Extending R-value for cathedral ceiling
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 |
Extending R-value for cathedral ceiling
On 11 Dec 2006 08:11:12 -0800, "Chris" wrote:
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 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. |
Extending R-value for cathedral ceiling
Chris wrote:
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. Why not put up a good plastic film vapor barrier under 8" of insulation? Nick |
Extending R-value for cathedral ceiling
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Extending R-value for cathedral ceiling
Chris Friesen wrote: wrote: Chris wrote: 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. Why not put up a good plastic film vapor barrier under 8" of insulation? You're supposed to leave space for airflow between the insulation and the underside of the roof. Chris 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. |
Extending R-value for cathedral ceiling
Chris Friesen wrote:
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. Why not put up a good plastic film vapor barrier under 8" of insulation? You're supposed to leave space for airflow between the insulation and the underside of the roof. Maybe not, if there's no chance of condensation from moist house air leaking up under the roof. Nick |
Extending R-value for cathedral ceiling
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Extending R-value for cathedral ceiling
DK wrote:
Here in the southern US, not leaving a place for ventilation will cost you a roof job about 10 years before it should. 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 |
Extending R-value for cathedral ceiling
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Extending R-value for cathedral ceiling
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Extending R-value for cathedral ceiling
Goedjn wrote:
Here in the southern US, not leaving a place for ventilation will cost you a roof job about 10 years before it should. 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. You don't stop water-vapor on the far side of the insulation, you stop it on the near side. You stop it on the warm side. Plastic film under insulation would do that. Nick |
Extending R-value for cathedral ceiling
wrote:
Here in the southern US, not leaving a place for ventilation will cost you a roof job about 10 years before it should. 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. Because that's how cathedral ceilings are built base on years of construction experience... You might say the same about shark repellent in Kansas :-) Nick |
Extending R-value for cathedral ceiling
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. |
Extending R-value for cathedral ceiling
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Extending R-value for cathedral ceiling
marson wrote:
where I'm from, Duluth, MN, the airspace is required by code. IIRC, ERVs are also code, and a big waste of money. 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. "Chutes" encourage "windwash," no? I suggest that you ignore the code and take control of your own life. Nick |
Extending R-value for cathedral ceiling
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-f...fs-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, 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? |
Extending R-value for cathedral ceiling
Chris wrote: 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 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. |
Extending R-value for cathedral ceiling
Chris wrote: 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 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. |
Extending R-value for cathedral ceiling
Well that's bizarre ... a cross posting to the same group :-) |
Extending R-value for cathedral ceiling
Nick Pine wrote: marson wrote: where I'm from, Duluth, MN, the airspace is required by code. IIRC, ERVs are also code, and a big waste of money. 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. "Chutes" encourage "windwash," no? I suggest that you ignore the code and take control of your own life. Nick 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. |
Extending R-value for cathedral ceiling
Chris wrote:
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 With a cathedral ceiling you get one chance to do it right. Put in the max. Stuff in 6" of fiberglass then add as much foam below as you can afford, then add a bit more. I'm in Manitoba, Hydro is recommending R50 now. I put in R60 two years ago. Yes R-Six-Zero. http://www.mts.net/~lmlod/Cabinfront6.jpg Temps. around here drop below -40 Deg F. in the winter. With a good fire going in the wood stove the inside temp. near the ceiling peak is probably 80 or 90 deg. F. even with a fan going. A lot of heat to keep snow off the roof. As for economy and the 95% - 96% boys, where were they years ago when R12 was enough, then R20, R40. The cost of heat is only going to go up. The bean counters will whine today because "You won't get the return for your money" then tomorrow they will cry "It costs so much to heat, put in more insulation". In reality that bunch doesn't know their ass from a hole in the ground. If you are building to sell and make a quick buck then go bean counter minimum, otherwise think 20 years down the road. If not for yourself then for your kids if you want them to be able to afford the place. Lots of ventilation. I put in metal ridge vent right across all three sections. One added bonus of the metal ridge vent is that birds don't like to land on the things. they stay away from my roof and crap all over my neighbors roof (ridge vent covered with a shingles) Seagulls, crows etc. the stuff covers half the top of his roof and hardly a drop on mine . By the way I built that place, I hired out the foundation, shingles and HVAC, the rest I did with help from family and friends, not a fricken bean counter among them. :) :) :) and as they say "Put my money where my mouth is". LdB |
Extending R-value for cathedral ceiling
bowgus wrote:
... R20 conductance is .050 ... 95% efficient, and R28 conductance is .036 ... 96.4% efficient or not much of a difference. But a 2000 ft^2 R20 roof in a 10,000 DD climate will leak 24 million Btu/year, vs 17 million at R28. Nick |
Extending R-value for cathedral ceiling
marson wrote:
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? Get an architect or PE to sign an "unvented" drawing? ... 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. 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 |
Extending R-value for cathedral ceiling
L d'Bonnie wrote:
Temps. around here drop below -40 Deg F. in the winter. With a good fire going in the wood stove the inside temp. near the ceiling peak is probably 80 or 90 deg. F. even with a fan going. A lot of heat to keep snow off the roof. How strange. Why would the ceiling be so hot with a ceiling fan going? Is the stove red hot, with no insulation in the house walls? :-) You want lots of R1/inch snow on the roof, no? How can it melt with that 2" air gap? Nick |
Extending R-value for cathedral ceiling
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. |
Extending R-value for cathedral ceiling
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Extending R-value for cathedral ceiling
marson wrote:
L d'Bonnie wrote: Temps. around here drop below -40 Deg F. in the winter. With a good fire going in the wood stove the inside temp. near the ceiling peak is probably 80 or 90 deg. F. even with a fan going. A lot of heat to keep snow off the roof. How strange. Why would the ceiling be so hot with a ceiling fan going? Is the stove red hot, with no insulation in the house walls? :-) You want lots of R1/inch snow on the roof, no? How can it melt with that 2" air gap? Nick Nick, you seem to have no experience heating a house in a cold climate. Wrong. ... of course the heat rushes to the peak of his vault! ever heated a house with a wood space heater? Sure. Ever studied basic physics? Nick |
Extending R-value for cathedral ceiling
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