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Hotbox Test R-value Database

SIPA - Structural Insulated Panel Association


Description Of Wall

Hot-box testing presented below was performed on the specific Structural
Insulated Panel (SIP) technology which was selected by the Structural
Insulated Panel Association for the thermal evaluation project with ORNL.
Since thermal properties of numerous SIP technologies can be significantly
different from the case presented on this page, the test data presented
below can not be used as a general thermal performance data for all SIP
technologies and it can not be utilized for getting code approvals.

Structural Insulated Panels (SIPs) have become a widely used alternative construction material for homes and other buildings. The acronym SIPs now usually refers to panels made from a thick layer of foam (polystyrene or polyurethane) sandwiched between two layers of Oriented Strand Board (OSB), plywood or fiber-cement. The result is an engineered panel that provides structural framing, insulation, and exterior sheathing in a solid, one-piece component.

During the time period from September to December 1998, a series of six hotbox tests were performed at the Oak Ridge National Laboratory - Buildings Technology Center on three SIP walls under steady-state conditions.

The following three panel configurations were tested for SIPA (Structural Insulated Panel Association): Panel # 1 - 3.5" foam core with spline, Panel # 2 - 3.5" foam core without spline, and Panel # 3 - 5.5" foam core without spline.

Two tested SIP wall panels contained 3.5-in. thick cores made of EPS and one had 5.5-in. thick EPS core. In all three cases two 7/16-in. thick layers of OSB were used for exterior and interior wall surfaces. SIP panels were joined using splines and solid wood profiles. One SIP wall containing 3.5-in. thick EPS core had 7/16 x 3-1/2-in plywood splines in the center of the wall. The other two walls were connected using solid wood profiles. During the Hot Box testing, the SIP walls were not covered by any finish materials.

Thermal conductivity of EPS foams used in test walls was measured at the ORNL BTC Laboratory using ASTM C518 procedure.


Sample thickness in. [cm]

Test mean
Temperature F [C]

k a Btu-in./hft2 F


from 3.5-in. panel
3.5 [8.78]
75 [23.8]
0.26908 [0.0388]
from 5.5-in. panel
5.5 [13.81]
75 [23.8]
0.2670 [0.0385]



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