Structural Insulated Sheathing at 800 South Main
Building

south-main-buena-vista-colorado-green-building-2-260South Main Building Company is putting to use on 800 South Main Street a nifty green building product called Structural Insulated Sheathing (SIS). This product combines insulating properties, structural bracing and moisture resistance, making it preferable to the standard OSB (plywood) sheathing system. The inch-thick panels, with their closed cell foam core, provide an additional R 5.5 of insulation to the wall system, addressing a problem known as “thermal bridging.” While a800_south-main-street_schematic-elevation_260 wall system may have a highly efficient insulation package, framing studs can still conduct interior heat energy through the wall. If OSB sheathing is used, that too will conduct the heat energy to the exterior of the building, diminishing the overall efficiency of the wall system no matter how tight the insulation package. SIS creates a barrier to any heat energy that might move through the studs from the interior of the building, thereby increasing the efficiency of the building’s envelope and saving heating energy and cost.
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The panels’ structural bracing property makes them an effective replacement for OSB in stabilizing the wall system. For 800 South Main Street, the project’s superintendent consulted with an engineer to determine how many SIS panels would be needed for structural stability. The remainder of the wall was covered with a more affordable foam board, which lacks the bracing properties of SIS but still provides the insulation necessary to address the thermal bridging problem.

Finally, the SIS panels are effective in managing moisture in the wall system, addressing a potential rotting problem, which can occur when OSB sheathing is used. Lacking any insulating properties, OSB conducts cold from the outside; in the event that warm interior air makes its way through the wall system, moisture in the air condenses when it comes in contact with the cold, interior surface of the OSB. Such condensation can create a rot problem in the wall cavity.

SIS panels present a solution to this long-standing construction dilemma. Since they do not conduct cold from the outside, and since interior air cannot migrate into the SIS panel, the dew point is within the foam core, and no condensation can take place.

At 800 South Main Street, the joints between each SIS panel have been taped, and a Tyvek building wrap outside of the SIS panels guarantees an impermeable weather barrier. Siding and shingles will be anchored to strips of OSB which have been nailed through the Tyvek and SIS into the wall studs. OSB strapping is oriented vertically on the building’s first story, where siding will be used, and horizontally on the second and third stories, where shingles will be used.

All told, this wall system is warmer, tighter and technically superior. It’s just one of the ways that South Building Company is raising the bar for green building.

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