Hot enough to fry an egg? Possibly, but….
You’re probably less interested in cooking eggs on your deck than you are walking barefoot on it. Your concern is reasonable – especially if the bare feet on your new deck will look like these:
To find out, I put a wide selection of synthetic decking  on 2xs in the hot August sun and measured the surface temperatures. I admit, I am limited by geography. The Boston suburbs this August barely reached the low 80s on sunny days. OK, temperatures did get to 88ºF on a few cloudy days, but overcast is no test of hot decking. You need the relentless sun to raise the temperatures to their realistic maximum. In all, I measured 63 synthetic deck samples from six manufacturers, in 17 product groups. For comparison, I included several wood species, concrete, brick, and asphalt.
The results include some real surprises.
But first, some unsurprising results:
- Within each manufacturer’s series, darker colors get warmer than lighter colors. This relationship remains surprisingly constant, with few exceptions among the 63 samples. The darkest colors in a series average about 13º (9%) warmer than the lightest. But the amount of variation is not consistent; it ranges from a max of almost 24º to a low of only 2º warmer.
- The sun is king. It makes deck temperatures 34º to 76º hotter than ambient air. When passing clouds hide the sun for several minutes, deck temperatures drop 6º to 8º on most decking, and as much as 24º on one deck board. And a corollary to the sun’s supremacy is the time of day. The shallow angle of sunlight at 4 pm lowers temps over 20º from 1 pm highs.
Ok, this is boring. I am reading this blog for the good stuff. Surprise me.
- All synthetics get hotter than concrete and concrete pavers. This is interesting: I didn’t know patios are cooler than decks.
- Wood decking is cooler than almost all synthetic decking. Light colored wood, like Alaskan yellow cedar and STK red cedar, is cooler than all synthetics. Pressure Treated (PT) decking and even ipe are cooler than all but a few synthetics.
- Denser boards are not hotter. Brick is very dense, but also relatively cool. Among the deck boards, the denser, heavier boards with wood/plastic cores are consistently cooler than all plastic boards.
- Cap stock decking is cooler than all-PVC deck boards. Cap stock has a composite core — wood mixed with plastic. It is heavier than all-PVC but noticeably cooler (roughly 20º cooler).
- Indeed, the warmest deck boards are also the lightest. The light weight all-PVC boards from Zuri, Azek, and Fiberon are the hottest.
Well, you finally mentioned manufacturers. Thank you, I was starting to think you were afraid of them. I am trying to learn which company makes the coolest decking. Do you have more?
Yup. I summarized the test results in the table further on. But first permit me a few observations.
Cool. The coolest deck boards are from DuraLife and TimberTech — the Landscape and
Hardwoods series from DuraLife and the Reliaboard from TimberTech. Ironically, these are old technology composites, with core material that is a combination of wood fibers and plastic. Why is this old technology cooler? Well, wood holds and conducts less heat than plastic. The newer, all-PVC boards perform better — resist food stains, mildew, and scratches better — but they are hotter. My temperature readings show this, and the published values of thermal conductivity endorse it.
So, why is DuraLife the coolest?
Duralife has a hard plastic cap, and it’s slightly cooler than TimberTech’s all composite boards, which have no plastic cap. I suspect Duralike’s advantage is in the materials: it uses polypropylene plastic for its cap and in its core. The other manufacturers use polyethylene or PVC, both of which conduct more heat than polypropylene.
Hot. The warmest boards are Azek Vintage and Zuri. Azek’s Dark Hickory (which is pretty dark) reaches nearly 144º, and Zuri’s Brazilia is the winner at just over 156º. Wow, let’s not set this little guy down on Brazilia:
So, what’s going on with Zuri?
Its core material is PVC, like Azek and like Fiberon’s Paramount.
Unlike these and any other synthetics, however, it has an acrylic coating. But acrylic is clear and has a thermal conductivity value nearly identical to PVC.  I can’t explain what’s happening here. I measured Brazilia in the sun to be 76º above ambient air, which agrees with Zuri’s published “heat buildup” factor of 78º.  I did notice that hot as Zuri is, it cools down quickly when clouds pass overhead.
