Wall design Zone 4a
Hi all,
would really appreciate any advice on the following wall design for new residential construction in zone 4a.
Thank you for all of your input.
Brick (painted) – majority
Hardie siding – minority
Rain screen
Zip 7/16 standard/no insulation on Zip (Zipflashing tape for seams; zip stretch for window & door sills; zip flashing extending up the full length of jams)
Siga Fentrim tape to flash base of zip to concrete foundation
2×4 framing 16 o.c.
R15 Roxul insulation
1/2″ drywall
2 coats latex paint
Not trying to design/build a complicated wall system; however, would like to:
Improve on a typical wall assembly for the area (which probably uses zip flashing tape at the mudsill joint & R13 fiberglass Batts (maybe Kraft faced)
In hopes it would lead to:
Improve air tightness at mud sill joint
improve R value
Possibly reduce exterior sound penetration
Improve ability for cavity insulation to better handle moisture and dry without it being damaged from moisture
Reduces chances of mold growth in wall cavity
Reduce chances of framing timber getting “wet” and rotting over time.
Therefore questions are:
Concerns with design?
Does wall need a vapour barrier/smart barrier behind drywall (or could it benefit from a smart barrier to justify the cost)?
Also, please provide any additional input that could improve things reduce chances of future problems.
Ps: it is hoped an ERV, and possibly a whole house dehumidifier, would be added.
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Replies
Hi Roger,
I will give your post a bump. One question. Have you considered using advanced framing?
Steve
Hi Steve,
Thank you for the bump. Did consider advanced framing but assumed it would be more expensive and framers would prefer to stick with 2x4 16 o.c. framing. Not sure though.
Also, some youtube comments from advanced framing videos seem to talk it down.... yes, comments need to be taken with a grain of salt :-)
Roger,
Your proposal does not even meet the current code requirement for insulation. The IRC/2018 requires R20 wall insulation, or R13 cavity insulation plus R5 exterior insulation. Of course, your jurisdiction may still be on an earlier code, but with your stated goals of "improving on the typical wall system," at least meeting the current national codes should be a priority. You could add ZIP-R6 sheathing to be a little bit better than code with few changes to your other details. Exterior insulation gives a significant bump in total wall performance and it also reduces condensation risk within the wall cavities.
That said, if you pay careful attention to air sealing details and meet less than 1 ACH50 leakage rate, you will be far better in that respect than most production homes. By itself, this measure will significantly improve energy and moisture performance. With this level of air tightness, and HRV/ERV will be required. You may also need dehumidification in some seasons.
Vapor retarders are not required in zone 4a, but are still not a bad idea. Vapor retarder paint is Class III and would probably work just fine. So would a smart membrane. Even with exterior air sealing, your should try to seal the interior too, either with a smart membrane, or with airtight drywall details.
>"The IRC/2018 requires R20 wall insulation, or R13 cavity insulation plus R5 exterior insulation."
That's right! In fact it doesn't even meet IRC 2o12! We would have to go all the way back to IRC 2009 for a 2x4/R15 wall to meet code!
A simple improvement would be to move to 2x6/R20 cellulose to bring it up to code. That's greener than R23 rock wool since it has a negative carbon footprint, and performs better than rock wool during the shoulder seasons due to it's thermal mass characteristics.
A better improvement would be to also incorporate some advanced framing features, such as 24" o.c. stud spacing and single top plates with the rafters/trusses aligned directly over the studs.
How cold does it get where you are, and are you willing to share your location? That might help fine tune some of the recommendations, specifically concerning vapor barriers. Zone 4a contains a few varied microclimates.
In our neck of the woods (also in 4a), you've already improved on the "typical" wall by incorporating a rain screen, sealing the bottom of your Zip, and selecting Rockwool. Here in Tennessee we languish in the 2009 IECC, so everything here is hardie board nailed to a housewrapped OSB, 2x4 studs, and 1/2" drywall. No rain screen, and if there is Zip it is not sealed at the top or bottom.
I'd strongly encourage you to be very careful about your brick veneer and make sure any mortar droppings behind the brick don't cut off your ventilation channel and provide opportunities for water to perch.
I'd also strongly encourage you to think through how you will handle the wall-to-ceiling air barrier transition. Are you pursuing a vented attic, non-vented, modern single pitch roof/ceiling assembly, or something else?
Thank you for the replies and good suggestions/heads ups.
It would be for a home in the North West corner of Arkansas. It appears the IRC would be 2012 and the IECC would be 2009, hence the 2x4 with R13. The attic would be the "standard" vented type.
Follow ups (and apologies for not being as advanced as most with regards to building practices - just trying to understand, as best one can, in order to build the best one can within a certain budget):
i). "A simple improvement would be to move to 2x6/R20 cellulose ": Would that imply framers would frame the house the same way they would with 2x4s, they just use 2x 6s instead? Or, would it cause the framers to have to rethink things and possibly be an upcharge for labor?
ii). "R13 cavity insulation plus R5 exterior insulation": Do framers frame the house as normal, with 2x4s, and then R5 is "attached" (nailed, glued, or what?) to the exterior framing and then leave a 1" gap for brick work.
iii) Would it be preferable to use zip R6 instead of OSB sheathing and then an R5 XPS 1" product. If either is ok, would one need a WRB between the exterior OSB and 1"XPS
iv) "think through how you will handle the wall-to-ceiling air barrier transition": Could you expand on this, please?
