Designing a perfect residential wall (Part 2)
Continued from Part 1
Behind the cladding, we’ve combined ZIP System® products from Huber Engineered Woods and theHome Slicker® ventilating rainscreen from Benjamin Obdyke to provide air, vapor and thermal control layers; ZIP System® R Sheathing + ZIP System Tape + Home Slicker®
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 Sheating has four (4) layers of air control that work in both negative (infiltration) and positive (exfiltration) pressure situations.
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 HomeSlicker, 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.
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.
Read part 1 of Designing a perfect residential wall.
Read more about the Proud Green Home at Serenbe.