Extending R-value for cathedral ceiling

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I have to add fiber glass insulation on my cathedral roof, for a small cottage 16' x 24', new construction, in Canada. The roof rafters are 2" x 8". For that size, I should use R20 (6") in order to keep 2" for ventilation on the top of insulation. I though I could to be able to use add another 2" x 2" lumber in order to extend the rafters wide to 2 x 10 (instead of 2 x 8) and to use R28 (8") insulation instead of R20 (6"). Is that a big difference for R-value? The cottage is used to spend weekends and some vacation days and not (yet!) for permanent living. What would you recommend to me? And another question: In order to keep the 2" for ventilation I though to use raft-R-mate. Is it wrong to put this raft-R-mate the entire long of rafters or only at the beginning should be enough?
Thank you, Chris
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You can add rigid foam insulation crosswise over the joists, as long as you cover it with sheetrock afterwards. That's why sheetrock screws come in a 3" (75mm) version.
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Why not put up a good plastic film vapor barrier under 8" of insulation?
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

You're supposed to leave space for airflow between the insulation and the underside of the roof.
Chris
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Chris Friesen wrote:

I would extend the rafters down so you can use the thicker insulation. I'd also use the air channel baffle material the entire length, as that is the best insurance that you will have a clear space for air movement.
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Maybe not, if there's no chance of condensation from moist house air leaking up under the roof.
Nick
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On 11 Dec 2006 13:27:57 -0500, snipped-for-privacy@ece.villanova.edu wrote:

Here in the southern US, not leaving a place for ventilation will cost you a roof job about 10 years before it should.
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Why would you say that? Seems unlikely to me, if there's no chance of condensation from moist house air leaking up under the roof.
Nick
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On 11 Dec 2006 15:09:45 -0500, snipped-for-privacy@ece.villanova.edu wrote:

You don't stop water-vapor on the far side of the insulation, you stop it on the near side.
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You stop it on the warm side. Plastic film under insulation would do that.
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

Because that's how cathedral ceilings are built base on years of construction experience. You provide a space between the insulation and the sheathing so that air can move from soffit to peak, where it exists via a ridge vent or other venting mechanism. I doubt you;d get a building inspector to pass off on an unventilated cathederal ceiling. Besides moisture, what do you think having no air flow under the sheathing is going to do to the life of the roof when it gets to 150 in the summer?
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You might say the same about shark repellent in Kansas :-)
Nick
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where I'm from, Duluth, MN, the airspace is required by code. I think it is important to use a chute the entire length, not only because it ensures airflow, but it also reduces "windwash" which strips heat away from your insulation. I like to use cardboard vent chutes (available at good lumberyards). I also think adding a layer of foam to the underside is a good idea. even a layer of 1/2" polyisocyanurate (thermax is a good brand) would reduce thermal bridging and provide an air/moisture barrier.
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IIRC, ERVs are also code, and a big waste of money.

"Chutes" encourage "windwash," no? I suggest that you ignore the code and take control of your own life.
Nick
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Nick Pine wrote:

no, chutes don't encourage windwash because the chutes keep the cold air from the insulation. it's easy to say "take control of your own life" but when a building inspector is standing there telling you you must have vent space period, what are you going to do? we have had trouble getting hot roofs with 12" of icynene by our building department, let alone fiberglass.
ironically, i basically agree with you. if i was building my own house and i didn't have to deal with inspectors, i wouldn't vent the roof either...but i would also be very anal retentive about my vapor barrier. in practice, cathedral ceilings do fail (condensation raining in the house), and unless someone is there who really knows what they are doing, the safe bet is to ventilate.
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Get an architect or PE to sign an "unvented" drawing?

Sure. And there are degrees of venting. With near-perfect vapor barriers on both sides, a small vapor barrier or roof leak could make a lake. Perhaps the roof should be slightly less airtight than the ceiling, with a ridge vent but no wind-whistling chutes.
If I were building a new house, I might put insulation in the attic floor and make the steep south roof clear Dynaglas polycarbonate plastic and make the underside of the north roof white and collect warm air and light in the attic in wintertime, with a few simple skylights in the attic floor, with no venting in wintertime.
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

And you might say that you're an idiot, giving out advice based on equations, without knowing anything about the real world, construction, or how roofs are properly ventilated.
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Research your code and recommended energy code and guidlines. R 20-R28 are substandard and inneficient. Your ceiling is where most your heat loss is. R 35 in Zone 5 is considered not optimal, R 60 is. I dont know your Zone but going to R 100 is getting more common in cold areas. You need an airspace but I dought you need 2", 1" should do, Foam chutes are cheap and easy to install. Furring out 2" is a good idea. For the best performance spray on foam, there are types with R7 per " , twice that of fiberglass. Also Fiberglass looses efectivness at extreme cold, maybe near 50-60% at below zero temps. A vapor barrier is needed on fiberglass, not foam, and inside next to the sheeting. 6.5" of closed cell blown in foam is R 45, it may impress you but its still not an optimal R value. Research, www.Energystar.com will help. Insulation manufacturers Dow, Owens Corning etc will recommend maybe R60. I will bet your Govs green sites might be even higher, R 70++, I dought your code is under R35 unless you are on a coast with completly moderate temps, but in canada? What you do now will be it, so if you can do it go for the maximun as it will have a big payback and increase your houses value.
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I don't wish to take sides in this argument and I certainly have no practical expertise in this area, but the following article discusses both "cold" and "hot" (i.e., vented and unvented) roof design in colder climates; more specifically, Alaska.
See http://www.north-rthn.org/french/f-factsheets/Attics-roofs-french.html
As to whether or not one should follow code, that's a personal call but it would seem somewhat foolhardy to do otherwise. If this should become an issue with the inspector, who do you think will have the final say?
Cheers, Paul
On 11 Dec 2006 14:20:34 -0800, snipped-for-privacy@optonline.net wrote:

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Chris wrote:

Fwiw ... R20 conductance is .050 ... 95% efficient, and R28 conductance is .036 ... 96.4% efficient or not much of a difference. And you didn't say but just in case, a ridge vent is the perfect partner for that raft-R-mate (although I would think if the fiberglass is installed carefully the raft-R-mate should not be required). And of course a completely sealed vapor barrier.
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