers would have no argument to lean on.However Gene has cautioned against using SIP's for roofing for other reasons. De-lamination??-pm
*Expense must be one. I just got a price of $3.25 sf for 4'x8'-16' R-32 5/8" OSB EPS SIPs (enough acronyms?). No formaldehyde, CFCs, or R-value decay. This is the "retail" price, whatever the heck that means (I doubt they'll be selling these at Lowes' any time soon!). I suppose labor savings would help balance that out, but I have access to fairly cheap labor (me). Also, these R-32 panels weigh 3.72 lbs/sf -- 180 lbs. for a 4x12 -- all but requiring a crane for setting the roof I would think....Past problems have included panel creep (esp. with EPS?), buckling, edge swelling. They of course claim to have worked these things out. It is an appealing system in terms of simplicity and efficiency. You can get the panels done in cement board, T&G, and others for an instant finished wall.
*andrew-Remember the method I mentioned for cathedral ceilings? I've used it since 1982 on about a dozen roofs(close to 40,000 sq ft-including a church roof and a school roof). I've installed it over conventional rafters and plywood, tectum, 3 1/4" double T&G, T1-11 on open rafters. All these roofs were reroofs, the stuctures suffering from problems such as ice damming, premature shingle failure, high energy costs. This method has helped on all counts. Costs were reduced for heating and coolingI use Iso board, usually two layers of 1 1/2" with all joints staggered, on the existing deck(whatever it is). 1 1/2" Iso costs $12.00 per 4x8 sheet and has about an R10. Over this I install 1x3(sometimes 2x3), 16" OC screwed down to the existing roof deck. These create the ventillation cavities. Over this I install the plywood deck and install roofing and vents as I would on any roof.If you have cavities to fill between rafters, go ahead and blow cellulose in tight and "fuhget about it". You will have whatever R value the cel yields, plus R20 from the 2 layers of Iso and a ventilated, warranted, "cold" roof.Each job had many unique situations and solutions, but the basic method remains the same. Roof pitches varied from 2 1/2 /12, to 10/12. John
*John,So this adds 3 inches to the exterior of the existing roof. Has this caused you any difficulties in retrofits with complex rooflines?Steve
*andrew d. I forgot to mention that the SIP buckling when used as roof panels is true only of ESP SIPs. It doesn't happen with Polyurethane (PUR) cores. Thanks for bringing it to my attention. GeneL
*Steve,Actually the method I described would add 4 1/4"(3" for the iso, 3/4" for the 1x3, and 1/2" for the plywood).The additional thickness is an issue. On a job with closed in rafters, the need for extra insulation above the original deck is less, so you could go with any thicknes of iso, or none at all, if you felt satisfied with with the R value available in the rafter space. I've gone with as much as 4" of iso where the ceiling was exposed 1x8 V-groove siding laid good side down(not much R value there!)Where dormers tie into the roof presents a little problem, I just cut the dormer overhang to accommidate the thicker material. Where the roof intersects sidewalls, it requires cutting the siding material away and step flashing up under the siding. Chimneys, soil pipes, etc. must be flashed to accommidate the new thickness.By the way, when running the 1x3(did I mention I screw them down?) I leave them a couple of inches short around skylights, dormers, etc. to allow lateral venting around the obstruction.Some type of trim detail has to worked out for covering the extra thickness at the eaves and rakes. On homes with large overhangs it is not necessary to run the iso board all the way to the eave or rake. You just stop the iso board a little past the outside wall and build up your firring to match. It can save a lot of money in unnecessary insulation, especially on a job with 3 ft overhangs.John
*JRS - did you ever hear of vented nailer board? I did the same thing on my roof that you prescribe, only to find out a month later that I could buy the whole works as a complete assembly for scarcely more than the cost of the iso board.Isn't it a pain that cordless drills won't run the insulated panel fasteners in?-Rob
*For a long time now the Standard Practice for constructing an open beam cathedral ceiling in So. Cal. has been (from the bottom up) beams, 1x or 2x t&g decking (depending on the span), vapor barrier, foam insulation, plywood and roofing. This is accepted by the building authorities. No on can say that it is because of the cool climate here prevents overheating. No vents! I have yet to see a roof failure from overheating. I recently did a retrofit with 4" foam over an existing hotmop compsition roof. I used 6" metal drip edge to cover the foam down to the existing fascia. I have never been required to vent a "flat" roof deck without an attic, either in Calif. or Washington.
