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Old 02-15-2010, 06:21 PM   #1
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Radiant Barrier + Insulation with Metal Roof


Hi,

So I've spend hours and hours and hours researching insulation in an attempt to educate myself and also to finaliz my building material list for my 30'x40' workshop. I need some expert advice. Right now, I've decided to go with a combination of Prodex radiant barrier (non perforated so serves as a vapor barrier too installed under the purlins (1.5" air space gap) of my metal roof + R19 batting + R10 2" polyiso foam panels.

I want to ensure that this particular combination of materials will be effective for insulation, moisture and radiant barrier. Would you mind look at the image & pointing out any concerns? I'm most concerned about the batting installed in between the 2" polyiso board and the radiant/vapor barrier.

Ultimately, when you look up at the roof you will see the polyiso board. I'm not finishing further. Hoping this will provide good insulation characeteristics. I'm located in Austin, TX so the summers are quite brutal and we do get humid at times.
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Last edited by customfab; 02-15-2010 at 06:24 PM.
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Old 02-15-2010, 06:35 PM   #2
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Radiant Barrier + Insulation with Metal Roof


How is the roof going to vent with the R-19 blocking the path?

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Old 02-15-2010, 08:54 PM   #3
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Radiant Barrier + Insulation with Metal Roof


actually air moves pretty good thru the edges of batt insulation,those edges should be blocked off
what would concern me is having the fg in between 2 vapor barriers
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Old 02-15-2010, 09:46 PM   #4
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Radiant Barrier + Insulation with Metal Roof


In more moderate climates, the condensed water could evaporate and pass through the radiant barrier into the attic space. While most uniform aluminum coatings do not allow water vapor to pass through them, many radiant barrier materials do allow passage of water vapor. Some allow water vapor passage through holes or perforations, while others have substrates that naturally allow water vapor passage without requiring holes. However, excessively large holes will increase the emissivity and cause a reduction in the radiant barrier performance. The ability to allow water vapor to pass through radiant barrier materials is not needed for the roof applications.

A radiant barrier reduces the amount of heat radiated across an air space that is adjacent to the radiant barrier. The primary function of conventional insulation is to trap still air within the insulation, and hence reduce heat transfer by air movement (convection). The insulation fibers or particles also partially block radiation heat transfer through the space occupied by the insulation. From: http://www.ornl.gov/sci/roofs+walls/radiant/index.html



http://docs.google.com/viewer?a=v&q=...oCq-P91xd-Qiaw

http://www.gravesroofing.com/Insulation.htm

Careful of the fasteners: http://www.eco-structure.com/cool-ro...hats-cool.aspx

Polyiso vs. XPS: http://www.roofingcontractor.com/CDA...00000000484938
The radiant barrier is in the correct spot, the foam is not.

Be safe, Gary
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Old 02-16-2010, 08:20 PM   #5
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Radiant Barrier + Insulation with Metal Roof


Quote:
Originally Posted by GBR in WA View Post
The radiant barrier is in the correct spot, the foam is not.

Be safe, Gary
Gary,

Could you elaborate on this further for my education?
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Old 02-16-2010, 08:22 PM   #6
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Radiant Barrier + Insulation with Metal Roof


Quote:
Originally Posted by tomstruble View Post
actually air moves pretty good thru the edges of batt insulation,those edges should be blocked off
what would concern me is having the fg in between 2 vapor barriers

"fg"?

Sorry, my drawing for the soffit vents isn't accurate. The foam board certainly would not be blocked vent flow.
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Old 02-16-2010, 09:12 PM   #7
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Radiant Barrier + Insulation with Metal Roof


What thickness is the R-19 Batt insulation?
I'm not familiar with that, but it might be the stuff we put into a 2 x 4 wall.
That should allow 2" (+/-) above it for ventilation?
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Old 02-16-2010, 09:25 PM   #8
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Picture on page 3: http://danperkinsroof.com/1108_JCL_Perkins_A.pdf

Be safe, Gary
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Old 02-16-2010, 10:47 PM   #9
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Radiant Barrier + Insulation with Metal Roof


Quote:
Originally Posted by customfab View Post
"fg"?

Sorry, my drawing for the soffit vents isn't accurate. The foam board certainly would not be blocked vent flow.
fiberglass
in your illustration air could flow thru the ends of the fiberglass
reducing its effectiveness ,the ends and sides should be blocked off from airflow
use a perforated radiant so any moisture can get thru to be vented

you could go with a product that specially designed for your type of project
http://www.insulation4less.com/


Last edited by Tom Struble; 02-16-2010 at 10:51 PM.
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