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insulation in a cathedral ceiling


lori7's Avatar
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02-06-04, 09:03 AM   #1  
lori7
insulation in a cathedral ceiling

Our home has a cathedral ceiling in the living room. Since we built it, we have had condensation problems. Last year, we put up 1 inch foam board with 8 inches of fiberglass below it. Both ends are sealed and the air flows above the foam board for roof ventilation.
We just had a warm spell and still have condensation problems. We have tried about everything except having no roof ventilation and foaming the space completely.
Could there be a reaction between the foamboard and the fiberglass? There is a vapor barrier on the interior side of the fiberglass that is also taped at seams.
Help!

 
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02-06-04, 03:14 PM   #2  
Where exactly is the condensation occurring? On the underside of the roof? The ceiling? Where on the ceiling? etc... Are you experiencing Ice Damms? Do you have recess lighting in this ceiling?

 
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02-06-04, 04:52 PM   #3  
lori7
The condensation occurs like rain when it is really warm, when we just have a mild warm-up, it runs down the wall after running down the ceiling behind the vapor barrier. We do not have drywall on one portion of the flat ceiling where the 1x6 tongue and groove meets it, and we saw the insulation developing wet spots. There is a 4 mil plastic vapor barrier over the insulation.

 
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02-06-04, 04:56 PM   #4  
lori7
Sorry, I forgot the second part of the question. We never have had ice dams nor do we have recessed lighting. We have 2x10 beams with 1" foamboard continuous from the wall to the attic. Under the foamboard is 8 1/2 inches of fiberglass. Both ends of the foamboard are sealed from any air infiltration.

 
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02-06-04, 06:26 PM   #5  
http://www.eere.energy.gov/consumeri...heets/bd4.html

On this site, the topic you want to read is "Perm Ratings".

Your problem is the foam board has too low of a "Perm Rating" and it is acting as an "Air Boundary".

The 4 mil plastic should be your vapor barrier and air boundary. On the cold side of the vapor barrier, all materials should have a five times greater "Perm Rating" and there should not be an air boundary. Do not confuse Tyvek, which is an air barrier with an air boundary. The "Air Boundary" is the surface area of the structure that prohibits air as it expands from being heated to enter the insulation. In most cases, the vapor barrier serves as the Air Boundary.

Remove the foam board and your problem should go away.

 
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