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NotoriousAPP
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# Posted: 13 Aug 2013 12:01am
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Quoting: MtnDon Lstiburek has spent decades working on climate and how to build the best structure for the climate.
I've found no less than 10 experts on this subject....and none of them specifically cover cathedral roof insulation and venting for a cabin which will be used for 20-40 days in the year and is located in a hot southwest climate. They may cover it for a structure which is occupied year round but never for a part time cabin.
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NotoriousAPP
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# Posted: 13 Aug 2013 12:03am
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Quoting: MtnDon That type of roof, with no overhang, is a problem to vent and to adequately insulate. The problems usually begin with the fact that the framing is often 2x4. Too small to get vent space and insulation.
My rafters are framed in 2x6" lumber.
Quoting: MtnDon Then apply rigid sheet foam on top of that in two layers or more, seams staggered and taped. Then another layer of OSB sheathing with long screws through to the rafters
I have no option to place any additional materials above the roof decking (OSB) except 30# felt and metal roof.
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MtnDon
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# Posted: 13 Aug 2013 12:43am - Edited by: MtnDon
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Then I'd foam the underside with a couple inches and finish with batts. No vents. Or foam the whole thing like a friend here did with 2x6's. No vents.
Quoting: NotoriousAPP I have no option to place any additional materials above the roof decking (OSB) except 30# felt and metal roof.
Have them sheath the roof and stop at that. You do the foam, etc. It really does make a super roof
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PatrickH
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# Posted: 13 Aug 2013 07:55am
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This is from the city handout--
Roof ventilation is required. Enclosed attics and enclosed rafter spaces formed where ceilings are applied
directly to the underside of the roof rafters shall have cross ventilation for each separate space by ventilating
openings protected against the entrance of rain or snow. Ventilating openings shall be provided with
corrosion-resistant wire mesh, with 1/8 inch (3.2 mm) minimum to ΒΌ inch (6.35mm) maximum openings.
Minimum area: The total net-free ventilating area shall not be less than 1:150 of the area of the space
ventilated exempt that the total area is permitted to be reduced to 1:300, provided at least 50 percent and not
more than 80 percent of the required ventilating area is provided by ventilators located in the upper portion of
the space to be ventilated at least 3 feet (914mm) above eave or cornice vents with the balance of the required
ventilation provided by eave or cornice vents. As an alternative, the net free cross-ventilation area may be
reduced to 1:300 when a vapor barrier having a transmission rate not exceeding one per (57.4 mg/s-m2-Pa) is
installed on the warm side of the ceiling. It might be necessary to add ventilation with your new roof to meet
these standards.
How can you determine if the roof is properly ventilated? An attic needs to breathe. An effective ventilation
system will help prevent attic heat build-up, attic moisture and condensation, ice dam build-up and weather
infiltration such as drifting, snow and wind driven rain.
Research has shown that proper ventilation is necessary if the shingles are to last their design life .
The inspector also made us drill 1" holes in the rafters of a portion of the roof that was not directly vented by the main roof to get air flow into that section. And the inspector told us no foam wasnt going to make a big deal over the whole thing to me venting makes more sense
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Sustainusfarm
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# Posted: 13 Aug 2013 08:11am
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Sprayed in foam is far superior to fiberglass batts! There is no air movement thus no reason to vent....since ours was done we have not had a single ice dam on our roof! In fact, the roof insulates so well that the snow doesn't even melt off the roof unless the air temp outside is actually doing the melting! 
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MtnDon
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# Posted: 13 Aug 2013 09:59am
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Everything the building official said is true, when looking at conventional, traditional construction methods where there is an unconditioned attic above the conditioned living space. However, sometime there can be more than one truth. I don't have time to get into a discussion right now as I'm out the door and on the road in a few minutes.
What Sustainusfarm states is true. Give buildingscience.com a look.
Sometimes building officials and department rules are slow to change with the times. Sometimes we have to educate them. More than one way to skin a cat!
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NotoriousAPP
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# Posted: 13 Aug 2013 03:42pm
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I've already been to buildingscience.com, they don't specifically or generally address my situation: hardly used building in hot climate.
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NotoriousAPP
Member
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# Posted: 26 Jun 2014 11:38am
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Hi Purplerules. How did your cathedral insulation job work out for you? did you ever have moisture problems?
Do you have soffit vents?
Did you have roof paper covering your ridge vent? If so did you leave it covering the vent or cut it along the ridge vent.
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