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Looking for help designing a durable & efficient wall for a house in the mountains near Asheville NC at 4,000 ft. elevation

I am trying to use articles from this site and from Building Science to design a durable and efficient wall for a residence in the mountains near Asheville NC at 4,000 feet of elevation.

Which would be a better wall; a 12" AAC wall with interior plaster and exterior stucco, or an 8" AAC wall with 3" of closed cell spray foam or XPS sheets on the exterior and then a 1" air/drainage space and then hardiplank? Would the increased complexity and cost of the second option be worth it? Are there any good spacers for the air/drainage plane that are not thermal bridges and will not corrode?

Asked by Stuart Murray
Posted Feb 11, 2013 4:30 PM ET
Edited Feb 11, 2013 6:50 PM ET


19 Answers

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As much as they make of the dynamic-R effects of AAC (and ICF), it still doesn't trump higher steady-state R in as climate as cool at that. An 8" AAC wall steady -state is a ~R10 wall, which is about the same as a 2x4 studwall with R13 cavity fill, but outperforms the studwall slightly during the shoulder seasons for a lower net-annual energy use, and some lowering of the peak heating & cooling loads.

Similarly, a 12" AAC is comparable to a 2x6 studwall with R20 cavity fill. But any R10 wall with R15-R18 of continuous foam on the exterior is going to BLOW AWAY 12" AAC wall on thermal performance by more than 2x. Even though it's only 2/3 the thermal mass, it's twice the steady-state R value, and unlike an straight-ahead AAC wall, the thermal mass is mostly inside the insulation, which gives it an even higher mass benefit.

You can use 1x wooden furring (through screwed to the AAC ~16"-24" o.c.) to establish the rainscreen gap. It would be important to use masonry screws of a material compatible with the fiber cement though, and I'm not sure how deep you'd have to drill into the AAC to achieve the necessary mechanical strength.

Both XPS and closed cell spray polyurethane have very high life-cycle greenhouse gas issues due to the blowing agents used. At 3" it's likely to be net-negative from a 50-100-year greenhouse effect point of view, whereas if you used 3" rigid polyisocyanurate you would get similar performance at about 1/200th the greenhouse gas impact. Using two staggered seams layers and taping/sealing the seams on both layers then sealing the edges with spray foam makes it pretty air-tight. Using foil-faced goods for the outer layer gives it a little edge during the cooling season too.

Answered by Dana Dorsett
Posted Feb 11, 2013 6:40 PM ET


As Dana points out, autoclaved aerated concrete (AAC) has a very low R-value. Hebel makes three types of AAC block; they range from a low of R-1.1 per inch to a high of R-1.3 per inch.

Between the options you list, choose an option that includes real insulation (closed-cell foam or rigid foam).

Or -- considering the very low R-value of AAC blocks -- you might consider an entirely different wall system with a higher R-value per inch.

Answered by Martin Holladay
Posted Feb 12, 2013 5:31 AM ET



Reach out to Rob Moody at Mathis Consulting Company in Asheville, NC. His e-mail address is rob@mathisconsulting.com. Between him and the rest of the staff, you should be covered.

Answered by Mike Collignon
Posted Feb 12, 2013 1:03 PM ET


A 12" AAC wall would still meet code min in NC without additional insulation.

For an 8" AAC wall they have to use the dynamic mass benefit aspect rather than the raw R value, but it still squeaks by code in climate zone 4 (but not marine-zone 4), which would cover the mountains around Asheville:


I've been told that AAC + exterior foam is a more common method than ICF in parts of northern Europe at comparable R20-R25 whole-wall values. An 8" AAC + 3" of foam is a mid-20s R-value wall, which is WAY above code-min, but not quite at "pretty-good house" performance. If the alternatives are CMU or poured concrete/ICF, it's not necessarily a bad option for cost/performance/flexibility. For R40 walls you'd probably want to do something else.

Answered by Dana Dorsett
Posted Feb 12, 2013 3:51 PM ET


Hi Stuart, be aware that Yancey and Mitchell counties are in Climate Zone 5. If your site really is at 4000' elevation, I would build to IECC 2012 zone 5 at least. Be sure to check out Eco-Panels, our most local SIP maker, if you havent already. The 6" wall panel they offer is one of the easiest ways to get to R40 in my opinion. They also offer an exterior skin with ZIP sheathing as a nice upgrade too. We are finishing up a home that came in with a pre-drywall blower door of .282 ACH 50 with help from their 4" panel + ZIP.

