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That study on green roof cost vs. benefit had some numbers that don't add up | Debate the Data - Vivisector

That study on green roof cost vs. benefit had some numbers that don't add up

I saw a report claiming green roofs cut energy costs by 25% over a 5 year span. But when I checked the raw data, most of those savings came from buildings in cooling dominant climates like Phoenix. For a place like Portland where it's mild half the year, the payback period jumped to 15 years instead. The study averaged all the locations together to get that 25% number. So is the real story that green roofs work great in some places and are a bad bet in others? Or am I missing something in how they weighted the data?

3 comments

3 Comments

emma_garcia2mo ago

You changed my mind, I used to think any green roof was a win.

jordan_hill2mo ago

Honestly, that "averaged all the locations together" part is where stuff gets muddy. I remember when some city I was looking into put green roofs on like four municipal buildings and then claimed a city-wide energy reduction that was basically just from one well-placed building. They used that same trick of lumping everything together even though two of the roofs barely did anything because they were in the shade half the day. Your point about Portland vs. Phoenix is really what people need to hear because it shows the real cost depends on your specific weather, not some fake national number. Ngl, if I saw that report I'd be checking the raw data for each city myself before I put a single plant on my roof.

fiona_kim972mo ago

lmao your "shade half the day" bit reminded me of my friend Sarah who got all excited about putting solar panels on her roof. She was looking at this national average that said they'd pay for themselves in like 8 years, but her roof faces north and she's got huge oak trees blocking the sun for most of the afternoon. After she actually tracked her own sun exposure over a month, she realized that 8 year number was more like 15 years for her specific house. She ended up just planting a couple of shade trees instead to cut her AC bill. Feels like that green roof stat you're talking about would work the same way, where you gotta look at your own literal shadows and weather patterns before trusting the average.