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Old 01-28-12, 01:21 PM   #11
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Quote:
Originally Posted by S-F View Post
...I imagine that there will be a BSC article about this soon.
That is where the last link takes you.

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Old 01-29-12, 06:55 PM   #12
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do you think that using cellulose for the entire roof would work fine for a hot roof installation? I am seeing mixed reviews online about doing that.
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Old 01-29-12, 07:32 PM   #13
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do you think that using cellulose for the entire roof would work fine for a hot roof installation? I am seeing mixed reviews online about doing that.
People do it successfully every day and have been for decades.
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Old 01-29-12, 08:33 PM   #14
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Really? That may change my mind about this. I am stuck between paying big dollars to have the whole thing sprayed. Or figuring out some DIY way to achieve a high quality job at a smaller price point. I would like to use the mooney wall concept to give myself at least 6 inches of insulation, and to break the thermal bridge for the entire roof structure. I guess maybe it wont be sealed 100 percent air tight but honestly who knows what condition the insulation is in the rest of the house and right now I dont want to figure it out. So even with an air tight second floor I am sure I'll still have a somewhat leaky house. If I went cellulose could I go unvented completely?
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Old 01-29-12, 08:39 PM   #15
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If you went with any insulation you could go completely unvented. I do this all the time. Every week I do this at least once. It's because this is how the auditors out here like to deal with these damn capes.

Don't overlook the air sealing because the rest of the house isn't passivehouse tight. Don't throw the baby out with the bath water! Top and bottom are where you need air sealing the most. Basement and attic. The middle of the house being leaky has a much smaller effect.

One step at a time. If you do it right, slowly every step of the way you will one day have a comfortable house that will last a long time and help you weather the tides.

I think BSC had an article recently about hot roofs. You might want to scare that you to give you some peace of mind and a little more information.

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Old 01-30-12, 08:07 PM   #16
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I read a study about hot roofs and cellulose and it depends on what your climate is. If you live in Indiana and want to put cellulose in to make a hot roof, you will end up with moisture on the roof deck and in the cellulose which will rot your roof deck and cause mold. They suggested 2 or 3" closed cell spray foam then do cellulose.

I have 2x8 rafters with r19 cathedral ceiling and r11 on the knee walls. I have thought about replacing the cathedral ceiling insulation with foil faced polyiso leaving 1" vent against the roof deck and stacking up the polyiso to give me over r40 but it is hard to justify the cost and effort of tearing down all that drywall.
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Old 01-30-12, 08:16 PM   #17
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There is no difference between a roof and a wall as far as insulation is concerned. I have opened up many hot roofs full of cellulose that have been in place for decades and I have never seen rot caused by this.

What was this study? Was it just theory or was there empirical evidence?
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Old 01-31-12, 05:08 AM   #18
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Let me know if I interpreted it wrong or something.

http://www.buildingscience.com/docum...d-roof-systems
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Old 01-31-12, 04:36 PM   #19
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That's the article I was mentioning earlier. It says that cellulose is fine but does list precautions for all types of hot roof assemblies.
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Old 01-31-12, 06:34 PM   #20
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There is another approach to this insulating problem...

A hybrid system, where closed-cell foam is utilized, next to the weather side, for it's better R-value, and also for it's exceptional air sealing qualities. Also used in this system is cellulose, next to the living space side... where it's R3.8/inch provides insulation at a lower cost than closed-cell foam.


There is controversy with such a system, because as moisture-laden air is migrating through the cellulose part of the declining temperature gradient, the possibility exists that the dew point might be reached in the cellulose layer, thus dropping moisture within the wall (or roof).
The fix is to make the closed-cell foam layer thick enough that the moist air will be immobilized before it reaches the dew point.

The last word on this issue seems to be this document.

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