*Hi Mike,The retrofit you mentioned doing, with the 4" foam over the roofing membrane is called an inverted roof system. We have been doing this for decades on ICI projects.The concept is simply that by putting the insulation over the roofing membrane, the membrane temperature stays closer to that of the interior of the building and therefore stays cooler and lasts longer.Gabe
*Rob-the only vented assembly I'm aware of is AJC's "Inventsabord". It is OSB bonded to a polystyrene(sp?) channelled(horizontally and vertically) 4x8 board that has a negligeble R factor. It costs about $25.00 per sheet, and for additional R value you still need to install some type of rigid insulation under it. It is special order, which means you must be very accurate in estimating your order and add a fudge factor. My system uses off the shelf materials which suits me much better.It would be nice to eliminate cords on air and electric tools entirely, but I guess I'm resigned to the fact that this won't happen in my lifetime. They sure come in handy when they can be used.Johnp.s. Inventsabord is also available with plywood, perlite, gypsum, and fiberboard. It is marketed as a way to ventilate flat roof systems as well as pitched roofs. I think it may shine most in flat roof applications.
*Why polyiso, exactly? Doesn't it degrade over time? Can the foam go on the inside, behind drywall of other ceiling covering in a closed rafter assembly?Nice simple approach. Lots of money for the PIC, though!
*AndrewIt has been reported here in months past that polyiso degrades enough to lose about R2/" and then stabilizes. Yes it can go on the inside behind drywall . . . I've done it several times and it works great, you just need longer d/wall screws, and tape the seams with special foil tape!!-pm
*Around here it is a layer of felt-faced iso board in whatever thickness you prefer, with OSB sleepers and sheathing hot glued to it. The top sheet is staggered from the rigid so that it can install easier. The rigid panel fasteners are sent through the sheathing/sleepers/foam to hit the framing. Oddly enough, a few sheets of this priced cheaper than the individual components. And this didn't take into account the time to gang-rip the OSB sleepers with the worm drive.-Rob
*John I like the idea of the foam on top for the reason that the eventual leak will show itself at the soffit instead of making a horrendous mess of the rafter bay isulation. But a double layer of foil-faced iso -- if this isn't a nearly impermeable vapor barrier (retarder) then what is. Once the foam is in, the vent channels have nothing to do but please the inspector, and perhaps cool the deck a few degrees.
*Andrew,I'm sorry, I use the foil face iso so rarely,I didn't mention that the iso board I use is made specifically for roofing applications. It is a black felt faced board that is needle punched, available from commercial roofing suppliers, if not from your local lumber yard. It is not a vapor barrier. I'm sure it's cheaper than the foil faced.By the way, R value for iso is given as aged R value, not initial. In your application, if you blow the rafter cavities solid with cellulose, you probably don't need to run your iso board in two layers.Depending on your target R value you could use 1 1/2", 2", 2 1/2",whatever your supplier has available. If you have trouble findig a supplier with the thickness you want in stock, see if you can piggyback on someone else's order(sometimes they may require a minimum order that may be more than what you need). There's probably more I could tell you, just keep firing away with the questions as they occur to you.John
*I see, said the blind man. Sounds like a plan.
*"I see" said the blind man,So he picked up his hammer and saw.
*"I see" said the blind man,To the deaf and dumb woman,As he picked up his hammer and saw.Rich Beckman
*"I see" said the blind man, To the deaf and dumb woman, As he picked up his hammer and sawAnd promptly tripped over a pile of 2x4's and fellflat on his face.
