In search of the elusive “perfect wall” assembly
Building in climate zone 5, have read extensively on the different wall assemblies to maximize value and thermal performance. To start, the “perfect wall” would fulfill certain criteria;
– designed to allow correct “drying potential”
– would not contain mold fodder or substrate for decay organisms
– Good air and thermal performance (within reasonable cost)
– Good structural integrity and expected lifespan
– Local work force can be trusted to get it done easily
To fulfill the above I explored steel, masonry, wood stick framing, ICF etc…..
I know there are as many opinions/ideas on this as there are people. Having said that, what are your thoughts on: (from exterior to interior)
stucco/stone/brick veneer (requirement to fulfill building design – Tuscan farmhouse) this could be portland cement based stucco vs EIFS over drainable plane
CMU core with interspersed durawall
3″ EPS taped and adhered to CMU (variable thickness, pending final R-value with a target of >=30 or so, can be increased or decreased also based on final cost)
2×4 galvanized studs – creates a serviceable mechanical space and addtl insulation
Mechanical space can be insulated with fiberglass batts vs cellulose vs rock wool vs open cell flash & batt – it must allow the internal wall to dry to the interior, while everything outside the CMU dries to the exterior.
Gypsum with latex paint
Total thickness around 12″ wall.
What do you think?
Thanks in advance for constructive criticism and insightful comments.
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Replies
Sal,
This wall is expensive.
I would put all of the desired insulation on the exterior side of the CMUs.
By the way, it's hardly worth installing any insulation between steel studs. There is so much thermal bridging through the studs that the insulation is basically worthless.
To hit R30 as a whole-wall R (all thermal bridging included) in a 12" thick wall is doable, but not with 8" CMU. Even with 6" CMU and R6.5/inch polyiso or R7/inch closed cell spray polyurethane it's coming right up against the stops, and you'd have no space for the mechancical routing. You just can't get around the low-R and thickness of the CMU. An 8" CMU is still barely more than R1, and a 6" is less than R1. For a bunch of money you can inject non-expanding foam into the hollow cores and maybe hit R2-ish with a 6" CMU. That leaves you 6" for the insulation, gypsum, and stucco. Allowing 2" for stucco + gypsum + drain plane cavity, you have 4" for all insulation, including the 3.5" depth of the 2x4s. Even without the thermal bridging it then demands R7/inch to hit R30, and WITH the thermal bridging of steel studs you'll be at about half that best case, with an all closed cell polyurethane fill.
Steel is orders of magnitude more thermally conductive than wood- a nominal 16" o.c. studwall with R13 cavity fill comes in at about R9.5 after thermal bridging with wood studs, but a bit less than R7 with steel, despite how thin the steel webbing is.
Your best bet: A 12" SCIP with 2" of shot-crete concrete both sides would have room for 8" of EPS, for a whole-wall R of about R30 after the thermal bridging of the tie-wires. Done right you can stucco directly on the shot-crete, and texture the interior shotcrete as the scratch-coat for a hard plaster. What you DON'T get is an easy way to route the electrical, which is usually roughed-in with conduit on the interior side before the shotcrete goes on.
You could also get to ~R30 in a 12" thick wall with a combination of 8" AAC (~R10)with stucco applied directly to the AAC and 3-3.5" of rigid polyiso (~R18-R21) on the interior, and no mechanical routing cavity.
Should have clarified, the galvanized studs (in essence only to hold up the sheetrock) would be installed internal to the continuos EPS layer that acts as a barrier between the CMU and the steel. Agree any direct connection between CMU and the steel invalidates much of the thermal benefit of insulating the mechanical space. With typical ICF walls its an R 28, and not cheap. The labor component is mostly in-house, masonry and framing, so not too pricey relative to contracting it out.
The "outsulation" concept is agreeable, with internal wall drying, don't know how I feel about the mechanics of hanging stucco/brick/stone veneer 3-3.5" away from a CMU wall. Dana I had looked over SIPs at length, including SIPs plus an interior mechanical space wall. Use and reliance on adhesives and USB was a turn off, will look at SCIP. Looking at different ways to skin the cat. Any input appreciated.
Sal,
You wrote, "I don't know how I feel about the mechanics of hanging stucco/brick/stone veneer 3-3.5" away from a CMU wall." This is a standard commercial construction technique: CMU wall, rigid foam, and then brick veneer. Many commercial contractors build these walls routinely.
The steel studs still thermally bridge whatever you put as fiber insulation, with very pathetic thermal performance for that layer.
SCIP construction is in no way comparable to SIPs, other than the insulation is all in the core. The concrete is applied on-site, not at a factory, and there are no seams to seal, etc. The panels of EPS are typically shipped with the reinforcing steel wire pre-installed, which get tied/reinforcet together on site, then shot-crete is applied in a continuous fashion on each side to the desired/specified depth. It isn't very common in the US, but it's a preferable method to building with concrete than ICF, particularly at higher-R, and particularly in termite country- the EPS is encapsulated in concrete where it can't be used as a super-highway for the insects. A substantial fraction of the thermal mass of the concrete is accessible to the interior, giving it far better mass-effect performance gains over ICF too. For something that want's a stucco or masonry finish on the exterior and a hard plaster interior it can be cheaper than a CMU cavity wall with outsulation + interior studwall, since it saves both labor & material on the finish layers. More common in cooling dominated climates, R8-R12 SCIPs are sometimes used for upscale masonry houses in Mexico, but there's no reason it can't be used in much colder or wetter climates.
SCIP roofs have a HUGE benefit over most alternatives in cooling dominated climates, but for zone 5 you may want to take a different approach on the higher-R roof, and would have to consider carefully in the design how you're going to thermally break the transition at the top of a SCIP wall and a trussed or rafter & joist attic/roof.
Not necessarily advocating these manufacturers, but the pics are something of a primer on how it's done:
http://www.fastfoldscip.com/gallery/how-it-works/
http://scconcretehomes.com/
Sal,
As I understand it you want a masonry (brick) exterior finish. What I'm doing is using thin brick adhered to steel substrate (Loxon) panels. It's certainly not cheap but gives an authentic brick appearance with less than 1" of thickness. A double wall system of 2x4 walls would seem to fill all of your requirements. Sheathing the exterior of the inner wall with plywood and the exterior of the outer wall with fiberboard (covered by house wrap then the metal substrate for the thin brick) gives excellent drying and air sealing ability. and a service cavity for wires etc. If the walls were spaced 2 1/2" and Roxul comfort bat (intended for steel studs) were used between walls and Roxul used between all the 2x4's the cavity r would be r 40 and whole wall well over 30. with a total wall thickness including drywall of under 12". I have found NO lower cost way of achieving decent thermal performance!
I think your right Jerry, from what I read, the double 2x4 wall is likely the least costly and not too foreign. Was looking to remove wood from the equation, thus the CMU/EPS/galvanized studs. I like the SCIP concept, will look into pricing, not sure how favorable that will be. Martin, by me, dealing with mostly spec residential homes, I have never seen 3" EPS layer specified in the wall. Nor is there any ICF construction. Stick framing and batt insulation is the norm, even on high end homes. I talk to friends, acquaintances,masons, framers, even architects about "perfect wall" theories and concepts, I get mostly blank stares, most consider it crazy to deviate from the norm, AND pay more.