Designing a perfect residential wall (Part 1)
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 residentialand 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:
- Rain - Because water can destroy a building. Period.
- Air – Because air carries heat, cold and moisture.
- Vapor – Because vapor is water (gaseous)…I’ve already covered this.
- 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.
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
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®
Read more about the Proud Green Home at Serenbe.