*Andrew-see what you started?
*Gene: I recently posted asking about an article that supports non-vented roof design. Do I understand that you have or will have a book that covers this? If so, please keep me posted. Jon T.
*Jon H, or Jon T. I did not see your post regarding unvented attics.I wrote an article on "Hot Roofs"--unvented--for the April 1995 issue of House Magazine.Joe Lstiburek's book, Builder's guide: Hot-Dry & Mixed-Dry Climates has information and illustrations on unvented cathedral ceilings. Check his web page at http://www.buildingscience.com.My book, _Complete_ Building_Construction_ page 514, asks the question,ÏS Attic Ventilation Necessary? "If the ceiling sheetrock sair retsarder is continuous and unbroken--no penetrations--venting of the cathedral ceiling is not necessary.. .The sealing of the air retarder must be supervised by someone who is experienced. do not make the mistake of assigning it to one of the crew. EVEN IF THE CEILING IS VENTILATED, THE AIR RETARDER SHOULD BE AS LEAKPROOF AS POSSIBLE" (Emphasis not in original) The book was first published in 1993. Hope this helps. GeneL
*Okay everybody - A new twist to the same question.In a roof with a valley - two intersecting roofs - how do you vent between the valley rafter and the valley jacks anyway ??? As soon as the last common rafter is vented the remaining jack rafters are blocked off from the ventilation system. Thus, in this type roof what should be done to ventilate ? Top and bottom edge cutouts would seem to weaken the rafters ? If warranty of roofing shingles is a concern and packing with rigid foam (no ventilation) is the way to go then are there guidelines as to what parts of the country you can or should not do this this and what locations is might not be a problem for the roof shingles. As mentioned, Elk allows shingles on SIPS, are there other manuf doing the same or limiting their approval to certain locations.
*Bob...I used to drill holes...nobody does though and as I now feel, I would never drill again...Less air movement and keeping the interior moisture down are what I'm into now...near the stream,ajTakes me awhile to add to posts sometimes...
*Bob. CertainTeed gives a 10 year warranty for their shingles installed on an unvented attic. GeneL.
*on valley and hip rafters i set my saw to 45 and max. depth and cut Vees laterally from the common rafter bay to the jack rafter bays bringing air from the continuous soffit vent to the ridge vent (Shingle Vent II ridge vent only) .... i also use this technique on dead end bays like skylight frame -outs... the jack rafters are oversized compared to the common rafters so the 2" deep Vee doesn't really weaken anything ... and the sheathing can easily span the top of the vee....less time consuming and more effective than drilling holes which is what we used to do....
I'm building a new home here in the North San Francisco Bay Area and I'm looking for some guidance on the open-beamed ceiling I'm building. I have the beams in, 2x6 T&G on top. It has been raining so we covered that with 30# felt. The question is this: Am I setting myself up for problems by laying a 2x4 framework on top of the felt, with rigid insulation between the 2x's, plywood on top of that and then more felt and asphalt shingles? Am I creating a sandwich of moisture between the 2 layers of felt? I'm long-winded aren't I?
Chris, welcome to Breaktime. That question wasn't long-winded at all.
There have been lots of discussions in the past on this topic, so you might start by using the Search function at the lower left of your screen.
My short answer to your question would be that if you provide for ventilation under your plywood deck you should be ok. If you don't want a vent space then you need to do something to completely block moisture from getting in there.
You might try a Google search on Nailbase insulation, a panel of foam adhered to a sheet of osb. Might work in your situation.
Mike
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I understand the need to ventilate and attic... but what about a cathedral ceiling. If there are no knee walls, and no attic area at the peak, is there any reason to attempt to provide ventilation between the insulation and the roof? Would installing a poly vapor barrier suffice for the moiture problem.. and not worry about the heat from the roof??
Or... should I attempt to put soffit vents in, vent props along the whole length of the rafters and a ridge vent at the peak?
Thanks
Chris