Designing A Perfect Residential Wall – Atlanta, Georgia

Designing a Perfect Wall in Hotlanta, GeorgiaAtlanta, Georgia has a reputation for getting pretty darn hot, but it can also get pretty dang cold (No snickering from you Northerners, please!). Believe it or not, Atlantans use more energy to heat their homes than they do to cool them. The design temperature set by ASHRAE for the Metro Atlanta area to use when designing heating systems, is 24° F, which I’m sure my friends from up North would think is balmy. On average Atlanta has about 2,500* heating degree days, which is a measure of the amount of time the outside temperature is below 65° F.

Atlanta also gets a lot of rain; average 50 inches per year*. That’s even more than Seattle, Washington that gets about 36.2 inches per year*. It also gets pretty humid; average relative humidity (RH) is around 82% in the morning and 56% in the afternoon* (No snickering from you deep-Southerners, please!)

These conditions make it somewhat challenging to design and build for. On rare occasions, Atlanta will experience a dry 35-degree day and wet 70-degree within the same week. In these conditions, building materials expand, contract, get wet, get dry, get hot, get cold. So, we design and build to protect our structures, and by protecting structures we’re protecting the people inside it.

*All weather data provided by NOAA
**Average based on the past 3 years of collected data from www.degreesdays.net

A Perfect Wall for Atlanta, Georgia, Proud Green Home

The Perfect Wall

The perfect wall is not a new concept. In a paper about ‘The Perfect WallJoe Lstiburek, PhD (pronounced “stee-brik”) explains best practices for designing and building a residential and commercial wall, roof or floor, that will protect a building and the people in it against unwanted moisture, air and heat. The fundamental basics of a perfect wall are cladding, control layers and structure. The cladding is the aesthetic component that keeps the rain and ultra-violet light out, and the control layers are placed on the outside of the structure and run continuously around the entire building to keep it warm (or cool) and dry. A break in this “control boundary” can reduce performance, or, in some cases, cause a building to fail. The four control layers identified in ‘The Perfect Wall’, in order of importance, are:

  1. Rain - Because water can destroy a building. Period.
  2. Air – Because air carries heat, cold and moisture.
  3. Vapor – Because vapor is water (gaseous)…I’ve already covered this.
  4. Thermal - Because heat belongs outside in the summer and inside in the winter.

At right is the above grade wall design at the Proud Green Home, which is being built just southwest of Atlanta in Palmetto, Georgia. We have cladding, control layers, and a structure; all in the “correct” order. The only difference difference between this wall and Joe Lsitburek’s Perfect Wall is that the structural sheathing (OSB) is on the outside of the thermal control layer.

Structure

We designed and built the exterior walls with 2×6 framing @ 24″ on-center. This is a common advanced framing technique used to conserve material and increase the amount of insulation we’re able to put in the wall, which will improve the thermal performance of the wall assembly

Designing and Building a Perfect Wall Atlanta, Georgia_Cladding Proud Green HomeControlling Rain

Cladding 1: The majority of the exterior cladding is cementitious (a.k.a. fiber cement) lap siding from Certainteed that is factory primed on all sides to help prevent any moisture intrusion that can degrade and even destroy the material.

Cladding 2: About one-third of the home, including a continuous the corner where you see ZIP System R Sheathing (green) in the photo at left, will be covered with a brick veneer made by Boral Bricks North America.

Cladding 3: As you can see in the photo, the entry tower has vertically-oriented corrugated metal siding by Metal Sales, who also provided the standing seam metal roof.

Cladding is such a big part of the aesthetics of the building. Along with proper flashing, ventilation (behind) and continuity, it’s also the most important control layer; Rain Control.

The Perfect Control Layer Combination

Behind the cladding, we’ve combined ZIP System® products from Huber Engineered Woods and the Home Slicker® ventilating rainscreen from Benjamin Obdyke to provide air, vapor and thermal control layers; ZIP System® R Sheathing + ZIP System Tape + Home Slicker®

Designing and Building a Perfect Wall Atlanta, Georgia_Cementitious Lap Siding Proud Green Home

Controlling Air

The ZIP System® Tape will be installed at the outside of every stud-to-stud, bottom-plate, and top-plate connections behind the insulated sheathing, as well as at every penetration, including windows, doors, pipes and structural members. The tape will take the place of caulks, gaskets, adhesives, spray foams, and other sealants often used.

