Designing A Perfect Residential Wall - Atlanta, Georgia

Atlanta, 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

The Perfect Wall

The perfect wall is not a new concept. In a paper about 'The Perfect Wall'Joe 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 2x6 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

Controlling 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®

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.

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 Sheathing 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.
For 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 2x4 stud, and about R-6/R-7 for a 2x6 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

Views: 1129

Tags: architect, building, design, durability, efficiency, energy, green, high, home, perfect, More…performance, proud, wall

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