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Old 10-19-2013, 10:24 AM   #16
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How much ice shield is recommended


Gary,

I have read the above listed papers previously and perhaps I should have been more specific in my designation. My reply was more in response to James (Jagans) post:

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Originally Posted by jagans View Post
Then maybe you don't want to change what has been working? Ice dams material will create a vapor retarder, or pretty close to a vapor barrier.
The roof, as we have discussed previously, certainly can push moisture into the attic via solar push.

http://www.owenscorning.com/NetworkS...tober-2011.pdf

My point about the Ice/Water question is that the roof is not venting through the plywood.

If a consumer is worried about ice damn because of the vapor permeance of the Ice/Water or that it will interrupt their roof venting, the venting schedule is dramatically flawed in its current deployment.

Assuming or counting on your roof to dry through the felt or plywood is a terrible idea and part of the reason that we always specify poly underlayments. Running poly keeps that summertime moisture push out of the attic and out of the structure.

Even if the plywood and felt are permeable, the shingles aren't so it isn't drying to outside regardless.

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Old 10-19-2013, 04:16 PM   #17
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How much ice shield is recommended


Yes, I have a similar article with Joe and the perm ratings of new underlayments from the same time period; https://docs.google.com/viewer?a=v&q...3cD6T-Jm7TyscA

I mentioned drying through the felt/sheathing as to the attic, if a leak or ? to saturate the sheathing.

My question to you was regarding the "no ventilation" in the article and Chart #2?

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Old 10-19-2013, 08:18 PM   #18
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How much ice shield is recommended


Quote:
Originally Posted by Gary in WA View Post
Yes, I have a similar article with Joe and the perm ratings of new underlayments from the same time period; https://docs.google.com/viewer?a=v&q...3cD6T-Jm7TyscA

I mentioned drying through the felt/sheathing as to the attic, if a leak or ? to saturate the sheathing.

My question to you was regarding the "no ventilation" in the article and Chart #2?

Gary
This table....

If that is correct then I take a bit of umbrage with it. I am sure the guys that wrote that are supremely bright but there just seems to be so many factors at play when it comes to ice dams.

Roof orientation, envelope, etc.

I have seen homes, as I am sure you have, ice dam with great venting and everything else done right.

I guess if the OP in this case is in a snow load zone of 35, this applies based on that study:

When the elevation is above 6000 ft (1830 m) ventilate all roofs. Below that elevation, ventilate
if the thermal resistance of the roof is less than R30 (5.2).

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Old 10-20-2013, 09:26 AM   #19
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How much ice shield is recommended


Just a couple of points of clarification. I have never had ice dams. Roof was installed long ago and has weird shaped shingles not 3 tabs. I'd guess they are probably 20+ years old and now having insurance go up because of roof age and companies decline coverage. I currently only have mushroom vents in the back. Ridge vents are now required by city code. Our attic does get to warm I guess in summer so new venting system is suppose to help. Also I do not have plywood on roof. It's still the original planks with a slight gap between boards from what I see in attic. Never paid attention if plywood was out on top of that or not.
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Old 10-20-2013, 09:31 AM   #20
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How much ice shield is recommended


What make up for in the intake ventilation...gable end vents?

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