Cool Roofs: An Option For Resilience
Help your house stay cooler. Help your roof last longer. Save energy cooling down.
If you are building or replacing your roof - choosing a white-colored, standing seam metal roof is one of the best, longest-lasting roofs you can choose. If standing seam is (understandably) outside of your budget, choose the lightest color, highest SRI roof you can get.
SRI - Solar Reflectance Index
The website coolroofs.org collects and calculates data on the “Solar Reflectance Index”, or SRI, for roofs. This measures how much heat and sunlight is reflected back and radiated out by different surfaces. The higher SRI the better and cooler your roof will be - with 0 being the lowest (all black, total absorption) and 100 being the best (total reflection, coolest surface).
The PNNL Cool Roofs website has a lot of detailed background information, and a decision flowchart (down at "How to Select and Install a Cool Roof or Wall Product"):
The flowchart helps you to choose based on your Climate Zone:
In terms of energy savings, cool roofs and cool walls are clearly appropriate for new and existing homes in Climate Zones 1A up to 3A and 3B.
In climate zones 3C, 4A, and 4B, the energy saving benefits are generally marginal for new homes, but worthwhile for older homes with less insulation.
For climate zones 4C and above, the overall energy benefit of cool roofs and walls is generally small.
Bigger Impact In Hot Climate Zones
As you can see from the flowchart, installing a cool roof has the biggest impact in hot climate zones, where you are most affected by extreme heat. Energy Vanguard and this Florida study agree - claiming that choosing a white metal vs black asphalt roof can lower your roof temp by 10 to 20 degrees… in Florida.
Bigger Impact On Older Homes
On the Green Building Advisor forum, building expert Martin Holladay says:
A "cool roof" saves energy when installed on a poorly designed home. It won't really save any energy when installed on a well-designed home.
If you follow GBA recommendations -- and all of your ductwork is located inside your home's conditioned envelope, and you have ceiling insulation that at least meets minimum code requirements -- you will see hardly any benefit from a cool roof.”
But Still Useful For Resilience
However the PNNL site notes details on cool roofs for resilience:
“For resistance to extreme heat, however, cool roofs and walls could be beneficial in all climates (heat waves can occur in even the coldest U.S. climates).”
More:
“Regardless of the amount of energy savings, cool roofs and walls can be a key player in helping homes be more resistant to extreme heat events.
In homes without air-conditioning, cool roofs and walls can result in lower inside temperatures and improved comfort in the summer.
From a community perspective, cool surfaces have been shown to reduce the urban heat island effect.”
Thus even if you live in a colder climate - or your power goes out - a light-colored, cooler roof can still makes difference when it gets hot.
Holladay seems to agree:
“The effect of solar radiation on your roof during the winter is less than it is during the summer, because the summer sun is higher in the sky than the winter sun. So you may still want to choose white roofing, in spite of its winter disadvantages.”
—Martin Holladay, Green Building Advisor forum
Improved Roof Longevity
I spoke with four roofing companies and they all told me me that reducing heat in the attic will directly improve the lifespan of your roof and shingles, so the cooler you can make it - and the more you can exchange air to lower the temperature - the better.
Solar Panels Also Reduce Heat
If your roofing project ever includes adding solar panels these will also help to keep your roof and house cooler, and reduce wear and tear on your roof. A study in in San Diego (Climate Zone 3) suggested that solar panels can keep the roof cooler by up to 5 degrees Fahrenheit:
“The [cooling] benefits are greater if there is an open gap where air can circulate between the building and the solar panel, so tilted panels provide more cooling.
The more efficient the solar panels, the bigger the cooling effect.” —Dominguez, Kleissl, Luvall
Choosing Cool Roof Materials - The Big Chart
So what material should you choose for the best SRI? Coolroofs.org has lots of data. I compiled numbers for many roofing types into a spreadsheet, displayed as an image below:
Here you can see that standing-seam white metal roofs rank the highest, and seem to hold up that way over time. I was surprised that some colors of asphalt shingles - e.g. dark brown - actually have a *lower* SRI rating than some black shingles. This suggests it is best to actually test or check the data rather than simply basing by color.
Unfortunately coolroofs.org does not have numbers for several types of roofing: slate, cedar shake, rubber, or even solar panels. If you have a good source for SRI data on these please let me know. (I also wish Substack had better options for formatting table data so I could format this data as text rather than an image.)
At Least Choose A Lighter Color
But what if metal standing seam is outside of your budget or just not in the cards? Even when all you can do is install standard asphalt shingles, it still seems worthwhile to choose a lighter color:
“A Florida utility study showed that choosing white shingles rather than dark shingles resulted in about a 4% reduction in measured space cooling at no extra cost to the home buyer or builder.” —PNNL
Cool Roof In Climate Zone 7
I live in Climate Zone 7, where it rarely gets above 95 F / 35 C. Even so - that is still too hot for me. We also have a vented attic, no ductwork in the attic space, and heavy attic insulation (R95).
With our vented attic and good insulation, I don’t count on any ‘bonus’ heating in the winter time from the sun’s rays. Our roof is usually covered in snow. When it’s not - any minor heat from winter sunshine won’t make it through our insulation to warm the actual house. I certainly *do* try to capture warm sunshine through our south-facing windows. But installing a cool roof won’t affect that.
Thus: selecting a lighter color, higher SRI shingle seems like an easy benefit with little downside. It costs literally nothing to simply choose a different color.
While installing a lighter, cooler roof won’t do much for our energy use or electricity bills (we don’t have air conditioning), I very much enjoy using passive cooling methods to keep our house cooler during the summer. A cool roof could help to make our house cooler in the hot summertime, make it more livable, and perhaps add some increased lifespan for the shingles. It seems like the most resilient choice we could make.
References
PNNL - Cool Roof info + decision flowchart (based on Climate Zone)
Cool roof product directory - lists SRI ratings for many products
Fine Homebuilding Magazine - “Save Energy With A Cool Roof” (PDF), 2011
Book: “The Resilient Farm And Homestead”
“Use steel or slate.” Strongest. Lasts the longest. Easy to maintain. Reusable.
“Asphalt as roofing should simply be against code in all climates” (p.233)
BeOpt - Building Energy Optimization Tool (downloadable + installable)
ORNL Roof Saving Calculator (dead).
Used to once be a calculator to show you the effects of cool roofs.
Unfortunately - seems to have been taken down, and only has links that redirect you back to the main page.
RoofCalc (dead, spam) apparently also used to exist.
Is now just full of spammy ads, with no actual useful content.
BuildItSolar Blog - experiment with white-washing your roof
Personally I would not do this. But they claim it reduced attic temperatures by 30-40 degrees F.