Apologies if this question has already been answered - but I've searched as thoroughly as possible and did not find a satisfiable explanation.
I am running a simple test with the following values; and the results received by both the formula found in the LEED Reference Guide (Equation 1), as well as by the "Weighted Average Calculator" (LEEDuser documentation tool) seem to be illogical.
Values:
Steep-slope roof area 1: 3000 sq ft. (SRI 26)
Steep-slope roof area 2: 7000 sq ft. (SRI 23)
Mech. Equipment area : 2300 sq ft.
Skylight roof area : 200 sq ft.
-------------------------------------
Total Roof Area : 12500 sq ft.
Net Roof Area : 10000 sq ft.
These values seem to me; obviously, would not qualify for SS Credit 7.2.; yet both the formula on the reference guide; and the Weighted Average Calculator, assert that the sample project complies with the requirements of SS Credit 7.2.
Seems I'm not understanding the calculations correctly, and I would appreciate input from the LEEDuser expert group.
Thanks in advanced for your time.
Gabriel
Mara Baum
Partner, Architecture & SustainabilityDIALOG
674 thumbs up
August 7, 2013 - 8:21 pm
Gabriel, the calculations aren't relevant in your case because both roof types have too low of an SRI - steep sloop roof SRIs need to be at least 29. The calculation is only helpful if one value is too high and one is lower.
Gabriel Moreno
1 thumbs up
August 8, 2013 - 10:37 am
Mara, thanks for your response. However, perhaps I'm not making myself clear enough.
For sake of clarity. Please consider the implications of changing the 3000 sq ft. of SRI 26 roofing material, to a material with SRI of 32.
Would this sample project then comply with SS Credit 7.2?
Gabriel Moreno
1 thumbs up
August 12, 2013 - 5:42 pm
The great thing about mathematics is that the proper equations/formulas, always prove accurate; no matter the circumstances.
It seems evident that the LEED Reference Guide (Equation 1), as well as by the "Weighted Average Calculator" (LEEDuser documentation tool) use an equation that is not accurate. In the sample project data offered above, the Equation should prove that the project does not comply with the requirements of SS Credit 7.2. Yet, the results actually state that the project does qualify.
Is anyone out there willing to verify?
Mara Baum
Partner, Architecture & SustainabilityDIALOG
674 thumbs up
August 12, 2013 - 6:02 pm
You're right about math - if the formula is right then it's accurate. If both the USGBC calculation and the LEED User calculation are giving something unexpected, then you may want to rethink the source of the problem.
I'm out of the office without my reference guides so can't comment more than that until later in the week.
Gabriel Moreno
1 thumbs up
August 12, 2013 - 6:28 pm
Yes, Mara. I agree with you. This is why I took the time to seek a second opinion with LEEDuser; which is one of the important reasons why I paid for the subscription.
I would really appreciate if you could take the time to actually do the calculations using the reference guide (Calculations - Step 3 - Equation 1); or simply enough, input the sample data I've provided, into the LEEDuser calculator - and think of the implications of such a simple example that shows less than 75% of the roof was covered with a material SRI > 29.
Hope you will find the time to check it out; otherwise it will continue to be difficult for you to understand the question I am bringing up.
Thank you for your time.
Gabriel
Mara Baum
Partner, Architecture & SustainabilityDIALOG
674 thumbs up
August 12, 2013 - 6:29 pm
Reading this comment I think I understand your question better.
This credit only requires 3/4 of the roof to have a high SRI, and a weighted approach over 100% of the roof is permitted. In other words, you could achieve the credit with having 75% of the roof at 29 and 25% of the roof at 1. Or, you could achieve it by having 100% of the roof at 28 (assuming steep slopes). However, if 25% of the roof had an SRI of 1 and 75% of the roof had an SRI of 28, then you wouldn't achieve the credit.
The credit language can be slightly confusing because the verbiage doesn't explicitly say this.
See also the FAQ above "We don't have enough SRI-compliant roofing to earn the credit, but we have a lot of relatively high-SRI roofing (SRI 70). Is there any way to earn the credit?"
Erin Holdenried
Sustainability Architect125 thumbs up
August 12, 2013 - 7:05 pm
Hi Gabriel, I think you need to change the way you think about how the weighted average calculation works. I admit, it confused me at first too. Just because a roof has an SRI below the minimum, does not automatically mean it cannot achieve the credit. What matter is how much below the minimum the SRI is, and the percentage of roof that each material covers. Consider two examples that achieve the same result:
1) a steep slope roof with one material can achieve the minimum SRI for 75% of the roof if 100% of the installed material has an SRI of at least 22.
OR
2) a steep slope roof with two materials where 75% of the roof has a material with SRI of 29 and 25% of the roof has a material with SRI of 0. This also meets the credit requirements.
I have had projects where over 100% of the roof was compliant because the SRI of 100% of the roof material was above the minimim SRI.
Gabriel Moreno
1 thumbs up
August 12, 2013 - 7:19 pm
Thank you Mara and EH for your input.
It is indeed confusing to read the language of the equations presented in credit requirements section, as compared to the language presented for the weighted average equation. They might seem to contradict one another - the former explicitly excludes any roof areas not meeting the minimum SRI requirements; while the latter evidently allows them.
However, both your explanations have helped me to understand the optional ways a project can fulfill the credit requirements.
Thank you for your time.
Regards,
Gabriel