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Old 12-03-2009, 12:21 AM   #1
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Need for venting in a Cathedral ceiling


I have an existing sun porch that I'm fixing up and it was built with a cathedral ceiling that wasn't insulated, but is finsihed off with car siding that I'd rather not disturb. I've always understood that there is a need to maintain an air space between the underside of the roof deck and the insulation, and have eave and ridge venting for this type of ceiling; as I did on a cabin I built.

But lately read and witnessed construction where they foamed the rafters full of insulation, leaving no air space. Will this cause early shingle failure due to heating - or moisture condensation in the rafter space? I like the concept if it would work because I could leave the car-siding in place.

Anybody have any experience or thoughts on this? The ouse is in the upper midwest. Hot in the summer and cold in the winter.

Thanks

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Old 12-03-2009, 11:02 AM   #2
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Need for venting in a Cathedral ceiling


Called a hot roof. It is supposed to work very well. The foam contractors know quite a bit about it. A job we just did was going to have it done. If it works like they say, it is a great idea for many older homes and critical areas.

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Old 12-03-2009, 11:09 AM   #3
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Need for venting in a Cathedral ceiling


Air space and venting is needed if using batt insulation or closed cell spray foam. No air space or venting is needed when using open -cell spray insulation. Be sure your local inspector will accept this, some don't.
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Old 12-03-2009, 10:13 PM   #4
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Need for venting in a Cathedral ceiling


I called an insulating contractor today and he doesn't do foam, but suggested high density blown fiberglass. Can I get the same effect with this method as with open cell foam? He also said that it would be difficult to foam in the closed ceiling, but I would think that foaming the closed rafter space would be just what foam is good for?
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Old 12-04-2009, 07:07 AM   #5
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Need for venting in a Cathedral ceiling


Blown insulation is not the same nor will it allow no venting and airspace. Foam is the best way to go for cathedral ceilings.
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Old 12-04-2009, 09:44 AM   #6
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Need for venting in a Cathedral ceiling


I've always heard of foam being sprayed on open walls/ceilings etc
They then shave it back to the studs/rafters after it has expanded
Foam expands - different rates...so I thought that was almost required (open studs etc)
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Old 12-04-2009, 01:00 PM   #7
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Need for venting in a Cathedral ceiling


There are pourable foams available meant just for this retrofit type of application.

No need for venting with closed cell. Closed cell does not allow moisture through. Open cell does.

Shingle life will not be compromised because insulation does not create nor hold heat. It simply slows the heat transfer process.

Closed cell is the best economic choice because it offers double the Rvalue of closed cell foams, rockwool, cellulose or fiberglass.

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