Roof Solar Absorptance Important Input for EnergyGauge USA
January 23, 2024
The roof solar absorptance is an important entry, especially for Florida homes, and it is important that contractors enter this correctly and that building officials check the values under roof inputs on Florida’s input code summary for Florida performance and Energy Rating Index code methods (R405 and R406). To illustrate the point we will share a recent investigation.
We were sent a pdf of a Form R405-2023, Florida Energy Efficiency Code for Building Construction report that indicated it was generated by EnergyGauge USA software. We entered the information into our software and produced a failing e-ratio of 1.03, whereas the report showed 0.94 e-ratio. At that time we indicated, based solely on the pdf report and inaccurate e-ratio, it appeared as though the report was altered. After communication with the builder and the contractor producing the report, we were sent an enb file that resulted in 0.94 e-ratio. So what was the change and which value is correct?
The answer is two-fold and involves roof solar absorptance. In our air conditioning dominated climate, roof solar absorptance is a large factor. Our research has shown a house with a white metal roof, one of the only roofs that has a solar absorptance less than 0.5, can significantly save energy relative to shingle roofs. Shingle roofs tend to have solar absorptance of 0.75 (“white” shingles) to 0.96 (dark shingles). The file and the report each had a solar absorptance of 0.4 listed. The builder indicated he uses the same shingle for all of his homes. Thus, the entry for solar absorptance was not correct based on the home using a shingle roof. For the house depicted in the R405 report, the difference between 0.96 roof solar absorptance and 0.4 roof solar absorptance is the difference in e-ratio of 1.03 to 0.94. We informed the builder that the 0.94 e-ratio was not correct if the rest of the fields entered were as depicted and recommended he plan on some upgrades.
EnergyGauge also has an input indicating that the solar absorptance was tested. If this check box is left unchecked, then for purpose of Florida energy code and ratings the solar absorptance value entered is ignored and the default value of 0.96 will be used. The enb file sent to us by the contractor had unexpected text in the entry for the tested solar absorptance check box. It was different from the exact “Yes” or “No” expected. This value resulted in the check box appearing grayed-in instead of the check mark that would occur when checked in EnergyGauge or the blank when not checked. The software then interpreted this as a checked field when running the Florida code calculation. The R405 report indicated a value of “N” for solar absorptance tested, thus we entered untested when we created our own file. Even with 0.4 solar absorptance entered, if untested, it will result in a 1.03 e-ratio.
We wanted to know how the unexpected text was in the file and noticed that the file originated from Wrightsoft. Wrightsoft sells a software product called Right-Suite with J-link. J-link is a feature Wrightsoft markets as a tool to export a file with the .enb file extension that will load into EnergyGauge USA and require input review and further entries. The contractor sent us their Wrightsoft file they used for Manual J. We imported that into Wrightsoft’s 2023 edition of their software, and running their 2011 EnergyGauge J link (the latest of the choices they have) obtained the expected “No” for this field. However, using another choice, the provided 2008 EnergyGauge J link in their software, we were able to produce the unexpected text that led to EnergyGauge USA interpreting it as tested for code compliance but setting it to “N” on the R405 report (should normally say “No”). Using the 2011 EnergyGauge USA link in Wrightsoft avoids this issue. Furthermore, EnergyGauge will be releasing a software change in version 8.1.01 that will be stricter on interpretation of unexpected text in that field from third party software. We have not received plans for this home and cannot indicate if anything else entered was correct or incorrect. What we do know is the roof solar absorptance entry was incorrect.
In conclusion, the roof solar absorptance entry is important to get right. In the example above it was not entered correctly. The contractor is responsible for making sure all entries are correct. Building officials should verify entries are reasonable and can verify tested results at https://coolroofs.org/directory/roof (see Figure 1).
You will see values like this for each product:
Figure 1. Sample of a product’s characteristics from coolroofs.org/directory/roof
Contractors should use the (1 – 3 Year Solar reflectance value) = Solar Absorptance for entry in EnergyGauge.
So for this product it would be (1-0.15) = 0.85.
Enter 0.85.
If you find the product listed, the tested box may be selected in EnergyGauge USA (see Figure 2).
Figure 2. Enter the solar absorptnace on t he roof screen and check “Solar Absorpt Tested?” if the solar absorptance is listed.
If the product is not listed it is important that you do not check the tested box. See Florida Building Code, Energy Conservation, Section R405.7.2:
R405.7.2 Installation criteria for homes claiming the cool roof option.
The cool roof option may be claimed where the roof to be installed has a tested solar reflectance of greater than 4 percent when evaluated in accordance with ASTM methods E-903, C-1549, E-1918 or CRRC Method #1. Emittance values provided by the roofing manufacturer in accordance with ASTM C1371 shall be used when available. In cases where the appropriate data are not known, emittance shall be the same as the Standard Reference Design. Testing of a qualifying sample of the roofing material shall be performed by an approved independent laboratory with these results provided by the manufacturer.
You can click on the Suggest box in EnergyGauge to populate an approximate value based on the type of material and color you enter but it will not be used for Florida code or ratings. Roof products must be tested and listed to earn credit.
The R405 report will show the entered values as shown in Figure 3. Building officials and plan examiners should verify these fields on code submittal forms.
Figure 3. Snapshot of section on an EnergyGauge R405-2023 report showing roof entries. Roof entries typically print on the second page of the report.
Some references on research we conducted on roof materials:
https://energyresearch.ucf.edu/research/buildings-research/roof-assembly/roof-finish/