ZERH construction detail questions
I am a homeowner/builder who is planning to build a certified ZERH [Zero Energy Ready House] in the next few months (breaking ground in late July/early August) in Dallas (CZ3A – warm/humid). Pre-fabricated wall panels (2×4 frame sections) and pre-fabricated roof trusses will be used. My main goal is to get the ZERH certification using “simple” assembly processes and economical materials (“simple” meaning techniques that the local labor force in Texas can perform).
Over the last year or so I have been researching different aspects of the wall and wall-to-roof details. I have drawn them in AutoCAD (see attached) and wanted to get some feedback from the GBA community; specifically:
1. Should I attempt to “air seal” the OSB wall joints AND ZIP-R joints or just liquid seal the ZIP-R joints?
2. Are the Heckman brick ties over-kill?
3. What are the benefits/drawbacks to Prosoco FastFlash vs. ZIP Liquid Flash?
4. If I air seal behind the York Flash-Vent do I need to mess with sealing the sill area gap (between the OSB/Zip-R and foundation) with a liquid flash product?
5. On the wall-to-roof detail: What is the best way to attach the ZIP-R section between the rafters/trusses (currently shown held up by liquid flashing)?
6. Have I committed any building science “fouls”?
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Replies
Have you verified a fastener schedule and fastener specification with Huber for installing the ZIP-R backwards, as drawn in your details?
ZIP-R is designed to be installed with the OSB (and factory applied WRB) on the exterior side, with the foam side in contact with the studs. Can't find any application in the installation manual where they flip it.
http://zipsystem.com/assets/user/library/ZS_R-Sheathing_Installation_Manual-(R3-12)v21.pdf
What does the acronym ZERH stand for?
Zero
Energy
Ready
Home (or House)
https://www.energy.gov/eere/buildings/zero-energy-ready-home
You are right Dana. I should have drawn the Zip-R section differently to better illustrate what is going on. The 7/16" OSB shown (against the studs) is applied separately. The 7/16" Zip/OSB side of the 1-7/16" Zip-R panel should face the exterior. The foam part of the Zip-R panel is sandwiched between the 7/16" OSB and 7/16" Zip/OSB.
>"The foam part of the Zip-R panel is sandwiched between the 7/16" OSB and 7/16" Zip/OSB."
The additional layer of OSB is buying you what? The 1.5" ZIP-R is structural if using the manufacturer's fastener schedule. If that's not sufficient for your local hurricane requirements, interior side flat metal X-bracing can get you there for a lot cheaper than a second layer of OSB.
When the ZIP-R is at the studs it's easier to air seal using ZIP tape on the seams, and lapped over the top plate. The bottom plate may still need a bead of caulk to the subfloor and to the ZIP at the bottom of each stud-bay if you're going for PassiveHouse type tightness, but maybe the Prosoco under the stainless flashing will just as well or better.
Quincy,
A few comments:
- Why use a termite barrier under the concrete slab? They have no pathway there.
- Why is the brick ledge dropped below the rest of the slab? How would you form that up to pour?
- How is the slab insulated? That will influence all the rest of the detailing you show.
- Why the double layer of sheathing?
- Why are all the members of the trusses shown as 2"x6"s?
- The attic is detailed as an unvented one with a continuously air-sealed exterior, and spray foam. Why then does it have railed-heel trusses? Aesthetic reasons?
In Dallas a brick veneer slab on grade doesn't get much benefit from slab edge insulation- there is always a big thermal bridge, and the subsoil temperature is room temperature, so there's little point to insulating under the slab. The IRC doesn't require slab insulation in zone 3, and it can hit Net Zero without it.
The 3' depth of the energy heel seems like an ambitious R100 overkill though. IRC code min is R38, anything over R50 probably isn't going to be needed to hit Net Zero. (Armando Cobo can probably fine tune that.) https://www.greenbuildingadvisor.com/profile/Acobo
A few quick responses:
@Dana - "The additional layer of OSB is buying you what?" The additional layer is just for air tightness (even though OSB isn't airtight...I figured it was better than just the Zip-R). The home is under/near one of the D/FW runways and I can smell jet fuel in the air occasionally (pretty bad to breathe, but at least Jet-A smells good!). Because of the outdoor air quality issue, I am shooting for a blower door of 0.6-0.75 ACH @ 50 Pa.
@Malcolm -
- "Why use a termite barrier under the concrete slab? They have no pathway there." I am pretty sure it is a code requirement here (I will verify). Most people spray the ground with a termicide prior to pouring; I like the Pango wrap better (it should last longer).
