Entering U Values of Mixed Assemblies in PHPP
Have you ever been working in your R-Values tab and seen this message at the bottom of your assembly?
This error message is alerting you that you have a highly conductive mixed assembly. This means that the overall R-value of the assembly has greater than 10% total difference between bridged and non-bridged sections resulting from differing components in the same plane having drastically different R-values. For example concrete studs with EPS insulation between.
The PHPP deals with this as per usual – conservatively, by adding in an additional 10% of heat loss when the error percentage is over 10%. Here is a way to get a more accurate R-value input for your PHPP.
First some basics, let’s start with this simple construction assembly:
As you know, the concrete in the assembly has a significantly lower R-value than the EPS it is encapsulated by.
As you enter the assembly into the PHPP R-Values sheet, the error message shown in Figure 1 pops up. Now you ask yourself, “HUH, what’s that?”.
Let’s examine the possibilities together. First, make your typical percentage of material calculation.
Figure 3 shows the initial calculation in plan. We then add for top and bottom plates to come up with our approximate percent of material to enter in the PHPP.
Next we want to examine the effect of the error message on the U value calculation in PHPP.
Now according to PHPP the assembly has an R Value of 18.7, but warns you of the variation possibility because of the dissimilar R-Value of the assembly. So what next? Well, enter the assembly into THERM. Make sure you are using the same boundary conditions and the same conductivities as in the PHPP.
- Interior – 0.74 h-ft2-F/BTU, 68°F
- Exterior – 0.23 h-ft2-F/BTU, 14°F
- Concrete – 1.1905 BTU/h-ft-F (R 0.070/inch)
- EPS – 0.0202 BTU/h-ft-F (R 4.125/inch)
Run the THERM model and examine the results.
As you can see, the actual composite assembly R-Value is 22.17, not 18.7. If you would like to correct the PHPP to the THERM model then you can adjust the “Percentage of Material” in Column J. Adjust the percentage until the R-Value matches the THERM results. In this case the percentage needed to be reduced to 6%.
Now simply input the thickness of the rest of the assembly to get the corrected R-Value for the composite assembly.
– While this method allows a little more accurate assembly calculation the following are some items to consider:
- With highly conductive materials like steel and concrete, the potential for condensation within the assembly increases as there is potential for air in contact with the material. Air seal detailing is important and hygrothermal modeling is recommended (WUFI).
- Minimum interior surface temperatures need to be considered in all assemblies, but in highly conductive assemblies this becomes a more critical element that is not specifically analyzed in the PHPP.
- You may lose the opportunity of “free” R-value by using highly conductive materials. Many localities allow the use of fire treated wood in non-combustible construction. In some assemblies this can represent a significant no cost R increase by simply changing the structural material.
- There is a reason the PHPP was developed with conservative methodology. By using the method above you are removing one of those “safeguards”. Consider the benefits and drawbacks of using this method before employing it.
- It is crucial to make sure the THERM inputs match the PHPP.
- If you employ this method, make sure you include THERM files with the PHPP for certification.