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ZIP-R panel on roof plus insulation


lt_sully's Avatar
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WV

01-15-17, 07:29 AM   #1  
ZIP-R panel on roof plus insulation

I have already installed ZIP-R (1.5") panels on my entire cottage (14x16), to include the roof. I know that it is not approved/code compliant for roofs....so please do not comment on that fact. It's already installed.

I have 2x8 rafters, and will NOT be installing a ridge vent or soffit. This will be an unventilated vaulted ceiling that I will be installing pine TNG on after I insulate further. I am also installing standing seam 26g steel on the roof.

My question is...With the sealed/air tight roof sheathing (1.5" R7), should I still install BATT insulation between the rafters? If so, faced or unfaced?

Again...I do not plan on leaving an air gap or ventilating this vaulted ceiling/roof at all. So my concern is with having foam on the outside and faced insulation on the inside...will this present a problem?

I will also be putting 2 coats of polycrylic on my pine TNG after installing.

Thanks for helping me out here!

Sully

 
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Bud9051's Avatar
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ME

01-15-17, 09:16 AM   #2  
WV is a combination of climate zones 4 and 5 from this map:
https://energycode.pnl.gov/EnergyCodeReqs/

From this link: Calculating the Minimum Thickness of Rigid Foam Sheathing | GreenBuildingAdvisor.com
There is a chart for combining exterior rigid foam with interior batt insulation to avoid condensation from interior moisture reaching the bottom of that roof. The article and chart are for vertical walls but should be close.

I'm not chasing code requirements but condensation issues.

From their chart in zone 5, R-7.5 would be ok with 5.5" of batt insulation on the inside. Where you have 2x8 rafters if you filled them with batt insulation the bottom of your roof could drop below the dew point. The solution would be less batt insulation or more rigid insulation.

One complicating factor would be the air gap between the T&G and the insulation where the air would circulate but unknown what affect that would have. I'm less concerned about final insulation value than moisture problems.

Note, T&G pine will leak a lot of air even when coated with the "polycrylic".

Not sure if this helps but it does give you more to read .

Bud

 
lt_sully's Avatar
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01-15-17, 03:52 PM   #3  
Thank you sir....I'll definitely check out these links.

As you said...my main concern is with the moisture potential between the two insulation layers (foam on the outside and faced-batt insulation on the inside).

 
lt_sully's Avatar
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01-15-17, 03:59 PM   #4  
Something I noticed about the article you linked....it speaks to exterior rigid foam....meaning "outside" the OSB. In my case, ZIP-R panels have the OSB on the outside, with rigid foam on the inside.

This would be same is hanging standard OSB roof sheathing, then spray foaming the underside of a vaulted ceiling. In my case, the foam is 1.5" on the inside. However, I still have a 2x8 rafter cavity that I can fill with more insulation.

So, I'm wondering FACED or UNFACED? I'm leaning towards unfaced to allow air/moisture to readily move through the fiberglass batt insulation all the way to the foam.

My concern being if I had vapor barrier just inside my pine TNG, followed by 8" of batt insulation, then rigid foam....what could happen?

 
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01-15-17, 04:55 PM   #5  
I would suggest you put an air barrier (drywall) on the ceiling prior to installing your t&g... not a vapor barrier. In the event there is ever any frost or condensation in the rafters it will need to dry to the inside since it won't be able to get out. You dont want to trap it.

 
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01-15-17, 06:38 PM   #6  
Your Zip-R roof sheathing has the same issues as the exterior foam referenced in the article. The issue is maintaining the inside surface of that exterior roofing or sheathing above the dew point. If you were to install (I'll exaggerate) R-100 batt insulation inboard of your roof sheathing then the inside condensation surface would certainly be almost as cold as the outside temperature, well below the dew point. Being batt insulation there will always be some air leakage will find its way through that insulation and then deposit its moisture. With less insulation, the warmth of the house is able to keep that surface warmer. From the article, your r-7 would want less than 5" of batt insulation in those cavities.

Now, it gets more complicated. If you leave the rest of the cavities empty, 9.5" – 5", then that 4" void will suffer convective air flow. One of the principles of physics is that moist air (at the same temperature) is lighter than dry air, so the humidity will collect adding more risk to the situation. See the link below on insulating cathedral ceilings.
How to Build an Insulated Cathedral Ceiling | GreenBuildingAdvisor.com

I'm not trying to make your life more difficult, just trying to lower the risk that it will have problems in the future.

My recommendation would be:
1. Add more rigid between the rafters to allow the remaining space to be filled with unfaced batt insulation.
2. Cover the bottom of the rafters with drywall (as X suggested) to act as a rigid air battier.
3. Then install the T&G pine.

Bud


Last edited by Bud9051; 01-15-17 at 06:40 PM. Reason: correction
 
XSleeper's Avatar
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01-15-17, 06:51 PM   #7  
Not that Bud needs anyone to back him up, but I agree 100%. Maybe add another 1.5" of rigid, air seal the edges of that foam, then you would be left with space equal to a 2x6... for some unfaced R-22 Roxul.

 
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