Isn’t there an intrinsic problem with SCIP building systems i.e. thermal bridging?
ClarkandGreen
| Posted in General Questions on
Isn’t there an intrinsic problem with SCIP (structural concrete insulated panels) that the reinforcing trusses bridge the insulation.
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There is no problem if the "advertised" r value properly includes the effects of heat conduction through all of the elements. The r value of the foam core alone is greater than the r value with the reinforcing but comparatively little heat is conducted through the small area of the reinforcing relative to the much larger area of foam. Even steel has a thermal resistance and heat flow is a function of that resistance and the area.
Yes, there is an intrinsic issue with thermal bridging with the steel trusses. As you know, steel is HORRENDOUS when it comes to thermal bridging, it is the worst material in regards to thermal bridging. Steel basically has no R-Value to it. With the R-value of steel at approximately 0.003 per inch, when compared to other construction materials such as wood (average R-value of approximately 2.5 per inch), steel basically has no R-Value to it and is a thermal bridging nightmare.
SCIP's have these steel trusses that are not thermally broken and they will thermally bridge and reduce the total R-Value of the wall. Now the million dollar question. How do you quantify it? ORNL did a Hotbox Test of a SCIP wall and it performed not so well. It dropped the R-Value of the wall foam by 35%. Although the SCIP guys will fight the test results, it makes sense because these trusses are everywhere and it all adds up.
Steel will thermal bridge and when you have a SCIP that has steel trusses that are exposed to the exterior by sitting inside of a shotcrete concrete wall and then that same steel truss bridges to the interior wall, it will reduce the R-Value of that wall assembly. Anyone who tells you otherwise is denying the scientific facts and has something to sell you. Even concrete has a better R-Value of an average R-value of approximately 0.1 per inch. Nothing great but better than 0.003 of steel.
Rebar in concrete slabs will thermally bridge. Engineering companies like Schock will use stainless steel rebar in places to help reduce that thermal bridge since stainless steel performs better than standard steel. Even with the stainless steel, there is still some thermal bridging going on in that rebar. Steel = horrendous thermal bridge
The term "thermal bridge" leads to considerable miss understanding! Unfortunately materials that are excellent thermal insulators are notoriously lacking in strength while materials that offer outstanding strength are very poor thermal insulators. Using a small area of a highly conductive but very strong material to reinforce a much larger area of a good insulator that is very weak is a compromise that is stronger than just the insulation but also conducts more heat than just the insulation. Such a design is not "intrinsically flawed" . A key question is what is the relative area of the reinforcement to the insulation. For example 4 pieces of 1/8" x1/8 steel ( area of 1/16 sq ") used to reinforce one square foot of XPS. Using Peter's value for steel and r5/" for the xps results in a composite that is r 2.9/". Calling it r 5/" would be FRAUD but calling it r 2.9/" would be fine and proper. We could find better materials, the steel could be stainless or possibly a slightly greater area of fiberglass but none of the choices have an "intrinsic problem". They are composites that are stronger than the basic insulator but insulate not as well.
Note to Peter common wood used in typical construction is between r 1/" and r1.3/".