Answered by Brian Knight
Posted Feb 12, 2013 4:31 PM ET



Here are some considerations for you:

One story? Two? This could certainly affect your decision. A two story AAC wall is more complicated.

Do you know of any local masons that have laid the AAC block? If not, you may have a tough time finding someone to build the walls at a reasonable cost.

I've worked with AAC before (12" block from Aercon). Used by itself, I don't think you would like the performance in your climate, at your elevation. Also, it is a friable product. I would be leery of fastening furring strips to the AAC, to support the siding.

One of the best features of AAC is that is can be finished with just stucco and plaster. This keeps the costs within reason, and makes for a simple wall system. As soon as you start adding insulation, furring strips, and siding---those benefits start to disappear.

If you are committed to using this material (perceived durability), I would suggest you investigate EIFS (Exterior Insulation Finishing System). The EPS foam could be installed with an adhesive. You would still have the stucco and plaster finish, but with much better thermal performance.

Of course, when you price this type of wall, you may find yourself moving in a different direction . . .

You've received a number of good comments to your question. Keep researching. Talk to knowledgeable and experienced designers and builders.

Answered by Daniel Ernst
Posted Feb 12, 2013 5:12 PM ET
Edited Feb 12, 2013 5:13 PM ET.


Brian: EcoPanels are polyurethane core SIPs, and unless one of the rare low-greenhouse gas blowing agents is being used, the HFCs used in their manufacture will have a far heavier greenhouse gas footprint than a lifetime of CO2 savings from the lower energy use (even a coal-fired grid) at a center-panel R of R40. The thinner goods may break-even in a century or two though... or maybe not- it depends on what the energy-source mix in NC looks like in 50-100 years.

There are newer lower-impact blowing agents emerging on the market, but right now HFC245fa holds the lions-share of the closed cell polyurethane market.

They are being pretty aggressive in their R-value statements, pretending as if SIPs have ZERO thermal bridging. (Even at window & door framing? How about bottom & top plates? ) Their scare tactics around styrene vs. polyurethane are also fairly off-putting. Overall their marketing approach makes me LESS likely to want to use them over a SIP competitor, even if they swore they don't use HFC245fa (or worse) as the blowing agent.

Yes, it's easy to make SIPs air tight and they DO go up quickly, but it's not usually the most cost effective path to a true R40 whole-wall building. (Then again, neither is AAC or ICF.) And it's not rocket science to build air tight structures using other methods, eg:

A 3 story w/full basement balloon framed 1905 antique in climate zone 5 came in at 464cfm/50 after a deep energy retrofit I was involved with, which is plenty tight enough from both an energy and moisture transfer point of view. That building also hit a true R40 whole using mostly reclaimed roofing iso on the exterior of the plank sheathing, at about 0.001 the greenhouse impact of an R40 polyurethane SIP. (The place is heated & cooled with one mini-split head per floor, and it sailed through -2F weather without a hint discomfort issues a few weeks ago, and it's probably good down to at least -10F when the mini-splits are approaching their limits. The 99% outside design temp is +5F, I'm not worried.) I'm sure the rehab on the wreck took quite a bit longer than just bulldozing it and erecting a brand new SIP house on the foundation, but that would be the opposite of green, in many or most cases.

Answered by Dana Dorsett
Posted Feb 13, 2013 8:10 PM ET


I agree Dana. Iam glad that the foam industry is moving in a better direction and I wish it would hurry up. Coal fired power plants and increasingly NG extraction have many hidden costs besides the CO2 component of emission equation. I dont feel its appropriate to discount most foams based on just one of their environmental costs when it can have such a big impact on so many others. Hopefully some better blowing agents will help the equation soon.

Answered by Brian Knight
Posted Feb 13, 2013 10:35 PM ET
Edited Feb 14, 2013 8:23 AM ET.


I would expect the phase-in of the VERY low GHG blowing agents to take a few years, but it will eventually become universal, and may even be regulated/legislated. In Europe all XPS is blown with CO2, which has a GHG footprint equal to 1x CO2 (go figure! :-) ), though it runs only R4.2/inch rather than R5/inch for the XPS in the US. But the 'merican goods are almost all blown with HFC134a, at about 1400x CO2, which is a pretty significant hit when going high-R.

