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Insulation on target

Exterior wall systems of retailer's Minneapolis headquarters perform triple functions

November 01, 2001 |

In the building team's quest to reduce energy costs, insulation is becoming an important component of building design because of its role in moderating fluctuations in outside temperatures. But insulation systems also control the spread of fire and smoke and prevent moisture migration to the exterior.

To keep its employees comfortable year-round and promote reliability of equipment that comprises the building's systems, retailer Target Corp. wanted to maintain 30 percent relative humidity year-round in its new 32-story corporate headquarters, Target Plaza South, in downtown Minneapolis. This presented a challenge for the specification of the building's exterior envelope insulation system.

"Office building interiors typically have less than 25 percent relative humidity," says Ray Usher, vice president of Fridley, Minn.-based Superl Inc., the project's insulation and fire-stopping subcontractor. "But in Minnesota, we have to be especially cautious because of the extreme climate in the winter."

According to Scott Kinkade, project manager for Minneapolis-based Ryan Companies, the project's design/build contractor, the vapor retarder is the key to preventing condensation on the building's exterior walls.

"The purpose of the vapor retarder is to stop inside humidity from migrating into the exterior wall system from the inside," says Kinkade. "When you have excess humidity, as we do in this building, the vapor retarder is that much more important; the detailing is greater."

Since vapor retarders are incorporated into the insulation manufacturing process, the project's exterior insulation had to include the proper vapor retarder to help maintain the desired indoor humidity. According to Usher, the building's insulation accomplishes this objective because key members of the building team collaborated to provide input for its specification. "The building was designed in a way that we could provide a continuous and monolithic thermal and vapor envelope including in the floor slab area, which is usually much more difficult," says Usher.

Extra required accommodations include taping over protruding nails, special care at corner intersections and caulking where needed. The insulation was specified to accommodate slab edges, roof edges, framing details, parapet conditions and interface with window systems.

A trio of systems

Target Plaza South, which was designed by Minneapolis-based A/E Ellerbe Becket, has three basic exterior wall systems.

In order to achieve the specified humidity performance in each of these wall systems, Superl Inc. specified systems and their individual insulation needs.

The first system is at the base of the building, consisting of steel stud framing with gypsum sheathing and serves as the backup for the stone façade. Insulation with vapor retarder was applied to the steel studs in those areas.

A second type of wall system consists of horizontal strip windows and precast concrete, both within a spandrel glass window system. Semi-rigid insulation with a factory-laminated foil scrim was applied to the spandrel panels and to the back of the precast concrete. The insulation is mechanically attached to the precast concrete. For the aluminum-framed curtain wall, Superl used pins with screws and also mechanically attached stiffeners.

The third exterior wall system is an all-glass curtain wall incorporating insulation that is applied behind spandrel glass. Metal spacers between the mineral wool and the building's outer skin provide an air void, allowing moisture to condense on the inside of the exterior spandrel panels and then drain to the outside.

With a proper air gap and drainage channels, condensed moisture can easily escape without creating environmental problems inside the building that would compromise the insulation system.

Mineral wool fills gap

The gap between the floor slab and the outer wall was filled with mineral wool, which is cut oversize to eliminate any voids or gaps in the safing system and to accommodate any joint movement.

Fire can easily spread in office buildings without a proper insulation and fire-stopping system in the perimeter gaps between floor slabs and the exterior walls.

After the safing was installed, a smoke barrier was added to the slab perimeter to seal openings against flame and smoke. This completes the system at the slab edge between floors and the exterior walls.

The goal of the system is to compartmentalize a fire in one area. Smoke/fire barriers and sprays, which are applied to the insulation, help it to achieve this goal.

The combination of the insulation, fire safing and smoke seal gave Target Plaza South the two-hour fire and smoke containment rating required by the city of Minneapolis.

Basic insulation types

Fiberglass and mineral wool are the two major options for building insulation, each with their own positives and negatives. Fiberglass insulation is made from silica sand, which is heated and finally melted into glass. Fiberglass insulation is not fireproof.

However, mineral-wool insulation is fireproof, as it is made of melted basalt, a fine-grained igneous rock, iron and copper slag. According to Usher, both provide the same thermal quality.

Charles Stobbie, product manager of St. Paul, Minn.-based 3M Fire Protection Products, says conventional insulation materials alone are not adequate for curtain-wall firestopping because of the high temperatures that may be encountered in a building fire.

According to Stobbie, glass-fiber insulation melts at 1,050 F and aluminum, copper and steel structural elements will melt at 1,800 F or less, while building fire temperatures can approach 2,000 F. Mineral-wool insulation withstands temperatures up to 2,080 F to protect underlying curtain-wall structures.

Using eight construction workers, insulation for the entire envelope was installed at Target Plaza South from the fall of 2000 until just recently.

The building's interior walls and ceilings have acoustical fiberglass insulation, which was installed by the drywall contractor, Minneapolis-based Berg Dry Wall.

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