- "Why is the brick ledge dropped below the rest of the slab? How would you form that up to pour?" It is pretty common to have the brick ledge offset in this area. I have never questioned why; maybe it is to better direct the water flow when flashing isn't used. Since I am planning on using SS flashing, perhaps I can pour it at the same level.
- "How is the slab insulated? That will influence all the rest of the detailing you show." Dana is correct. It just doesn't make sense (financially) to insulate the slab in our CZ.
- "Why the double layer of sheathing?" To reduce air leaks.
- "Why are all the members of the trusses shown as 2"x6"s?" The top chords are 2x6 for spray foam reasons (spray foam will cover full depth of 2x6 and 1" beyond it...to minimize thermal bridging). The bottom chords are 2x6 to fully enclose the recessed lighting.
- "The attic is detailed as an unvented one with a continuously air-sealed exterior, and spray foam. Why then does it have railed-heel trusses? Aesthetic reasons?" Exactly. Aesthetic reasons. The attached garage (my wife wouldn't agree to a detached one!) has a 12' ceiling. The rest of the house has a 9' ceiling. To prevent the garage from dwarfing the house (curb appeal) I put a 3' energy heel around the entire house.
Quincy,
-The top chords are 2x6 for spray foam reasons (spray foam will cover full depth of 2x6 and 1" beyond it...to minimize thermal bridging).
1" of spray foam won't do enough to minimize thermal bridging. Going with 2x4 top chords with 5.5" of spray foam would be higher performance. That would give you closer to R37 VS R30 with the 2x6.
- "Why the double layer of sheathing?" To reduce air leaks.
If you are worried about leaks through the sheathing, go with plywood. Plywood plus 1.5" rigid insulation would be cheaper and less work to install than OSB+ZIP R. You can also tape the seams on both the plywood and the foam for extra sealing.
Because of the VOCs from the airport, plan for an activated carbon filter on your HRV/ERV inlet.
>"The top chords are 2x6 for spray foam reasons (spray foam will cover full depth of 2x6 and 1" beyond it...to minimize thermal bridging). The bottom chords are 2x6 to fully enclose the recessed lighting."
At 6.5" open cell foam is only about R24, well below code minimum. Using HFO blown 2lb foam you'd be at about R45, but it's ridiculously expensive. The better option would be to go with plenum truss to house the ducts & air handlers, with at least a foot of depth (18" better) to accommodate blown cellulose insulation out over the top plates of the exterior wall. R50 open blown cellulose usually costs less than even 2" (R14) of HFO blown 2lb foam or 6-7" (R21-25-ish) half pound open cell foam.
https://www.greenbuildingadvisor.com/article/use-plenum-trusses-to-keep-ducts-out-of-your-attic
>" The additional layer is just for air tightness (even though OSB isn't airtight...I figured it was better than just the Zip-R)."
Actually, the extra layer of OSB would make it more difficult to fully seal the ZIP-R. ZIP and ZIP tape is actually pretty good from an air tighntess point of view, if you follow the instructions meticulously. ZIP-R is pretty expensive way to add R6 to the exterior though, compared to simply OSB/CDX detailed as an air barrier with an inch of foil faced polyiso with seams taped on the exterior.
I am planning to follow the performance path (rather than prescriptive) to "get away" with the R24 attic ceiling insulation.
We are planning on using the attic for possible future living space, so don't want to do the blown cellulose. However, I agree, the plenum truss with ducts in conditioned space would be the way to go if we had real estate in the attic to do it.
I appreciate the recommendation for OSB/CDX + 1" Foil Faced PolyISO. It is a very economical, simple, and energy effective solution.
In general, you have good details, although I think you are spending more money than necessary. Assuming I'm reading your details correctly, I have few notes:
1. The roof trusses bearing on your 2x4 top plate is 2", and most trusses require 3" bearing. I would check with the truss company.
2. You appear to show a conditioned attic, but does not show what type nor amount of insulation you are installing in the attic. If roofing material is shingles, you shall apply 2" ccSF under the decking and 7" ocSF under the ccSF for a total R38 minimum to avoid possible moisture issues on the roof decking. 36" heel trusses are more than you need, 10" would be all you need, unless you have an aesthetic reason. If you are using non-ventilated attic, you don't need ventilated soffit.
3. For ventilated unconditioned attics, all you need is 14" heel trusses. 12" for blown insulation, to include settlement, plus 2" space for baffles.
4. IMO, you need a horizontal 2-by- under the vented soffit to nail the 1x4 frieze board trim.
>"1. The roof trusses bearing on your 2x4 top plate is 2", and most trusses require 3" bearing. I would check with the truss company."
