Designing for a Cool Future in a Warming Climate

(Why AC Units May Soon Be the New Normal In San Francisco)

I don’t mean to complain, but it’s mid-November here in San Francisco and I’m uncomfortably warm in my apartment. It’s been this way most of the year.  Given that I’m a Certified Passive House Designer (CPHD), my first response was to look for the cheapest and simplest passive solution to solve this problem: better shading.

SF Shade Sail

Image 1: Source - Bronwyn Barry

Our building is a condo with a Home Owners Association (HOA.) That means any modification to the building exterior requires the approval of the HOA.  This limited my options somewhat but I did manage to find a temporary, seasonal solution that worked well for most of the summer. I ordered and installed a simple shade sail that blocked a good portion of the solar heat gain coming into our unit.  It was under $300, including the installation hardware, and I was able to easily install it myself. 

While this shade sail did show some cooling benefit, it wasn’t perfect. I needed a bigger, more expensive custom size to cover our excessively large window area.  It also took just one short rain shower (our only one thus far) to demonstrate that this wasn’t going to be a permanent solution.  During our one rain shower, the wind rattled the hardware against the building and kept both us and our neighbors up all night - not a smart move in a condo building. This meant that I needed to take the sail down early.  Which is how I come to be overheating here. In November. In San Francisco.

This is where the AC units come in.  Like most solutions-oriented people, my husband suggested we simply install one of those window mounted air-conditioning units. Duh. These are commonplace in New York City, but are unheard of here in the Bay Area.  Problem solved.  Except, as I stated earlier, I’m a CPHD. This goes against the grain of everything I advocate in both my work and volunteer efforts: a gizmo tech solution that doesn’t address source control and uses more energy.  Ugh.

Building Scientist, Dr. Allison Bailes III, deftly explains what a Degree Day is here:

“It’s just the difference between the outdoor temperature and the base temperature multiplied by the time it's at that temperature.”

For more info on Degree Days, read his longer blog post here.)

Now I know we’re not the only family in San Francisco that is overheating so I thought I would take the opportunity to try and understand what was happening more specifically.  The effects of global warming are no secret.  Experts have warned that cold places will get colder, and warm places will get warmer.  What I uncovered here certainly brought this prediction into sharp focus. 

The following data was obtained from a website called:  It allows anyone to download the data on their heating or cooling degree days for any place on the globe that has enough weather data available.  (Not all places in SF have the complete info so I used San Francisco Airport as my data point.)

The first chart I created illustrates the monthly Heating Degree Days (HDD) for SFO, using the baseline temperature of 65F. This is the standardized number typically used.  My graph shows that over the past three years, our HDD’s have dropped from 291 to 193 – a drop of 33%.  That roughly translates to us not needing to use our heating systems for as long as we used to. Bonus.



Image 2: Source - Bronwyn Barry, SFO Heating Degree Days

It was the next chart, showing our Cooling Degree Days, which had me sweating - not from the temperatures that caused this whole investigation, but from the implications that this info could have on our future building designs and energy use. The chart shows that our average annual cooling degree days (the orange bars) have increased more than three-fold over the past three years.  That’s shocking!

Image 2: Source - Bronwyn Barry, SFO Cooling Degree Days

What this means for San Francisco and much of the Bay Area is that our normally mild, marine climate, is rapidly becoming not so mild.  These ‘heat storms’ are exponentially increasing our need to supply cooling to our buildings in order to stay comfortable. If the trend continues, we can expect hotter weather with more families like mine desperate for cooler temperatures and scrambling to install air conditioning.  

Without digging too deeply into the implications this will have for our utilities (way over my pay grade) it could also potentially drive up energy costs.  Utilities already use peak day pricing to motivate customers to shift their energy usage to off-peak times.  Unfortunately this peak rate coincides directly with when you most need cooling. 

As a Certified Passive House Designer, this information is incredibly instructive.  It means architects, builders and designers need to be even more careful about how we design our buildings.  Our mechanical systems will need to include options for cooling.  We will have to specify lower SHGC windows and we will need to optimize every passive shading opportunity we can leverage, so our buildings can withstand the trending increase in cooling loads this data reveals.   That’s very sobering indeed.


Bronwyn Barry, CPHD

Design Director, One Sky Homes