Condensation in wall cavities
I’m hoping you experts can help me better understand the impact of potential condensation within a wall cavity in the following scenario:
Huber is selling their product called Zip R sheathing which is rated at a 6.6 R-value. It’s been summarized in the past that a safe guideline for the use of rigid foam “exterior” to the sheathing would be an R-value of 7.5 for zone 5 with 2×6 walls. This product places the 1″ polyiso layer “interior” to the 7/16″ Zip sheathing which would be affixed right up to the 2×6 studs. I was looking to insulate the wall cavity with either a higher density blown cellulose or fiberglass product followed by 1/2″ drywall and a latex paint. In a cold climate, the drying potential would primarily be inward with the polyiso exterior to the studs, correct? Is this a recipe for potential mold and rot issues down the road by slightly missing the 7.5 value? The condensation wouldn’t affect the wall sheathing since the polyiso is on the inner side of the wall … but would it then just settle within the blown wall insulation and possibly impact the wall studs and bottom plate over time? If that were the case, then this product would be limited to 2×4 construction in zone 5 where the required R-value is 5 or need additional closed cell insulation applied within a 2×6 wall cavity prior to applying any blown insulation, correct? I’d appreciate your comments.
Following is a link to the Zip R site where they provide several links on tabs to the right of the web-page related to overview, installation tips, etc.:
http://www.zipsystem.com/zip-system/rsheat.aspx
With the DOW SIS product being discontinued, is this the only other “all-in-one” alternative?
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Replies
Milan,
I’ve raised the same issue for a while. In my opinion rigid foam on a wall needs to be on the outside of the OSB/Plywood sheathing; that’s why I do not spec out Zip R Sheathing. I think their regular Zip panels for wall and roof sheathing are great, but I’m still skeptical on the Zip R Sheathing.
FYI, I received a letter from our Dow Area Rep. a few days a go, where it says that Dow has entered into an agreement with Ox Engineered Products, a subsidiary of Ox Paperboard, LLC, who will manufacture, market and sell STYROFOAM SIS™ Brand Structural Insulated Sheathing. Dow will continue to support the STYROFOAM SIS™ trade name and brand as part of their overall portfolio of insulation solutions and Ox Engineered Products will market and sell this technology under the STYROFOAM SIS™ brand name.
Armando,
Thanks for the follow-up ... I do recall your asking about the product as well in the past. It would logically seem to make more sense to join the sheathing directly to the studs for structural reasons and then keep the sheathing warm by insulating from the exterior. But this product seems intriguing as an all-in-one solution with simplified window installation, etc. Provides structure, insulating qualities for a thermal break and can function as a water & air barrier with the use of their Zip tape, although I've read that many builders still prefer to add a housewrap over it all to ensure proper window flashing. From an R-value perspective in zone 5 with 2x6 construction, would the 6.6 R-value of the product create some real issues within the wall if a proper air barrier was established and the cavities were filled with blown cellulose or fiberglass? Interesting to hear that the Dow SIS product will continue to be available.
Milan,
Q. "In a cold climate, the drying potential would primarily be inward with the polyiso exterior to the studs, correct?"
A. Yes.
Q. "Is this a recipe for potential mold and rot issues down the road by slightly missing the 7.5 value?"
A. Well, this assembly would be on the risky side of the guidelines. In most cases, the wall would probably be fine -- especially if the homeowners made sure that the indoor humidity level didn't get too high during the winter. But .. why choose a wall assembly that is on the risky side of the guidelines? If I were building a house, I'd want more of a safety cushion.
Q. "... then this product would be limited to 2x4 construction in zone 5 where the required R-value is 5."
A. My own theory about why the Dow SIS product had troubles is that its R-value is so low that it can only be used in a narrow range of climates.
Martin,
Thanks for the follow-up.
So ... to use it as is really limits its usage to 2x4 construction which would yield something like R-15 for 2x4 blown fiberglass insulation along with R6.6 from the Huber Zip R sheathing providing just short of R-22 discounting any framing factor penalty.
Sounds like a safer and better combination would be to replace the all-in-one product with individual components such as:
- Latex paint
- Drywall
- 2x6 studs
- Blown fiberglass @R-24
- Air sealing with OC Energy Complete
- 7/16" Advantech sheathing
- 1 1/2" XPS @close to R-7.5
- WRB like Tyvek, etc.
Net result is closer to R-32 ... and a safer wall, correct?
Re: 1.5" of exterior foam. https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing
Milan,
Your suggested wall sounds good. You didn't mention whether or not it would have a ventilated rainscreen gap between the housewrap and the siding; I would recommend that you include one.
John,
Thanks for the link to the full article. I've printed off most of these "How To" articles in the past, but hit PRINTER FRIENDLY which only gives the article ... not the follow-up Q&A. Although the main content covers the topic well, it's the Q&A which reinforces and helps to better educate.
Martin,
Thanks for the suggestion re: rainscreen gaps. Dr. Joe indicates in his Builder's Guide to Cold Climates, that all that's needed is a drainage plane/drainage space as the annual rainfall in central OH falls between the 20" and 40" range. But that changes to a rainscreen once you hit southern OH. The rainscreen then is just an additional insurance policy to ensure that the back of the cladding is ventilated and not just drained ... again, a safer wall system. Products such as Benjamin Obdykes Hydrogap, are essentially only providing a drainage space, not a rainscreen, right? To get the rainscreen, it would be best to use something like 1x4 furring between the cladding and the housewrapped foam, correct? Fiber cement cladding would then be attached directly to the furring.
The subdivision has a requirement for either a 42" tall brick or synthetic stone watertable below the fiber cement wall cladding. Would I just recess the upper wall slightly onto the poured wall foundation below to ensure that the metal lath affixed to the furring is in the same plane as the poured wall below since the synthetic stone is applied directly to the concrete portion? Really is a question of how best to deal with the transition between the wood wall and the foundation since the furring projects the wall outward of the foundation if it were all in alignment to begin with
Thanks again for the feedback.
Milan,
In regards to your using XPS rigid insulation, I know it has a significantly higher Global Warming Potential than EPS rigid insulation: https://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/avoiding-global-warming-impact-insulation
I know some builders use EPS as exterior sheeting, but have heard other builders suggest it has it's short comings in this application. I know very little about the durability of EPS as exterior insulation, but do know it has the GWP advantage. 2" of EPS would give you somewhere in the R8 neighborhood and wouldn't cost any more than 1.5" of XPS.
Anybody have any feedback as to why one wouldn't want to use EPS in lieu of XPS here?
Tyler,
The main disadvantage of EPS in this application is its fragility. It's more likely to crack and crumble than XPS. However, it's possible to order high-density EPS which is probably less fragile than the run-of-the-mill EPS you usually see.
Hi Tyler,
Dr. Joe wrote an interesting article a few months ago referencing some issues with shrinkage of EPS panels. He didn't see the same issue with the XPS foam used on the home. The EPS was very thick and only applied in 1 layer. He proceeded to replace it with multiple layers of Polyiso. I'm not certain though whether high density EPS would have performed any better. Most likely you wouldn't see the issue to the same extent if thinner layers were used.
http://www.buildingscience.com/documents/published-articles/pa-foam-shrinks