A word of caution here. If you want to know which decking will feel coolest to your bare feet, my measurements are not the complete answer. Ultimately, how hot decking feels is a function of both its surface temperature and its thermal conductivity. My measurements do not address “thermal conductivity”. That sounds like a term from physics. Perhaps some physics will help us:
Here’s Fourier’s Law of Heat Conduction: The time rate of heat transfer through a material is proportional to the negative gradient in the temperature and to the area. More clearly stated:
OK,….that didn’t help. Besides, I didn’t have any spare negative gradients hanging around when I measured the decking.
Let’s try something simpler. I stole this from the internet just for you:
“When you touch something hot, you don’t feel how hot the thing is; you feel how hot it makes your hand. Metal conducts heat more easily than wood. So if wood and metal are hot, the heat will flow more easily from the metal to your hand.” 
That makes sense. If you’ll agree that plastic conducts heat better than wood, we can move on to my chart of deck temperatures.
I’ve already mentioned some limitations – temperatures were taken on 80º days in August in Lexington, MA, just outside of Boston. Don’t read too much into a difference of few degrees between two deck boards. (My infrared thermometer’s error margin is about 2º.) Instead, consider the larger variations — 10º and more – among different manufacturers’ series.
The Chart: Coolest to Warmest
This article is dedicated to the Sun. It can raise a deck’s temperature by over 70º in less than an hour. And it does this from 93 million miles away. Impressive.
I measured surface temperatures with a Fluke 62 Max infrared thermometer. Fluke claims it is accurate to 1.5%, which amounts to about 2º F.
Real Heat. A few years ago, a colleague measured some deck temperatures in central Texas, in the mid-day sun of early August, when it was 100º in the shade. His results were consistent with mine, only hotter because the air was 20º hotter.
The Fine Print. (enlarged so we all can read it). My lawyer made me say this. These temperatures are not guaranteed, and merely reflect individual test results on 3 different days in August of 2017 in Lexington, MA. The temperatures of your deck under your bare feet may vary. Indeed, decking manufacturers will probably claim better results for their decking. Except DuraLife.
Final Thoughts. As I noted above, these surface temperatures are not the full story. Thermal conductivity plays a role in how hot your deck feels. The warmest boards I tested also use the most conductive materials (PVC and polyethylene), so the all-plastic deck boards will probably feel even hotter to your skin than their higher temperatures suggest.
Which decking should you choose? All-PVC decking resists stains, mildew, and scratches, but it is hotter than cap stock with its composite core. It can also be more expensive. If your family will be barefoot much of the summer in warmer, sunny climates – Florida, Texas, Arizona, etc. – consider a wood deck or a patio. Don’t burn your feet on PVC.
Or you could plant trees….
Or just wear….
Oh, can you fry an egg on a deck? That depends on how well-done you like your eggs. Eggs begin to cook at 130º and the whites harden at 158º. But an egg would partially shield the deck from the overhead sun, so you’d need air temps of maybe 100º. Then, if a Zuri deck in Arizona or Texas reached 170º or 175º at mid-day, you could enjoy a late breakfast.
A special thanks to little Konstantino, who helped to test a synthetic deck (TimberTech Tigerwood). And, no, we did not burn him. We photographed him on a cloudy day in the low 70s.
- Return to Main Text I use “synthetic decking” to include several artificial deck types: composite decking (the older technology which mixes wood fibers and plastic), PVC decking, and “capped composites” (which have a thick plastic coating bonded to and covering a less expensive core material).
- Return to Main Text In BTU / (hr ft ºF): acrylic is 0.20, and PVC is 0.19. See: http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
- Return to Main Text From Zuri’s 2016 maintenance booklet.
- Return to Main Text From The Physics Stack Exchange. See: https://physics.stackexchange.com/questions/185997/different-materials-have-different-temperatures , modified slightly.
A. Azek: ESR-1667: http://www.icc-es.org/Reports/pdf_files/ESR-1667.pdf
B. DuraLife: https://duralifedecking.com/news/article/2017/7/25/whats-inside-composite-decking-matters/
C. Zuri: http://www.designbuildersmd.com/blog/how-is-zuri-premium-decking-manufactured
D. Thermal Conductivity: http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
E. To better understand thermal conductivity, attend Kahn Academy for about 10 minutes: https://www.khanacademy.org/science/physics/thermodynamics/specific-heat-and-heat-transfer/a/what-is-thermal-conductivity
F. Bill Nye, the Science Guy, addresses sidewalk cooking here: https://www.livescience.com/37860-hot-enough-to-fry-an-egg-on-the-sidewalk-this-weekend.html
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