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Insulated Sandwich Panels

By: Darryl Dixon
Composite Technologies Corp.


In the U.S., it is estimated that buildings consume 65 percent of the electricity generated and produce
30 percent of the national output of greenhouse gases. This has produced a shift in design and construction practices towards methods that significantly reduce the negative impact building operation has on the environment. One such design area addresses the energy efficiency of the buildings themselves and the thermal resistance (R-Value) of the building envelope.

While Tilt-Up concrete itself integrates a high amount of thermal mass into the structure, most don’t realize that 7 inches of concrete has the same R-value as a typical pane of glass; approximately R-1.4. Today, most model energy codes have implemented legislation or require a building to exceed minimum thermal performance requirements. One such way to meet and exceed minimum insulation requirements is with an Insulated Sandwich Wall Panel.

Tilt-Up Sandwich Wall Panels have been used extensively for years, but given current code requirements and the shift towards panelized construction and sustainable systems, sandwich walls have grown in popularity and frequency. Whether it is schools, offices, correctional facilities or houses of worship, Tilt-Up sandwich panels provide the durability, speed of construction, and design flexibility of Tilt-Up while providing significant R-Value and moisture protection.

In general, sandwich wall panels are comprised of two layers, or wythes of concrete, separated by a layer of rigid insulation, and are tied together with a series of connectors or fasteners. The performance of the Tilt-Up sandwich panel, both structurally and thermally, depends greatly on the capacity of the connector and the detailing of the insulation.

A sandwich wall panel is formed up to the total wall thickness, which will include the exterior layer of concrete, the integral insulation, and the interior layer of concrete. The panels themselves are prepared in a fashion similar to that of an un-insulated Tilt-Up wall panel. Architectural features, such as reveals and rustications are laid out and a bond breaker is applied.

Specific to a sandwich wall panel is the thickness of the exterior layer of concrete and the rigid insulation. The minimum exterior wythe thickness is 2 inches plus the depth of any reveal or rustication. Therefore, if incorporating a 3⁄4-inch architectural reveal, the minimum exterior concrete thickness is 2.75 inches. Also unique to a sandwich wall panel is the mix design and anchorage capacity of the connector. Since the exterior concrete layer of a sandwich wall panel is typically non-structural and the connectors are installed in a plastic mix, pursuant to ACI 551R, the concrete is typically reinforced with a 6x6xW2.9xW2.9 mesh positioned on plastic bolster strips or chairs, and a slump of five to seven is recommended to ensure proper consolidation of the connector. Additionally, minimum size aggregate is 3⁄4-inch and a 4000 psi minimum concrete is typical.

The insulation thickness is a function of the desired R-Value and interior condition (ambient, cooler, freezer). Ambient facilities typically require 2 inches of extruded polystyrene insulation, while coolers require 3 inches, and freezers utilize 6 inches or more. In all cases, an isothermal analysis and dewpoint analysis should be performed to confirm R-Value and condensation control respectively.

In order to achieve a high R-Value and consistent thermal and moisture protection, the insulation must be placed edge-to-edge, completely separating the two layers of concrete. If the insulation is not continuous, thermal bridges will occur, resulting in a loss of R-Value as well as increasing the likelihood for moisture migration.

The casting of a sandwich wall panel typically spans two pours. The exterior wythe pour and insulation/connector placement occurs simultaneously. The second step, involves the installation of the interior concrete reinforcing, lifting and bracing hardware, and any inserts isolated to the interior wythe of concrete. Prior to installation of these items, however, is the testing of the connector anchorage. A pull-out test on the connector should occur once the exterior concrete achieves 25 percent of its 28-day strength. Depending on outside ambient air temperature, this could be 12 to 36 hours. Additionally, in some cold climate locations, this could require a cylinder test. Below is a reference chart:


Ambient Temperature Range (oF)
70 and higher
60 to 70
40 to 60
Below 40

Minimum Time from Casting to Anchorage Test (hrs)
Field-Cured Cylinder Test Required

Once the anchorages of the connectors are confirmed and the interior concrete components are in-place, concrete placement proceeds and is placed and finished, erected and braced, in a manner consistent with un-insulated Tilt-Up wall panels.

The finishing and sealing of sandwich wall panel varies by climate location and interior conditions. Panel joints are sealed both inside and out with a closed cell backer rod and caulk. Joints may also be foamed in place with an expandable urethane when deemed necessary by the designer.

Once in place, sandwich wall panels provide an ideal solution to the design community by combining concrete’s thermal mass and the performance of integral insulation, all while maintaining the architectural and functional integ- rity of Tilt-Up concrete walls.

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TILT-UP TODAY, a publication of the Tilt-Up Concrete Association, is THE source for Tilt-Up industry news, market intelligence, business strategies, technical solutions, product information, and other resources for professionals in the Tilt-Up industry. A subscription to TILT-UP TODAY is included in a TCA membership. Subscriptions for potential TCA members are also available. If you would like to receive a complimentary subscription to the publication, please contact the TCA.