| Author |
Message |
David R. Combs, CSI, CCS, CCCA
Senior Member
Username: davidcombs
Post Number: 307
Registered: 08-2004
| | Posted on Wednesday, August 12, 2009 - 04:54 pm: |    |
NEW SCENARIO . . .
We have typically NOT used polyiso in cavity wall applications, for fear that it is not really suitable for wet/damp locations, and will lose its R-value when it gets wet.
Do others share this appoach?
Reason for asking:
We are hearing from a product rep (for a certain polyiso manufacturer) that this "fear" is no longer warranted, since there is now a newer generation of polyiso that CAN be used in cavity wall applications.
Has anyone else heard of such a family of products?
As always, a sincere Thanks in advance for the comments and feedback. |
George A. Everding, AIA, CSI, CCS, CCCA
Senior Member
Username: geverding
Post Number: 479
Registered: 11-2004
| | Posted on Wednesday, August 12, 2009 - 05:41 pm: | |
The polyiso with an appropriate facer (foil? if I remember correctly) is acceptable for cavity walls. The idea is the little water that gets in will run down the face. I think you need to tape the seams too.
Some architects prefer polyiso because you get more bang for the inch with thermal resistance than polystyrene, others are still concerned about the possibility of getting wet in the cavity and stick with polystyrene, so I have specified both products recently.
PIMA would be a good place to ask the question.
George A. Everding AIA CSI CCS
Cannon Design - St. Louis, MO |
J. Peter Jordan
Senior Member
Username: jpjordan
Post Number: 350
Registered: 05-2004
| | Posted on Thursday, August 13, 2009 - 05:00 pm: | |
Polyiso is an open cell material which makes it more prone to absorbing moisture and losing it's thermal resistance. Closed-cell products would work better in the long run, IMHO. |
Sheldon Wolfe
Senior Member
Username: sheldon_wolfe
Post Number: 351
Registered: 01-2003
| | Posted on Friday, August 14, 2009 - 12:56 pm: | |
Dow and OC offer extruded polystyrene with R value of 5.6 per inch. |
(Unregistered Guest)
Unregistered guest
| | Posted on Friday, August 14, 2009 - 12:11 pm: | |
Polyiso is NOT an open cell insulation. It is closed cell. You can read all about it here:
http://www.pima.org/ContentPage/ContentPage5,45.html
R-value is constant with foil facers front and back (see Dow Thermax). Products with permeable facers are the ones that need to be looked at in terms of LTTR (usually only with roofing insulation). |
ken hercenberg
Senior Member
Username: khercenberg
Post Number: 33
Registered: 12-2006
| | Posted on Wednesday, August 19, 2009 - 06:26 pm: | |
The Dow system was presented to me last week as follows:
1. Eliminate the gyp sheathing by attaching the polyiso directly to face of stud and taping the seams to create a vapor barrier.
2. Follow up with a spray urethane foam insulation applied to the back side of the assembly to completely seal gaps, voids, openings, etc. The spray foam is closed cell and seals around the studs as well as the back of the insulation panels, and the decking.
3. Attach an interior gyp board as your thermal barrier to comply with IBC 2603.5.5, presuming your system has been tested in accordance with NFPA 285. The Code requirement for NFPA 285 testing is supposed to ensure fire safety. The claim is that we get our air, vapor, water, and thermal barrier all in one fell swoop.
I can certainly see where this saves a lot of money, but I'm not sold on a number of levels, even with claims that polyiso doesn't hold water. Let me know when someone has it in place for several years so we can open up the wall and see how it performs. |
(Unregistered Guest)
Unregistered guest
| | Posted on Thursday, August 20, 2009 - 12:49 pm: | |
The system you have described certianly does not sound like it saves money. This will be a very expensive system to install. Instead of polyiso, consider XPS or EPS as the insulating sheathing material. Lstiburek has an excellent paper on Insulating Sheathings free on his web site. Spray foamed insulations are very expensive... It is not necessary to use these products to get a properly performing air barrier.
There is at least one exterior insulating polyiso sheathing product that I am aware of that you could also check into. I have used a silimar product (heavy asphaltic facer) only once, horizontally, at a plaza deck. it was the only solution to the particular problem we faced and it worked very well. |
Chris Grimm, CSI, CCS, LEED-AP, MAI, RLA
Senior Member
Username: tsugaguy
Post Number: 219
Registered: 06-2005
| | Posted on Thursday, August 20, 2009 - 03:58 pm: | |
A Dow rep pointed out to me recently that the NRCA in an attempt to end the LTTR controversy recommends not counting on any higher than R 5.6 per inch for polyiso due to decrease in R value over time. (See pg 74 of the 2007 edition, and pg 99 of previous edition). Whether in a roof or a wall, it is the same insulation. However, I think ASHRAE still allows calculations to use a whopping R 7.20 per inch. |
Anne Whitacre, FCSI CCS
Senior Member
Username: awhitacre
Post Number: 893
Registered: 07-2002
| | Posted on Thursday, August 20, 2009 - 04:20 pm: | |
Ken-
in many of the systems I've used, the gyp sheathing is also used to provide some rigidity to the system -- and the combination of polyiso and sprayed foam certainly isn't going to give you any additional stability to an exterior wall.
as for polyiso: I generally use 6.0 or 6.2 for the aged value of it, and think its a better product than most of the eps or epx foams out there -- it has smaller cells, and doesn't break down as easily over time; is also isn't as brittle during handling. |
ken hercenberg
Senior Member
Username: khercenberg
Post Number: 34
Registered: 12-2006
| | Posted on Thursday, August 20, 2009 - 05:19 pm: | |
If that's true, why are contractor's asking us to change our wall systems and offering credits? I don't interpret Dr. Lstiburek's writings that way. He seems to base many designs on polyiso insulation is his book "Spray Polyurethane Foam in External Envelopes of Buildings". I cannot find this conclusion in his "Guide to Insulating Sheathing" either. Perhaps we can get Dr. Lstiburek to weigh in on this himself. I would never feel comfortable speaking for him.
Please note that I am not advocating the polyiso system, just relating what was presented to me by the manufacturer who is promoting it. Frankly I would like to see it work on many other peoples' projects first. I am concerned about designing around this system. I still like the redundancy of having my impermeable thermal barrier outboard of my sheathing barriers and for that purpose I still prefer XPS, for now.
Anne, when we delegate design of CFMF the sheathing is not included in the performance. The studs must resist loads and deflection without benefit of the sheathing. I agree that adding sheathing adds strength to the assembly, another plus to the system we're already using. |
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