Curtainwall and storefront systems share similar functions and appearance. Both are designed to protect the building and its occupants from weather, while providing daylight and views of the outside. That is where the similarity ends. The systems are designed to best suit the application for which they are intended. Reviewing the features and differences of these systems with the product manufacturers and other building team members will help ensure the building envelope meets its structural, air and water, energy efficiency, durability, aesthetics, and budget objectives.
Most storefront systems are designed as “flush glaze” with no projecting stops, giving it a clean look. Commercial storefront systems are intended to be single span structures, with anchors at the head and sill only. The frames are installed between and anchored to the building structure. Frame heights of 10 to 12 feet can be expected of most aluminum-framed storefront systems. Some systems with a deeper profile can be used to heights of up to 14 feet.
The key difference between storefront and curtainwall is the way water is managed in the system. In curtainwall, each lite of glass is weeped separately. In a storefront system, the entire elevation is weeped at the sill. Any water entering the system is directed down the vertical mullion to the sill, where it is then weeped to the exterior of the building. Consequently, this can lead to the system becoming overloaded when the exposure to weather becomes excessive. For this reason, storefront typically is limited in height. Ideally, it should be located below the fourth floor of a building in areas that are protected from weather elements by an overhang of the roof or other overhead structure. The more protected storefront is from the elements, the better it will perform.
Throughout North America, storefront systems are composed predominantly of extruded aluminum framing and insulating glass units (IGUs). Basic dimensions for storefront systems include 13/4-inch or 2-inch sightlines, and 41/2 to 61/2-inch depths, accommodating 1-inch IGUs. The glass is typically held in place using glazing gaskets and snap in glass stops. Some higher performance systems use structural silicone to hold the glass in place and provide a higher level of performance against weather. Glass usually is centered, but may be offset to the front or back. A 0.375- to 1/2-inch glass bite is common. Glass bite refers to the dimension of the glass captured by the framing system, which provides the seal area to prevent air and water infiltration.
The frames are “stick built” from extrusion, and may be fabricated in the shop or in the field as needed. Pre-fabricated frames can be brought to the site and assembled into the openings. This reduces the amount of field labor required for a job, which helps to keep cost relatively low. In fabricating the storefront, it is important that water deflectors are installed at the intersection of each vertical and intermediate horizontal. The water deflector should be sealed completely, except for the outside corner to direct water down the vertical mullion to the sill flashing. This will prevent water from dripping on the IGU seal, which can result in water infiltration or seal failure. Sealant is used at the joints where horizontals meet verticals. Critical seals are at the ends and back of the sub sill, also known as sill flashing, sill pan or gutter; and at the anchors. Be certain not to seal the weep holes.
Aluminum is an excellent thermal conductor. Where thermal performance is important, manufacturers use a thermal break between the inside and outside surfaces of the storefront system. Two different systems are commonly used: a polyurethane poured and debridged (P&D) thermal break, and a polyamide strut system. Both help increase the condensation resistance factor (CRF) and reduce the system U-Factor. A higher CRF will reduce potential condensation and frost in cold climates, and a lower U-Factor will help meet code requirements and improve energy savings.
Storefront can accommodate a variety of features including entry doors, vents, and sunshades. If the system is installed properly with all of its required accessories, and the manufacturer’s instructions are followed, there will be a very low chance of leaks during rainstorms.
Unlike storefront, curtainwall can span multiple floors, often starting at the first floor, and may extend up several floors. At ground level, it may be used in place of a high-performance storefront system. Often curtainwall will be attached to and extending past the face and floor slabs of the building. The vertical mullions are anchored to the building structure. Horizontal mullions are not typically anchored. Curtainwall will also have higher performance air, water, and structural requirements.
Along with higher performance than storefronts, curtainwall may be unitized or stick built. Unitized systems are fabricated and pre-assembled into units in the shop to ensure performance requirements are not subject to uncontrolled conditions in the field. These unitized systems may be specified for an entire building, or used in combination with storefront systems and/or field fabricated stick-built curtainwall systems.
Similar to storefront systems, curtainwall framing is composed predominantly of extruded aluminum framing. Basic dimensions for curtainwall systems are bigger than storefront 2-inch or 21/2-inch sightlines, and 6- to 10-inch depths. Along with IGUs, curtainwall can accommodate metal and stone panels or other infill.
Heavy wall extrusions, called “back members,” form the framework for supporting the glass and anchorage to the building. For a captured system, the glass or panel is retained by a “pressure plate” or “pressure bar” that is fastened to the tongue of the backmember. Gaskets form the seal to keep air and water out. Face covers conceal the screw fasteners on pressure plates. Alternately, the glass may be held in place with structural silicone, eliminating the need for the pressure plate and cover. This may be done to the verticals, horizontals, or both. Back members and face covers may be ordered in a wide variety of depths and finished in different colors on the exterior and interior aluminum framing surfaces.
Curtainwall also offers longer spans and wider spacing between vertical mullions than storefront. Horizontal and vertical members can be the same profile. The large open tube structure of the backmember often allows for steel reinforcement. This can allow for a considerably longer span between anchor points than storefront. Spans of more than 20 feet are not uncommon and are limited by the available extrusion length and load considerations.
As with storefront, the curtainwall systems’ thermal performance is enhanced significantly with a thermal break that separates the pressure plate from the back members. Additional enhancements include fiberglass or polyamide pressure plates, polyamide struts, and P&D features.
Costs associated with both storefront and curtainwall systems vary by region due to the labor market. Generally, storefront is less expensive and is the product of choice for entrances and elevations on the first and second floor. Curtainwall tends to be more expensive from a material and labor perspective, but it allows for greater flexibility in design and perfor mance. Other common variables that affect price will include the glass type selections, thermal break methodology and finish choices.
Operable windows work within storefront or curtainwall to allow fresh air into the occupied space. This also may bring added value toward sustainable design criteria such as the U.S. Green Building Council’s LEED rating systems. Along with natural light and ventilation, operable windows within storefront and curtainwall can provide thermal performance that contributes to optimized energy savings.
Aluminum framing may be specified with recycled content and recycled at the end of its useful life. Durable finishes enhance the longevity of these systems. Selecting low-emitting finishes and finishing providers that minimize volatile organic compounds may assist with indoor air quality and other green building considerations.
Storefront is often desirable because of its overall lower cost, but is limited to shorter spans, low- to mid-rise installations and more modest performance requirements. Curtainwall is considered the best choice when performance is of foremost concern, for longer spans and for buildings taller than three stories. Both systems offer versatility in meeting aesthetic and sustainable design goals.