Energy Conservation Code R-value vs. U-factor

[Harrye]

I noticed that the R-value method TABLE ECC402.1.3 includes requirements for continuous insulation.

The U-Factor method however does not, TABLE ECC402.1.4. Why is this? Are they not the same standards?

Please advise.

 

[tmurray]

Typically U-Factor is a whole assembly approach where you take into account thermal bridging by repetitive framing members. You typically calculate the thermal resistance of the assembly by performing a parallel heat flow calculation and an isothermal planes calculation.

The added continuous insulation method is a prescriptive path for compliance. the U-factor method is more of a performance path.

 

[Harrye]

That is an excellence explanation. What does the COMcheck software provide? Would that be considered a prescriptive path for compliance or performance? The envelope compliance certificate spits out both R-values and U-factor figures. The U-factors seem to meet code but the R-values glaze over this requirement for continuous insulation.

 

[RLGA]

The U-factor involves the entire assembly from exterior air film to interior air film. U-factor is the inverse of the R-value and considers, as tmurray indicated, the thermal bridging of stud elements. For example, a wall with 3-1/2" metal studs at 16" o.c. with R-11 insulation has an effective R-value of 5.5 (0.18 U-factor) due to the thermal bridging of the metal studs. The R-value table only takes into consideration the R-values of the insulating materials without considering how the insulations' performance is altered by the application in which it is used. Thus, the R-value tables into consideration the thermal bridging inherent in stud framing. Since metal studs are greater conductors of heat, the R-value table requires continuous insulation to overcome the thermal bridging. For wood-framed walls, the wood is a much better insulator, so the R-value table gives two choices: provide cavity insulation with continuous insulation to break the thermal bridging, or provide cavity insulation with a much higher R-value to compensate for the thermal bridging.

 

[RLGA]

That is an excellence explanation. What does the COMcheck software provide? Would that be considered a prescriptive path for compliance or performance? The envelope compliance certificate spits out both R-values and U-factor figures. The U-factors seem to meet code but the R-values glaze over this requirement for continuous insulation.

I would only use COMcheck when considering the component performance alternative of Section C402.1.5 (2015 IECC). This uses a give and take approach where the overperforming wall assemblies can offset the deficienies of underperforming assemblies. If your building envelope components (i.e., walls, roof, slab-on-grade, fenestration, etc.) comply with the prescriptive requirements of either the R-value method per IECC Section C402.1.3 or the assembly method (U-factor, C-factor, F-factor) per IECC Section C402.1.4, then there is no need to use COMcheck.

 

[Harrye]

It is my understanding that COMcheck is used as a compliance tool for the prescriptive path and an energy model is used for the performance path.