Designing a Perfect Residential Wall Atlanta, Georgia_ZIP System R Sheathing

The ZIP System® R Sheathing is Huber’s insulated wall sheathing product that combines their traditional ZIP System wall sheathing with a layer of rigid polyisocyanurate foam in to a single panel. The wall sheathing has an integrated weather resistive barrier. The tape will be installed at every seam and penetration. With the air barrier properties of the foam, as well, the R Sheating has four (4) layers of air control that work in both negative (infiltration) and positive (exfiltration) pressure situations.

 

Controlling Vapor

Controlling vapor in a wall assembly is more about giving it a place to go than it is about just stopping it. In fact, trapping moisture is what causes a lot of the damage in walls.
Designing a Perfect Residential Wall Atlanta, Georgia_Home SlickerFor the air-borne moisture outside that gets behind the cladding, the integrated weather resistive barrier in the R Sheathing is a drainage plan. In the parts of the house where we have brick, 1″ gap behind it allows the moisture to drain and find it’s way down the wall and to the various drainage systems we have in place to direct moisture away.

For those areas that we have lap or metal siding, we’ve installed Home Slicker, a 3/16″ thick yellow ventilated rainscreen seen in the photo at right. It has a three-dimensional matrix that provides a continuous space for drainage and drying vertically and horizontally. A gap at the top and bottom of the cladded areas, with an insect screen to prevent intrusion, promotes proper ventilation. Without the rainscreen, moisture can get trapped where the cladding makes contact with the R Sheathing. This would eventually degrade and/or destroy the materials.

Controlling Heat

Heat migrates to cold through conduction, convection or radiation. In a wall assembly, it’s mostly conduction and convection. We’ve installed open-cell spray foam, from BASF, to provide to control the convective heat, and the continuous Polyisocyanurate foam in the R Sheathing controls the conductive heat. Even though wood isn’t the greatest conductor of heat, it has a low thermal resistance capacity; about R-4 for a 2×4 stud, and about R-6/R-7 for a 2×6 stud.

Preventing the cold from ever reaching the structure at all is one of the main objectives of the perfect wall. Not only does the foam minimize heat loss through the structure by providing the necessary thermal break, but it also prevents any condensation that occurs if warm, moist indoor air from inside meets the cold surfaces of the wood framing and sheathing.

Does this work for all climate zones?

This is a “perfect wall” for Atlanta, Georgia, and many parts of the country. It may require additional exterior insulation if you’re in a cold climate, or less if you’re in a hot climate, but the principals of the perfect wall are the same.

Yes, the perfect wall concept works in all climate zones.

Image of ‘Made in Hotlanta’ hat, curtosey of wardtog.com

The Perfect Wall is a white paper written by Joe Lstiburek, Ph D at Building Science Corporation. It’s a highly recommended read for all building professionals, especially architects!

Written by Chris Laumer-Giddens

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13 Responses so far.

  1. Milan Jurich says:

    Chris,
    Great article! Thanks for the post! Couple of questions:
    Regarding the open cell spray foam in the wall cavities … what if it were replaced by a blown fiberglass product such as Spider by Manville which promises R15 in a 2×4 wall cavity or even BIB dense packed cellulose at R13? Any disadvantages you see to those as an alternative in terms of performance, cost and R-value?
    Re: the yellow mesh product used exterior to the Zip R sheathing … what if it were replaced by an additional water resistive barrier such as Pactiv’s Raindrop 3D which has provisions to still help shed water when lap siding is applied along with an extra layer of insurance protection in the flashing of “outie” windows to the Zip R sheathing in lieu of pure Zip tape? I realize it’s climate specific, but this would be for zone 5. Interested in hearing your thoughts.

  2. Thanks, Milan.

    Blown-in insulation would work fine. It would also eliminate the off-gassing from foam. In a “perfect wall” scenario, the main air barriers are outside of the structure, so the fact that the blown insulation products don’t air seal is not an issue. The critical thing is that the air control layer on the outside must be continuous.

    The Home Slicker provides a 3/16″ gap, which is just shy of the 5mm gap that Canadian building codes are now requiring for ventilation behind cladding. Codes are going to continue to get more strict on this. Raindrop 3D and HydroGap both provide a gap, but I prefer to see more.

    In addition to moisture control, the ventilation gap also improves the effectiveness of the insulation in the wall.

  3. Milan Jurich says:

    Chris,
    As a follow-up, did the project require any additional strapping or ties between the framing components to improve structural integrity when using the Zip R sheathing as opposed to typical OSB sheathing? One would think that there’s a slight strength penalty as a result of the polyiso break between the OSB component and the wall studs.

  4. Hi Milan,

    Here is the ICC-ES Evaluation Report addressing the structural compliance of the panel. This is a common question.

