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Corey Squire

How much sun do you block when you block 80% of the Sun?

Posted by on 8/26/13 at 10:08am

Ever since Herzog & De Meuron’s  De Yong Museum and Thom Mayne’s  San Francisco Federal Building, perforated sheet metal has been all the rage. There are plenty of strong opinions on the aesthetic merits of perforated metal, but we are more interested in looking at it from a performance standpoint. Specifically, we wanted to know the effects of a perforated metal canopy on the comfort of those sitting below.

To answer the question we set up an experiment in the parking lot behind the office. Our in-house data collector, Maite Bermudez, placed two sheets of metal, one 20% perforated and one solid, on bricks four inches above the asphalt. Each hour she used an infra-red thermometer to measure the temperature of the asphalt under each sheet of metal and an area in direct sun. By three in the afternoon we had our results.

dogrun1

The unshaded asphalt reached a temperature of 157°F, the shaded asphalt reached 115°F, and the area beneath the perforated metal reached 124°F. Shading the asphalt kept the surface 42°F cooler than if it had been exposed to direct sun, while Shading with perf lowered the temperature by 33°F. The difference between the two comes out to 20% – the same as the open area of the perforated panel. In the words of a famous director of sustainability, “Physics wins again!”

Dog run spotty hand

It makes sense that allowing 20% of the sun’s rays through the canopy will make the surface below 20% hotter, but will the space be 20% less comfortable? Being subjective, this is a much harder question to answer.  In 1986, E. Arens et al. attempted an answer in their paper, Thermal Comfort Under an Extended Range of Environmental Conditions. Based on their finding we came up with a graph comparing percent perforation of the canopy with perceived increase in temperature. Standing beneath a 20% perforated canopy at noon on a clear July day in San Antonio, the occupant would perceive a 4.5°F increase in temperature. If the Dry bulb temperature is 100°F, direct sun would feel like 127°, complete shade would feel like 100° and standing beneath a 20% perforated canopy would feel like 104.5°.

dog run graph

It’s a good thing we collected the data when we did. The next day the dumpster ended up on top of the experiment.

dog run dumpster

 

 

  • Rick

    I’m curious to hear what your takeaway is from this. Do you think the
    20% heat gain under the perforated sheet is minimal? Is the heat gain
    offset by the quality of light and possible air flow? Or is the perceived temperature higher because of the sun on the surfaces under the perforated metal? Thanks for your thoughts.

    • Corey Squire

      The sun generates a lot of energy. Even if you’re blocking
      80%, the occupants are still exposed to over 100 W/sf of direct solar
      radiation. This added radiation can make the occupants feel 5 to 7 degrees
      warmer than if they were under complete shade. If it’s 100 degrees outside, it
      will feel like 106 degrees under a 20% perforated canopy. The subtitle take
      away is this: if the goal is to provide a comfortable outdoor space, blocking 80%
      of the sun is not going to be enough.