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polyiso vs styrofoam

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Posted by Jeff Thies on November 14, 2005, 4:20 pm
   I've been looking for 1" foil faced polyiso at the usual suspects
(Home Despot and Lowes) and do not seem to find it. I do find 1" foil
faced styrofoam and thinner polyiso without the foil. Perhaps these are
temporary shortages???

  It seems that polyiso has about a 40% greater R value, but are there
any other properties of styrofoam that would make it unsuitable for a
solar collector? (probably air)

   Google is is unrevealing as to polyiso vs styrofoam comparisons. I've
seen some annectdotal comments that make me think polysterene
(styrofoam) is more flamable and more water resistant. Resources?


Posted by Jeff Thies on November 14, 2005, 4:25 pm
Jeff Thies wrote:

I just found this: <URL:
http://www.pima.org/technical_bulletins/tb201.pdf  >

which indicates vastly inferior performance (of styrofoam) at temps over
165 degrees as well as some other nasties.


Posted by Ecnerwal on November 14, 2005, 4:54 pm

Perhaps you should try an actual lumberyard. Mine has Celotex in any
thickness you'd like, and I've been noticing some interesting things as
I watch prices there and at HD - for instance, HD has a roll of 9" wide
flashing (Vycor) for about the same price that the lumberyard does. But
HD's is 33 feet long, and the lumberyard's is 75 feet long...

Cats, coffee, chocolate...vices to live by

Posted by daestrom on November 15, 2005, 9:55 pm

Be careful about the '40% greater R value' claim.  Polyiso, *with a foil
face* can have a higher R value than polystyrene, but it's the foil face
that makes the difference.  I bought some polyiso at Lowes, 1" thick with
single foil face.  Reading the printed information carefully, I found that
it is R-value 5.0 if installed between two other layers, and R-7.6 if
installed such that the foil faced a dead air gap.  Unless you're building
with a dead air gap between it and another layer of material, you don't
really get the full R-value of 7.6.

Unfaced polystyrene is a fire hazard and doesn't meet building codes in many
areas.  It's okay if you're putting a layer of drywall or sheathing over it
or something, but it can't be left exposed (much like kraft-faced fibreglass
batting).  Of course, a collector may not be subject to building codes, but
there you are.

The polyiso I bought didn't mention if it met fire codes on the un-foiled
side, so I don't know about it.

Polystrene probably wouldn't be the best choice for a collector if you
expect to get high temperatures from it, it tends to degrade/soften if
heated much past 150F.


Posted by waayne langille on November 16, 2005, 12:20 am
 I use Johns Manville Ap foil faced poly foam,,good to temp 250 f .

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