Can you build Double Stud Wall with Zip system?
AZVuilt
| Posted in Energy Efficiency and Durability on
Is it possible to build Joe Lstiburek’s double wall [https://buildingscience.com/sites/default/files/migrate/pdf/2014_HighR_Wall_Sheet_02_double_stud.pdf] with Zip system as A/V barrier without moisture control issues in a zone 5 region (Joe usually advises against OSB but praises strong air barrier construction for these assemblies)?
Further, how would the principles of the double wall assembly apply to cavity insulation on the underside of roof sheathing (i.e. attic insulation) if you were planning to do an unvented attic? Would this assembly still require exterior insulation boards at the roof deck to achieve a 40-60 R value?
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OSB is not an issue with double stud walls if you have vented cladding. CDX or gysump sheathing do work better and don't cost all that much more, so I don't see why one would build with OSB/ZIP for such a wall.
No matter how many layers you have, you can't build an unvented roof with fluffy only. Any unvented roofs require either rigid foam insulation above the deck or spray foam bellow it, in that case the permeability of the roof deck doesn't matter.
Thats helpful. A few follow up questions:
1. How do you ensure air tightness on a CDX sheathed structure? Would you still tape or flash seems? Which products are best for this?
2. Is there another method for a double wall where you do not sheath the interior wall frame but apply a more cost effective air barrier?
3. For the roof deck, how would you install the foam to prevent condensation issues? 1-2" of 2lb closed cell at the roof deck and then open cell over that? Or could you do 4" of GPS or polyiso on top of the roof deck and then spray 10-14" of open cell foam?
Or is all of this unnecessary and you just build a 2x6 wall with R10 continuous exterior and open cell or fluffy in the cavity (consistent with https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing)?
1. You can tape cdx and densglass the same as Zip. Zip tape actually works quite well for this, I've also used 3m 8067. You tape all the seams then install regular WRB and you flash to that the same as standard install. One more step over ZIP.
2. Walls need one solid air barrier. The simplest is the taped sheathing on the outside. It doesn't hurt to have an interior air barrier as well but doesn't have to be perfect. Air tight drywall or variable perm membrane both work, no need for extra interior sheathing.
I know the perfect wall puts a layer in the middle, but it really doesn't buy you much. Regular double stud walls with just fluffy inside work.
3. Take a look at this here for roof info:
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
I think how you build comes down to local trade preferences and building experience. Since R5+2x6 is code around me, exterior rigid walls are cheaper to build than anything else. If you are not in such area, a double stud is much simpler as it is still pretty close to standard build with just a bit more framing.
Anything with rigid or spray foam gets very expensive (also not very green) when you need large R values. You generally don't want to build an r50 assembly with foam.
Akos,
Thank you for the insights, they are very appreciated. Expanding on your #2 response about air barriers, it makes sense that a sealed exterior is the greatest insurance policy against air leaks driving moisture to a cold condensing surface (i.e. exterior sheathing in the wintertime), but following Joe's design do you think there is any benefit to a taped A/V barrier behind the interior wall? I haven't investigated the vapor permeability of corrugated plastic sheet goods but I would imagine if glued and taped properly, these sheet board could stop the transmission of warm humid air through the wall assembly?
Could a 10-12" double wall assembly look as follows (outside to inside):
1. Exterior Siding (i.e LP Smart Side, Hardie Board, Cedar, etc.)
2. Rain Screen Vent (i.e. 1x4 furring strips installed vertically)
3. Prosoco Joint & Seem Filler + 2 coats of R-Guard MVP over sheathing
4. 1/2" CDX Sheathing
5. 2x4 advanced frame wall assembly
6. 3-5" air gap between double stud walls
7. Dense packed cellulose installed between exterior sheathing and through the 3-5" air gap to the backside of the interior wall assembly
8. Corrugated plastic sheets taped and glued to interior wall assembly to serve as an internal A/V barrier (replacing the CDX in Joe Lstiburek’s assembly)
9. 2x4 advanced framed interior wall assembly
10. Rockwool batt insulation installed in interior cavity
11. 1/2" gypsum board
12. Low perm interior primer and paint
Does the corrugated plastic sheet goods replace the CDX in the assembly, or does that section still need to provide for some vapor permeability (i.e. a vapor block at this section would be problematic for durability)?
You generally don't want a zero perm element in a double stud wall. Your assembly could work with one of the variable perm membranes though. There would be some sequencing issues as you would have to have the frames and insulators out twice.
Advanced framing doesn't buy you much in a doublestud wall as you get a big thermal break from the 3.5" gap. I would limit which elements of advanced framing you use, most are not worth it.
We are in the middle of doing double stud walls with CDX. Taping the seams with Zip. The garage is built with Zip panels. The tape sticks to plywood fairly well, but not nearly as well as zip panels. When it gets wet at all, the tape will not stick. The ends of the plywood really suck up water too, so here in the late fall/early winter, my crew is having a hard time getting the seams taped. I've used a roll or two of Tescon Vana when the plywood was still a bit wet and it worked better, but if it's raining or rained in the last hour, not a chance.
Hmmm, thanks Tim. Are you concerned that through frost and condensation cycles over time that the tape will fail? Does anyone know if a combination of Prosoco's Joint & Seem filler along with their R-Guard MVP would be a superior solution vs Zip or Siga tapes on CDX sheathing?
On a current project with double-stud walls that I designed, under construction now, I had spec'd the Zip system, which is reliable and predictable. I only recommend using it on double-stud wall systems in conjunction with dense-packed cellulose, an interior variable permeance membrane and a robust rain screen on the exterior because I'm not confident that the OSB would still be performing well after 50-75 years of moisture cycling otherwise.
The owner/builder wanted to use plywood instead, so I changed to that, with a self-adhered WRB/air control layer. That is a combination that I also like; I have found that plywood quality is much lower than it used to be and the seams need to be primed and divots filled before taping, but it's more vapor-open and therefor more resilient long-term.
He ended up air-sealing the plywood seams with Prosoco joint filler and then continued with R-Guard over the entire exterior, in large part so he could do it himself more easily than installing a self-adhered membrane. I don't recall what his shell air-tested at but it was below the Passive House limit of 0.6 ACH50. It was probably the most expensive of the options but should perform very well.
Tescon Vana Sticks like no other. I'm sure the Siga stuff is as good if not better. But the price is high. Using a caulk based item would take forever. My framers already want to skip the zip tape, they don't totally see the point and all the details become difficult. If using the expensive tapes, it's worth pricing out a self adhered in comparison.
As Michael mentioned, the quality of the plywood is not so great. We switched to OSB for the roof at the recommendation of my framer. He didn't think plywood would give as flat a roof. The siding it doesn't matter as much since it's got a 3/4" rain screen.
Our roof is 24" deep parallel chord trusses with a 36" deep 4ply girder truss. Free span is 30 ft at the longest, post at each end and roughly the middle. We are dense packing to R80 at the roofline, so our attics are conditioned. This roof design was about $10k cheaper than an Ijoist rafter option we were quoted.
@Michael Maines - what did you end up doing for the roof assembly design? CC spray foam under the roof deck or exterior foam board over roof or something different?
The main house has raised-heel trusses with cellulose insulation and a vented attic. The garage has a second floor of living space with vented scissor trusses.
@Michael Maines, if you were to choose an unvented roof assembly, what would you suggest for the assembly components when optimizing in the following order: 1. Energy Efficiency, 2. Cost, 3. Wild Fire Protection, 4. Environmental Impact?