    Huber Engineered Woods is working on the R Sheathing in a roof application.

    http://www.zipsystem.com/uploads/technical-tips/ESR-3373%20(6-1-12)-07022012085108.pdf

    On page 3, it says: “…in accordance with ICC-ES Acceptance Criteria for Racking Shear Evaluation of Proprietary Sheathing Materials Used as Braced Wall Panels…”

  5. bill says:

    Ok now you got my attention!!! i cant let this BS go on any longer!!, maybe what you should be doing, if you are going to talk about “advanced wall framing”, (what is so advanced about 2×6 construction) ,you guys are jumping through so many hoops it is “RIDICULOUS” , why dont you really take a very very very close look at ICF construction, you are spending more time and money on trying to make a wood wall work better and you are actually geting less performance for the dollar, come on at some point you have to say enough is enough, are you going to make a space rocket out of wood as well? We are pushing wood construction way beyond its practical application. if you are really serious about building a “Great” exterior wall system see this website
    http://www.gnphouse.com ICF construction is the only really great exterior wall! eliminates all those items builders and architects are worried about plus it really does give the homeowner a great energy efficient “sustainable” house not a so-called Wood Hybrid, hey it is still wood !!!
    please try and broaden your wall horizons!
    Next Generation Builder

  6. Milan Jurich says:

    bill needs to spend more time on this site, BuildingScience.com or Greenbuildingsdvisor.com where there’s more substance and less advertising (ICF).

  7. ICF walls are so superior in proformance & long term cost , I understand how bill feels , The best sites to visit are Building sites .Forget the smoke & mirrors , get out there in the real world

  8. Well I guess I have to jump in here as well. Its 2013 people – the 22nd century is here. Why does the article and the comments above focus on wood and concrete. Both of these products have been used for thousands of years and todays building science offers numerous better options. How about steel, polystyrene, multiple layer water and vapour barriers of plastic products, fibreglass studs and integrated multiple product designs? I mean realistically, does either Concrete or wood provide a “perfect residential wall” without rebar, poly, insulation or other products? Does any residential wall, even a foundation, really NEED a half ton of concrete for every lineal foot (either cribbed or inside an ICF product) for structure? Will any wood framed wall not twist, turn, and crack drywall as humidity and heat affect the organic structure of the wood fibres? How about some other perspectives from out there on what a perfect wall SHOULD be made of …..

  9. [...] Here we can see that the 7/16″ exterior sheathing that has a piece of 1″ insulation board before it makes contact with your 2×6 framing. This continuous insulation layer reduces thermal bridging between the exterior and interior conditions reducing heat transfer and making the home much more efficient. More info here. [...]

  10. LostInVA says:

    Thank you for the great write up. I am looking to build a similar (2×6 walls, R-sheathing) structure in central VA and was hoping you could answer two questions: Why did you choose open cell over close cell? What would you change moving to my climate?

  11. Thanks, LostInVA!

    For the same house in your climate, I wouldn’t change a thing. I would also not necessaryilty recommend the same assembly for every house in your climate zone, although it could be appropriate. The design temperatures and moisture & humidity levels in Climate Zone 4 make this a good assembly.

    The amount of insulation in the wall assembly in this post is actually a bit higher than necessary for our Climate Zone 3A (Atlanta), and bit more appropriate for most of 4. The approximate point of diminishing return (http://lgsquaredinc.com/2012/10/02/improved-energy-codes-diminishing-return-and-a-new-performance-baseline/) in Atlanta is about R-20 / R-21. More is not better.

    The most important thing you can do, as far as insulation goes, is to provide the continuous layer. For climate zone 4, it could be as little as R-3, and as much as R-10. There are reasons to stay away from putting ALL of your insulation on the outside of the wall, and none inside the wall in CZ 4.

    I chose open cell foam because it has a better drying potential than closed cell. I typically don’t recommend closed cell unless a home needed additional structural stability (minimal benefit), and was in a colder climate. Closed cell can also make retrofitting difficult, no matter where you are. That said, there are plenty of good reasons not to use open-cell foam, as well.

    Depending on the design of your home, this could be a perfect assembly.

  12. [...] Then, a ventilated rainscreen was installed over the sheathing and behind the cladding. In an earlier post, we discussed how this assembly performs like a “Perfect Wall” (J. Lstiburek, Building [...]

  13. [...] be designed and built as a single system, where every detail effects the other. In a post about Designing a Perfect Wall, we talk about this and how an effective well-executed building envelope can minimize infiltration, [...]