mix 2lb. with .5lb hybrid system | Ask Mason Knowles - SPF Consultant | Spray Foam Technical Questions and Answers | SprayFoam Magazine Q&A Forums

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Amy Snyder Posted: Jan 24, 2010 10:09 AM	mix 2lb. with .5lb hybrid system What are the standards for using 2lb. sprayfoam with a .5lb over it to insulate a vaulted ceiling in cold climate ( upper N.Y.) I have a customer that wants to go 3" of 2lb against the underside of the roof deck with 7" .5lb. Will he require a vapor barrier?
mason Posted: Jan 25, 2010 08:49 AM	No, with 3 inches of closed cell SPF, you can use a higher perm rated insulation over it (i.e. cellulose, fiberglass or open cell SPF) The exception would be if the interior climate was contantly cold such as a freezer or extremely humid such as a swimming pool. Then a vapor retarder would be installed on the warm side of the insulation.
Dennis Davidson Posted: Feb 01, 2010 10:17 AM	Mason, I have an engineer disputing what you've just said. We have a very large home to do installing 4" 2lb cc foam covered by 8" of cellulose in 12" deep cathedral ceilings. He said that condensation will occur on the surface of the foam when the outside temps reach 28 degs and below. I disagree, but I need to find something to show him.
mason Posted: Feb 02, 2010 11:59 AM	YWhat is he using for an indoor air humidity for his calculations? If he is assuming 60% humidity or higher that they should be introducing mechanical ventilation to reduce the indoor humidity. I am pretty sure a WUFI simulation with indoor humidity around 55% would not show an accumulation of moisture in the assembly over time. That is the best documentation that I can advise.
Posted: Feb 02, 2010 08:42 PM	It would depend on the combination of temp, snow load on the roof and indoor RH. 28 degrees with 24 inches of snow is a much different location for the dew point in the roof assembly compared to 28 degrees and no snow load.
Michael Fusco Posted: Feb 03, 2010 09:55 AM	RUN A WUFI.....it will mimick the system and answer your questions. As much as I hate hybrid systems, thats the one way to get hard data.
Matt Kenner Posted: Feb 03, 2010 11:07 AM	On a side note, the 2007 IRC Supplement states (in section 806.4) that 3"-4" of closed cell(dependent on what climate zone you are in...5 or 6) needs applied to the underside of the roof deck for condensation control.

Author

Comments

Amy Snyder

Posted: Jan 24, 2010 10:09 AM

mix 2lb. with .5lb hybrid system

What are the standards for using 2lb. sprayfoam with a .5lb over it to insulate a vaulted ceiling in cold climate ( upper N.Y.)
I have a customer that wants to go 3" of 2lb against the underside of the roof deck with 7" .5lb.
Will he require a vapor barrier?

mason

Posted: Jan 25, 2010 08:49 AM

No, with 3 inches of closed cell SPF, you can use a higher perm rated insulation over it (i.e. cellulose, fiberglass or open cell SPF)

The exception would be if the interior climate was contantly cold such as a freezer or extremely humid such as a swimming pool. Then a vapor retarder would be installed on the warm side of the insulation.

Dennis Davidson

Posted: Feb 01, 2010 10:17 AM

Mason, I have an engineer disputing what you've just said. We have a very large home to do installing 4" 2lb cc foam covered by 8" of cellulose in 12" deep cathedral ceilings. He said that condensation will occur on the surface of the foam when the outside temps reach 28 degs and below. I disagree, but I need to find something to show him.

mason

Posted: Feb 02, 2010 11:59 AM

YWhat is he using for an indoor air humidity for his calculations? If he is assuming 60% humidity or higher that they should be introducing mechanical ventilation to reduce the indoor humidity.

I am pretty sure a WUFI simulation with indoor humidity around 55% would not show an accumulation of moisture in the assembly over time. That is the best documentation that I can advise.

Posted: Feb 02, 2010 08:42 PM

It would depend on the combination of temp, snow load on the roof and indoor RH.

28 degrees with 24 inches of snow is a much different location for the dew point in the roof assembly compared to 28 degrees and no snow load.

Michael Fusco

Posted: Feb 03, 2010 09:55 AM

RUN A WUFI.....it will mimick the system and answer your questions. As much as I hate hybrid systems, thats the one way to get hard data.

Matt Kenner

Posted: Feb 03, 2010 11:07 AM

On a side note, the 2007 IRC Supplement states (in section 806.4) that 3"-4" of closed cell(dependent on what climate zone you are in...5 or 6) needs applied to the underside of the roof deck for condensation control.

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