This article reviews a new Exponent analysis of the December 2025 fire-hospitalizes-one-resident/”>fire at PG&E’s South of Market substation in San Francisco. The report ties the blaze and a major Bay Area blackout to humidity, condensation, and aging infrastructure.
While the focus is on San Francisco, Marin County folks should pay attention. This has big implications for regional reliability, weather resilience, and the ongoing debate over who should actually control critical power assets in the Bay Area.
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What Exponent Found and Why It Matters
The Exponent report says humidity and condensation likely caused the fire at PG&E’s South of Market substation. Moisture, air infiltration, and fire risk overwhelmed equipment protection that was supposed to handle these threats.
Investigators noticed burned spots and warping on a barrier insulating board, plus metal erosion on a circuit breaker a month before the fire. That breaker failed a November test and got replaced.
Exponent argues the damage probably relates more to the insulating board issues than to the water damage PG&E suspected at first. The substation’s ventilation system pulls in unconditioned outside air and doesn’t have heating, cooling, or humidity control. That leaves equipment exposed—especially if a fire room heater wasn’t running during the event.
For Marin County readers, this highlights a bigger issue: our own critical infrastructure sits at risk when weather and old equipment collide. In towns like San Rafael, Mill Valley, and Novato, people rely on a fragile chain of power, water, and communications. Heavy rains or heat waves could easily strain these systems, just as they did in San Francisco.
Humidity, Board Failures and the City-Grid Debate
The barrier insulating board, designed to shield equipment from moisture and air, takes center stage in Exponent’s findings. Officials flagged concerns about moisture buildup that could spark fires or degrade components. This shifts the conversation from a simple water leak to a deeper maintenance and design problem.
No climate control in the substation’s ventilation system means shifting Bay Area weather—like storm surges near Sausalito or Point Reyes Station—could leave equipment vulnerable. Marin residents, used to unpredictable coastal weather, might see the same risks in local infrastructure in Marin City or Fairfax. Everything depends on reliable power and decent moisture management.
San Francisco city leaders jumped on the report to push for more municipal control of the grid. That’s a topic plenty of Marin communities care about, especially those wanting more reliability and local accountability.
The Exponent findings echo questions from last year’s outages. Are state and utility investments actually keeping up with aging assets in dense cities, or would more distributed systems serve places like Tiburon and Larkspur better during big events?
PG&E Response and Upgrades
PG&E pushed back, highlighting about $3 billion in capital upgrades across San Francisco over the past twenty years. They say they’ve stepped up inspections, testing, and maintenance since the fire.
The company lists some real changes: heaters and dehumidifiers now run at the Mission Street substation, humidity and temperature monitors are in place, equipment’s been swapped out, and “weatherproofing” is ongoing for indoor substations. These steps aim to stabilize Bay Area substations, but Marin County folks are still watching. Outages don’t care about city lines from San Francisco to San Anselmo or Corte Madera.
San Francisco’s City Attorney David Chiu blasted PG&E for allegedly knowing about equipment damage in advance and not acting. That’s fueled lawsuits over outage responses. In Marin, people often ask how regional cooperation, state policy, and local resilience plans could stop similar scenarios from hitting towns like Novato, San Rafael, or Ross when storms and power surges meet old infrastructure.
Marin’s Take: Resilience, Municipalization Ambitions and Practical Steps
The Exponent report ramps up calls for a more resilient Bay Area grid. This idea is catching on not just in the Mission District but all over Marin County, from Mill Valley’s hillsides to the bell towers of Sausalito.
No Marin city has gone all-in on municipalization yet. Still, local leaders and civic groups keep discussing microgrids, battery storage, and community resilience hubs as ways to rely less on the unpredictable national grid during peak demand or wild weather.
Practical Takeaways for Marin County Residents
Marin homeowners and small businesses might wonder how this report fits into daily planning for resilience. Here are a few actions worth considering:
- Monitor local outage alerts and have backup power ready for essentials. This matters most during winter storms or thick fog that can push circuits to their limits.
- Support and join in on community resilience projects—think energy storage pilots, microgrid plans, or better weatherproofing for shared spaces around San Rafael or Corte Madera.
- Push for more transparent utility reports and quicker maintenance cycles. Marin’s climate is tricky, and equipment in towns like Sausalito and Larkspur can fail from moisture if ignored.
- Keep an eye on Marin County or city meetings about grid reliability, municipal options, and emergency prep. These conversations tie into bigger Bay Area infrastructure debates.
Here is the source article for this story: Humidity at PG&E Substation Likely Cause of Massive December San Francisco Blackout
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