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-   -   is there a problem with vapor barrier? (http://www.diychatroom.com/f19/there-problem-vapor-barrier-12872/)

walenta 10-30-2007 11:33 AM

is there a problem with vapor barrier?
 
I am remodeling my attic into liveable space.
as part of it I am insulating the roof/ceiling

I am planning to use 2 differnt materials for insulation to optimize cost and energy efficiency (ie my rafters are only 6 inch deep and I have a cedar shingle roof fixed on nailers running across rafters) - but I want to make sure I am not causing a potential vapor barrier issue by trapping mositure....

I am planning to put from the inside to outside...
1. Drywall
2. 2 inch rigid foamboard (on top of rafters)
3. R13 Fiberglass (in between rafters)
4. 2 inch rigid foamboard inside rafters (against nailers)
5. existing nailers and shingles

any views if this will cause any problems?

AtlanticWBConst. 10-30-2007 04:19 PM

As far as layering your insulation goes, there shouldn't be a problem as there is not vapor barrier stated anywhere in your description. See below regarding vapor barriers...



Standard Building practice


Problems:

1.) Sheetrock should be installed directly on the rafters, NOT over the foam board. If you do it the way you planned, every screw/nail is going to end up popping over time.

2.) A vapor barrier should be installed between your warm side (conditioned side)and the cold attic.

3.) You will need to install rafter vent for air flow in your rafter bays between the roof sheathing and the insulation.

Ron6519 10-30-2007 11:02 PM

You need a space for air circlation under the cedar shingles. Lose the last layer of foam against the nailers.
You also need a source of intake at the eaves and exhaust at the peak.
Ron

walenta 10-31-2007 08:35 AM

what is the right vapor barrier
 
thanks for the good ideas and advice...

I looked into the DOW foam baords - the TUFF R product actually has aluminum foil on both sides versus the Styrofoam which is just the Polyutherane...

advantage of the TUFF R is that it has a 6.5 R per inch versus the Styrofoam only has 5R per inch ...

however I don't know if the aluminum foil can cause any problems regarding vapor barrier, specifically if I layer multiple boards to higher thickness?

if the aluminimum is not a problem, do I need an additional vapor barrier - and what would that be?

Ron6519 10-31-2007 11:40 AM

It makes no sense to do multiple layers. You do not want to stack these sheets on the joist and then install sheetrock. You'll spend extra money for longer screws and the holding power will be diminished the more panels you put on. I would bump out the joists enough for correctly installed R-19 or R-25 and put one layer of foil panel over the top. You only want one vapor barrier. If using the foil panels, tape the seams with foil tape and use unfaced fiberglass batts.
Ron

AtlanticWBConst. 10-31-2007 04:15 PM

Quote:

Originally Posted by walenta (Post 70876)
...
if the aluminimum is not a problem, do I need an additional vapor barrier - and what would that be?

It is against building code to have a double vapor barrier (2 layers) as it creates an "Envelope" for moisture (from moist air) to be trapped.

walenta 11-01-2007 07:08 AM

ok - fair enough, however i am still not sure of the right course of action....
The TUFF R board comes with the foil, the Styrofoam does not....
is my best bet then to use styrofoam first (5 inches) and then a layer of TUFF R (towards the inside of the attic) - what about the point that the TUFF R has the foil on both sides (does it matter?)
......
btw I live upin NEW YORK, where it gets cold in the winter....

Ron6519 11-01-2007 10:12 AM

The answer to the question is in my previous post.
Ron

lee polowczuk 11-01-2007 01:26 PM

me too
 
I am considering doing this i my attic as well..

so it's R-25 or better between the ceiling rafters...then a moisture barrier on top.... then sheetrock.

Try to explain the vent issue again

thanks

AtlanticWBConst. 11-01-2007 04:56 PM

Quote:

Originally Posted by lee polowczuk (Post 71072)
...
Try to explain the vent issue again

thanks


Explanation:

http://secure.owenscorning.net/Porta...TA%20SHEET.pdf


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