Closed cell polyurethane is a wonderful product for it's air sealing and moisture control properties, but even without the greenhouse issue it's not a very green way to insulate to high-R (no foam is). But at intermediate levels of insulation or as one component of a stackup it's pretty good. As part of it's rollout of the Solstice lower-impact blowing agent late last year Honeywell cooked up this bit of lifecycle analysis:


But open cell foam has about 1/4-1/3 the total environmental impact of Soltice blown 2lb foam per unit-R, and dense-packed cellulose has less than 1/10 the lifecycle impact, making it a very favorable material for bulking out the thermal performance of high-R assemblies.

The greenest foam on the market is RECLAIMED foam (typically from commercial building demolition or re-roofing), where re-use avoids a disposal problem, the environmental hit from it's manufacture having already been taken.

BTW: There are at least two closed cell polyurethane vendors out there with products that use WATER as the blowing agent:

Icynene MD-R-200 is an ~R5/inch somewhat higher permeability foam (1.3perms @ 3"), not to be confuse with their higher-R/lower perm MD-C-200 product, which is blown with HFC245fa.


Aloha Energy is a regional player in the northeast with a range of water blown closed cell products.


Answered by Dana Dorsett
Posted Feb 14, 2013 12:42 PM ET
Edited Feb 14, 2013 12:50 PM ET.


Thanks to everyone for your comments! Very helpful! Let's say I like a masonry wall for several reasons including fire and pest resistance, and have chosen AAC because it lacks cavities and offers better insulation than concrete block. I do want a high R value for this wall because of the location of the house. What if I use 8" AAC block with plaster on the inside, and build a pressure-treated (PT) 2x4 wall 1" outside of the outer surface of the AAC block, and use closed cell (CC) spray foam to fill the 1" open gap and the depth of the 2x4 frame wall? This would give me 4.5" of closed cell spray foam outside of the AAC wall, and the 1" gap between the 2 walls filled with foam would minimize thermal bridging due to the 2x4s. The CC spray foam could be shaved off flush to the outer surface of the 2x4 wall, and PT 1x4s installed as furring strips for the attachment of fiber-cement board or any other siding material. The furring strips would provide a ventilated drained space behind the "rain screen" siding. The R value of this wall should be 35+, not taking into account the thermal mass effect of the AAC block. I prefer the idea of CC spray foam to sheets because I hate to rely on tape for air sealing, and I think it is probably easier to keep the insulation continuous from the under-slab insulation to the roof insulation using CC spray foam. Any thoughts on this approach?

Answered by Stuart Murray
Posted Feb 18, 2013 9:59 PM ET


1. The framing lumber doesn't have to be pressure-treated.

2. If I were doing it, I would find it easier to install a layer of rigid foam followed by vertical furring strips -- as long as the AAC blocks can hold screws. (I'm not sure whether they can.) That will cost less and be thermally superior.

Answered by Martin Holladay
Posted Feb 19, 2013 6:44 AM ET



Does anyone know how long the tape lasts that is used to tape rigid sheets of insulation together? I know there are several brands, but have any of them have been around for a really long time? Call me paranoid, but I am a little leery of using tape inside a wall (sheathing and possibly furring strips would have to be removed to get to it) as my vapor barrier. Could CC spray foam be applied in any other kind of frame besides 2x4 lumber that would provide a framework for the spray and an attachment point for exterior siding?

Answered by Stuart Murray
Posted Feb 19, 2013 8:38 AM ET


1. Foil-faced polyiso is easy to tape. When I did a backyard tape test, several types of tape stuck tenaciously to foil-faced polyiso, including common brands of housewrap tape.

If you are worried about tape longevity, installing two layers of rigid foam with staggered seams -- with both layers of foam taped -- may help you sleep at night.

Answered by Martin Holladay
Posted Feb 19, 2013 9:01 AM ET


If you install 2 layers of ISO, both layers taped, don't you create a space between the two layers, admittedly small, that cannot drain in either direction? There will be small amounts of air containing water vapor in small gaps and seams around the second layer, and this would be trapped and could not drain either in or out. Is this a concern? I just read an article by Joseph Lstiburek called "Mind the Gap, Eh!", and apparently he likes the double layer.

Answered by Stuart Murray
Posted Feb 19, 2013 9:26 AM ET


1. The "gap" between two layers of polyisocyanurate isn't really a gap. If you choose to call it a gap, you can -- but it's extremely tiny.

2. If you tape the outer layer of foam, and you have a rainscreen gap, it's hard to imagine that water will get between the sheets. The water will want to follow gravity, and drop down the rainscreen gap. That's where it wants to go.

