Storefront and Curtainwall Systems

The term curtainwall usually refers to aluminumframed systems carrying glass, panels, louvers or occasionally, granite or marble. Curtainwall selection and design criteria include code compliance, structural integrity, weatherability, durability and aesthetics, plus other, application-specific needs. Storefront framing systems are often specified for curtainwalls, but should only be used for lower structural performance applications. Storefronts are… Continue reading

Storefront and Curtainwall Systems

The term curtainwall usually refers to aluminumframed systems carrying glass, panels, louvers or occasionally, granite or marble. Curtainwall selection and design criteria include code compliance, structural integrity, weatherability, durability and aesthetics, plus other, application-specific needs.

Storefront framing systems are often specified for curtainwalls, but should only be used for lower structural performance applications. Storefronts are non-load-bearing glazed systems that occur on the ground floor, which typically include commercial aluminum entrances. They are installed between a floor slab and the building structure above it. Commonly field-fabricated and field-glazed, storefronts employ exterior glazing stops at one side only. Provision for anchorage is made at perimeter conditions.

Another important difference between curtainwall and storefront, and reason for not mixing the two, is the way each handles infiltration of rainwater.

Storefronts control water infiltration by directing all internal moisture to the sill flashing (1). Most manufacturers offer sill flashing that fastens to the bottom of the opening prior to installing the storefront framing. A watertight seal is applied over the fastener heads (2) to direct water to the exterior through two weep holes at each lite of glass (3). Optional sill flashing also have a much higher back leg for better water performance
(4).

Assuming that the sill flashing is installed correctly, the most important element in controlling water penetration with storefronts is the proper fastening and sealing of end dams (5). Without end dams, water will intrude into the jamb locations of the building. To ensure water bypasses the lite of glass, deflectors must be installed wherever a horizontal mullion occurs in storefronts. Without water deflectors, moisture settles on the top of the glass, causing possible failure of the insulating glass unit seal.

The distinctions between curtainwall systems are not absolute and are often difficult to differentiate.

“Stick” curtainwall systems are shipped in pieces for field fabrication and/or assembly. These systems can be furnished by the manufacturer as “stock lengths” to be cut, machined, assembled and sealed in the field, or “knocked down” parts pre-machined in the factory for field-assembly and -sealing only. All stick curtainwalls are field-glazed. Frame assembly requires the use of either:

a) “Shear blocks” to connect vertical and horizontal framing elements, or

b) “Screw-spline” construction, in which assembly fasteners feed through holes in interlocking vertical stacking mullions into extruded races in horizontals.

Many stick curtainwalls are called “pressure walls” because exterior extruded aluminum plates are screw-applied to compress glass between interior and exterior bedding gaskets. A snap-on cover, or “beauty cap,” is then used to conceal pressure plate fasteners. Performance of any field-assembled or field-glazed curtainwall is only as good as field workmanship allows and is limited by variables such as weather, access and job site dirt and dust.

With pressure walls, many critical seals are necessary, even in systems that are designed to drain or “weep” rain penetration from the system back to the exterior. Compartmentalization of each lite is strongly recommended to isolate glazing pockets. A fairly recent development in these curtainwall systems is the incorporation of fiberglass pressure plates for improved thermal performance. Significant increases in condensation resistance and reductions in thermal conduction also have been recorded.

To accomplish as many critical seals as possible in controlled factory conditions and minimize dependence on field labor,
“unitized” curtainwall systems have been developed. Unitized curtainwalls are factory-assembled and -glazed, then shipped to the job site in units that are typically one lite wide by one floor-span tall.

Most unitized curtainwall systems are installed in a sequential manner around each floor level, moving from the bottom to the top of the building. Only one unit-tounit splice, usually a translucent silicone sheet or patch, needs to be field-sealed. Only one anchor per mullion needs to be attached to the face of the floor slab. Interlocking unitized curtainwall frame members are weatherstripped to seal to one another, both horizontally and vertically. This accommodates thermal expansion and contraction, inter-story differential movement, concrete creep, column foreshortening and/or seismic movement.

“Window wall” systems span from the top of one floor slab to the underside of the slab above it. Window wall employs large side-stacking window units contained in head and sill receptors, also called “starters,” which accommodate movement and drainage, but require field-applied perimeter sealants. Slab covers can be fabricated from aluminum extrusions, sheet, panels or even glass. Window walls easily accept operable windows, and unlike curtainwall, can be installed non-sequentially.

“Hybrid” systems combine characteristics of multiple wall types. For example, some four-side silicone wall systems use stick wall grid frames, with factory-glazed carrier frames.

The project’s final, balanced design and curtainwall selection should be based on all applicable criteria, not on any specific single number rating system. Specialty glazing contractors and manufacturers can provide design input, budget pricing, sequencing and schedule information to help building teams achieve their projects’ specific requirements.

Tom Minnon, LEED AP, CDT, is the eastern region sales manager for Tubelite Inc., Walker, Mich., serving clients from Maine to Florida. To learn more, visit www.tubeliteinc.com.