Would really appreciate any links that could provide solid details on the above questions or suggestions made earlier by others.
Thank you.
> zip R6 instead of OSB sheathing and then an R5 XPS
Either should be OK in your zone, but the latter will provide some warming of the sheathing - a more conservative design.
Either would be OK....
.... except for the fact that all XPS sold in the US is blown with HFC blowing agents currently banned under the Kigali Amendment to the Montreal protocol, due to the extreme global warming potential...
...whereas polyiso is blown with much lower impact hydrocarbons.
The "...R5 XPS..." is actually only warranteed to R4.5, and after 50 years of service would likely be closer to R4.2 (if you're hoping to cash in on that warrantee...) This is because as the climate damaging HFCs diffuse out over several decades it's performance declines to that of EPS of similar density. (EPS is also blown with hydrocarbons, with very little residual blowing agent left by the time it leaves the factory, and has stable performance over time.)
Since you've got a vented attic your air barrier is going to need to be at the attic floor level, or in other words the ceiling of the rooms beneath the attic. The wall assembly you've described relies on the Zip sheathing as the air barrier located at the outside side. That air barrier needs to turn the corner across the top of the exterior wall and start heading horizontally across the plane of the attic floor. Standard practice is to just hang drywall from the rafters. If that's all you have, then you've got a leak between the sheathing and the top plate, and then between the top plate and the drywall. Already we have 2 significantly leaky joints and we haven't even made it into the field of the attic yet. Ok, you could get up in the attic later with some gun foam and squirt it into the plate/sheathing and plate/drywall cracks. But you can't get under the joists where they lay across the plate. And what if that joint is out in a really tight eave, or behind a big rigid duct, or obstructed in some other way? Just a little bit of forethought can yield a lot of benefit. Some sort of air barrier material could be laid across the wall plate before the ceiling joists are set, such as a tape or fully adhered membrane or even rips of OSB, to connect the Zip sheathing to the ceiling. Some builders go so far as to construct an air barrier ceiling out of OSB or the like, tape all the joints, then hang wood strapping from that which acts as a service cavity for wires and such, then hang drywall as a finish layer. Basically anywhere 2 different things come together in your envelope, there will be a leak. A little strategic planning around these joints and penetrations will help minimize their impact on your air leakage.
Thank you Nathan for the great info.
Do you have any links that go into greater depth on what you said or air barrier location options?
*** I am a little confused....
i) why did you say the zip layer is the air barrier point instead of the drywall layer?
ii) Can one choose the drywall as the location/point for the air barrier?
iii) If so, does that mean one can avoid the concern for where you mention "turning the corner where the top of the zip sheathing and top plate meet, as tape and mud would be used to "seal" the drywall wall and ceiling joint?
iv) If yes to (iii), does that now mean sealing all the drywall holes from outlets etc. becomes a serious component.
v) if yes to (iv) does a smart vapor barrier help at all?
Thank you.
I (probably others too) would really benefit from in-depth info and photos showing what you touched on.
Thanks!
The exterior sheathing is a more rugged and robust primary air barrier than drywall, but ideally BOTH would be detailed as air barriers.
A smart vapor barrier doesn't necessarily help for detailing the interior side air barrier, but it doesn't hurt. It's not particularly easier to seal the electrical & plumbing pentrations of a membrane than it is to seal the penetrations of drywall.
In a zone 4A climate nothing more vapor tight than standard interior latex on drywall is needed for controlling wintertime moisture drives from the interior into the assembly, and any amount of exterior R makes it even less necessary.
For (I): yep, do everything normally but use 2x6s instead of 2x4s. This makes for a thicker wall (a more solid wall too), and more room for insulation due to the deeper studs.
For (ii): yep, normal framing here, but then rigid foam insulation gets installed over the structural sheathing. Usually nails or screws are used, but sometimes glue. I’m not a big fan of glue here myself. You get some extra work with some of the exterior detailing (around windows, sometimes with siding), but the framing is the same inside the walls.
For (iii): this isn’t such an easy answer. This being a green building side, XPS isn’t particularly popular. EPS or polyiso are better on the green side of things. Zip uses polyiso. You will get the same thermal performance with zip or the same amount of polyiso installed over regular wood sheathing. Zip tends to be a better grade of OSB compared to the usual stuff that would be used, and you get a factory applied WRB which, when taped, seals up very well. Zip basically saves a few steps in the construction process.
You can use XPS as a WRB, but it doesn’t tape very well so I wouldn’t trust it in the long term. If you use foil faced polyiso instead of XPS, then taping the joints between sheets is no problem so you’re safe.
For (iv): you need to make sure you can keep your air barrier intact. Some things are easier than others to actually implement here. I like to go with the airtight drywall approach, with a bead of sealant between the top plate and the drywall, and paper tape and mud between the wall and ceiling drywall.
Note that if you go with 24” stud spacing, I highly recommend going with 5/8” drywall instead of 1/2”. 1/2” drywall with 24” stud spacing makes for too much flex in the drywall in my opinion. If you go up to 5/8” drywall you keep a stiff wall (I use 5/8” all the time for this reason, regardless of stud spacing). It’s very little extra money to go up to 5/8” drywall.
Bill
Thank you for taking the time to reply Bill.
If possible, do you have any thoughts on the "smart vapor barriers"? Essentially, I'm wondering if I should consider one, and if so, are their any brands or green ones to look out for.
Thanks.