3. Polyisocyanurate foam is not hurt by small amounts of water. Moreover, it should have a foil facing to protect the polyiso.

4. If you're the type to worry a lot, invest in closed-cell spray polyurethane foam. Of course, spray foam has its own set of worries. There is always Valium.

Answered by Martin Holladay
Posted Feb 19, 2013 9:51 AM ET
Edited Feb 19, 2013 9:56 AM ET.


Thanks Martin! I will forgo the valium, but I do worry. I want to get this wall right, and I know there will be lots of opportunities to screw it up, especially around windows and doors and tying it to the foundation and roof insulation. One big aspect of being green, I believe, is to build structures that will last, and not have to be rebuilt or repaired often. Thank you again for all of your advice!
If I use fiber-cement board for my rainscreen, and have a 3/4" gap behind it, how much gap do I need to leave at the top and bottom of the fiber-cement layer to allow air flow? How do you keep insects from nesting in and blocking the rainscreen cavity?

Answered by Stuart Murray
Posted Feb 19, 2013 10:07 AM ET


Stuart: High-R foam between studs is largely wasted, due to the thermal bridging of the studs. You also can't trim cc foam flush the way you can with open cell foam- the stuff is too dense and very difficult to work with that way. The best you can do is ~3", not 3.5". And at 3" even R7/inch foam only adds about R1 to the "whole-wall" R above what you would get with 3.5" of open cell foam at a fraction of the cost.

At a 20% framing fraction, without counting the R of the siging or rainscreen gap or the interior layers, a 2x4 wall with 3.5" of open cell foam trimmed flush comes in at a bit over R8. The same stud-layer with 3" of closed cell foam comes in a bit under R9.5 How much are you willing pay for R1-1.25?

With the R10 AAC plus the ~R10 studwall (counting the siding & rainscreen here), the 1" of closed cell adds about R6 or R7, so you're in the high mid-20s, not "...R35+..." in your proposed stackup, an artifact of the R1/inch framing over 20% of the total wall area.

If you flip the studs (but not the stud plates) sideways and flush with the exterior and use an all open-cell solution it'll be a lot higher whole-wall R, despite a lower center-cavity R due to the much lower thermal bridging. If the AAC is the structural wall it frees up what you can do for exterior studs/trusses etc quite a bit.

Not to mention the previously discussed lifecycle greenhouse-gas impact of the blowing agents used with closed cell foam in high-R assemblies. The rigid polyiso approach is FAR greener in most respects.

Most rainscreen vents get critter-proofed with either a combination of Cor-A-Vent + conventional bug screen materials, (see: http://www.cor-a-vent.com/siding-vent-sv-3.cfm ) or a coarse polypropylene wool matting that looks a bit like pot-scrubber material ( eg: http://www.greatlakesstone.com/Mortairvent.pdf ) There are many variations on the theme out there- a well-trodden path, but many products are marketed directly toward stucco-siding (which is often a moisture-disaster when done without vented rainscreens.)

Answered by Dana Dorsett
Posted Feb 19, 2013 4:46 PM ET


OK - here's the wall I want to go with, from inside to outside: 8" AAC block, 3" ISO taped, 2" ISO taped with staggered seams relative to the 2" layer, 1x4 furring strips, and then exterior cladding attached to the furring strips. Two questions: (1) Would you put a layer of housewrap between the ISO and the furring strips? (2) If I want to avoid exterior wood (I don't want to provide any exterior termite food), what material could I use for the furring strips? Would something like Trex decking work, or would I be better to go with pressure-treated 1x4s?

Answered by Stuart Murray
Posted Feb 22, 2013 8:35 AM ET


At least five manufacturers sell plastic furring strips:

  • BattensPlus manufactures BattenUp furring strips. These pieces of polypropylene strapping measure 1 1/2 inch wide by 1/2 inch thick by 48 inches long.
  • El Dorado makes plastic battens that measure 1 5/8 inch wide by 3/8 inch deep by 8 feet long.
  • VaproShield makes a plastic batten called VaproBatten.
  • Cor-a-Vent makes plastic furring strips called Sturdi-Strips.
  • DCI Products makes plastic furring strips called CedarVent strips.

All of these plastic furring strips have a big downside, however: they don't hold fasteners, so you can't attach siding to the furring strips. That's why I think you should stick with wood.

Answered by Martin Holladay
Posted Feb 22, 2013 9:22 AM ET

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