R-value is a measure of thermal resistance — the higher the R-value, the better the insulation’s resistance to heat flow. For SIP panels, R-value is determined by core material type and thickness. Climate zone determines the minimum R-value required by the energy code. Panel selection starts at the intersection of those two variables.
How SIP panel R-values are calculated
The R-value of a SIP panel is approximately the R-value per inch of the core material multiplied by the foam thickness. Panel facings (OSB) contribute a small amount to total R-value, but the foam core is the dominant contributor.
- EPS core: approximately R-4.0 per inch at 75°F mean temperature
- Polyurethane core: approximately R-6.5 per inch at 75°F mean temperature
Note that these are nominal center-of-cavity R-values. The whole-wall R-value — which accounts for the effect of framing at panel joints — is slightly lower. IECC compliance is evaluated on a whole-assembly basis, which is typically what the energy code table values represent.
R-values by panel thickness
| Nominal Thickness | EPS R-Value | Polyurethane R-Value |
|---|---|---|
| 3.5″ | R-11 | R-23 |
| 4.5″ | R-14 | R-29 |
| 6.5″ | R-21 | R-42 |
| 8.25″ | R-27 | R-54 |
| 10.25″ | R-35 | R-66 |
| 12.25″ | R-40 | R-80 |
IECC climate zone requirements (2021 IECC)
The 2021 International Energy Conservation Code sets minimum assembly R-values for walls, roofs, and floors by climate zone. These are the values that panel selection must meet or exceed for code compliance:
| Climate Zone | Min. Wall R-Value | Min. Roof R-Value | Min. Floor R-Value |
|---|---|---|---|
| 1 (Very Hot) | R-13 | R-30 | R-13 |
| 2 (Hot) | R-13 | R-38 | R-13 |
| 3 (Warm) | R-20 | R-38 | R-19 |
| 4 (Mixed) | R-20 | R-49 | R-19 |
| 5 (Cool) | R-20 | R-49 | R-30 |
| 6 (Cold) | R-20 | R-49 | R-30 |
| 7 (Very Cold) | R-21 | R-49 | R-38 |
| 8 (Subarctic) | R-21 | R-49 | R-38 |
Nominal vs. whole-wall R-value
Nominal R-value is the center-of-panel R-value — the value that comes directly from the core material calculation. Whole-wall R-value accounts for the thermal bridging effect at panel joints, where dimensional lumber or SIP splines interrupt the foam core. In a well-detailed SIP assembly, whole-wall R-value is typically 90 to 95 percent of nominal R-value — significantly better than stick-frame construction, where thermal bridging at studs reduces whole-wall R-value to 65 to 75 percent of nominal.
When specifying panels for code compliance, confirm with the local building department whether they require nominal or whole-assembly R-value compliance. Most energy codes are written on an assembly basis.
What R-value to specify
Code compliance is the minimum, not the target. A building that just meets minimum R-value requirements will perform at or slightly above the code minimum — which may be well below the project’s energy performance goals or the owner’s long-term operating cost targets. Specifying above the minimum by one panel thickness step typically adds modest cost and meaningful performance improvement. The right target depends on the project’s energy goals, the local utility rates, and the owner’s planning horizon.
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