I will check with the truss company and appreciate you mentioning it.
>"2. You appear to show a conditioned attic, but does not show what type nor amount of insulation you are installing in the attic. If roofing material is shingles, you shall apply 2" ccSF under the decking and 7" ocSF under the ccSF for a total R38 minimum to avoid possible moisture issues on the roof decking. 36" heel trusses are more than you need, 10" would be all you need, unless you have an aesthetic reason. If you are using non-ventilated attic, you don't need ventilated soffit."
The attic is non-ventilated and semi-conditioned (I don't have dedicated supply/return registers in the attic and will be relying on the conditioned space below and leakage through home ceiling to "condition" the attic space). I was planning on using the ZIP roof deck as my air barrier (I think it is 12-16 perm, so I don't think it qualifies as a vapor barrier or vapor retarder). I was thinking that using the ZIP for the roof deck gets rid of the need to spray ccSF. Is this correct?
>"3. For ventilated unconditioned attics, all you need is 14" heel trusses. 12" for blown insulation, to include settlement, plus 2" space for baffles."
No comment.
>"IMO, you need a horizontal 2-by- under the vented soffit to nail the 1x4 frieze board trim."
Good point. I will consider adding this.
1. What is the reason for the 2x4s outside of the heel trusses? If is about bracing, it’s done with long 2x4s nailed to compression webs perpendicular to trusses, and using metal connectors at truss ends to the top plates. See truss manufacturer’s installation guide and details.
2. There is no such thing as “semi-conditioned attic”, is either conditioned or not. Fire code does not allow passive vents, and there is not enough diffusion thru the ceiling drywall for proper attic ventilation. You need a supply vent in the attic delivering 1cfm/50 sf of attic area.
Going under the Performance code to install ocSF and less insulation under the roof decking is very risky, even in CZ2-4. Open cell foam maybe air-permeable at 5.5”, but it is not MOISTURE IMPERMEABLE. There is plenty of documented cases of rotted roof decking here in DFW, Houston and the Southeast US. This is a sales tactic that insulation companies use here in DFW, and other regions, to be more competitive and get a sale, but the laws of physics will always win sooner or later. Do not fall in a trap!
See: https://www.greenbuildingadvisor.com/article/open-cell-spray-foam-and-damp-roof-sheathing
https://www.greenbuildingadvisor.com/article/its-ok-to-skimp-on-insulation-icynene-says,
I need to add, if the issue is costs, install a ventilated attic. Of course, now you need to design an HVAC system in the conditioned space... another set of issues.
Info about safely using only open cell foam in an attic (summary, it must be properly conditioned):
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-016-ping-pong-water-and-chemical-engineer
Jon/Armando,
Thank you for posting the links. I really appreciate the insight and knowledge given here and will condition the attic!
Should I: 1) install ccSF + ocSF + condition attic or 2) install ocSF + condition attic? Option 1 would reduce risk of underside ZIP roof deck rot to [likely] zero, but would cost a fortune. It sounds like Lstiburek is ok with not installing ccSF as long as you condition the attic.
Its all about risk management, and how much risk you are willing to live with. I design mostly expensive houses, and the last thing I or the Builders I work with want is to increase the chance to have problems and lawsuits as those clients have the money to do it. Statistically (NAHB), for every $1 spent to install a product, it costs $10 to fix it and $100 to settle a lawsuit. Are you willing to risk it?
And if you want even lower risk, design to un-vented specs and then add a vent above it. And use plywood (not OSB). Where does one stop with risk reduction?
More rationally, is there any data showing problems with attic open cell when the attic is properly conditioned (per Lstiburek)? Seems like this is the missing piece to concluding the open-cell in attic issue.
Regarding Prosoco vs. Zip liquid:
I have just finished my first project with Zip Sheathing (not Zip R). I did some experimenting with the gun grade flashing products.
The Zip black sausage grade (liquid?) flashing is spectactular (and even more expensive than Prosoco, but not by much)
Prosoco seam filler is great but harder to work with
Prosoco fast flash works pretty well as a seam filler since it is still pretty thick.
Henry makes some too but not worth buying.
Hands down, when working with Zip, I will spend the extra $100/case or so for the Zip sausage tubes due to it's great workability. Possibly still use the Prosoco when doing a liquid flash at my windows and doors. I also bought a can of Prosoco roll on liquid for my windows and doors which is thick enough that we used a trowel/spreader to apply.
Thanks for the feedback Alan; it is exactly what I was looking for.