Building Diagnostic-Service Panel Corrosion

The following post is a copy of the report I sent to the owners of this home having severe corrosion of their electrical service panels.

I was asked to investigate the source of corrosion and rust in the electrical service panels for a couple living in Northern Minnesota.  The original service panels were replaced in 2022.  Deterioration of the replacements has warranted a third set of electrical panels. 


The service panels are located in the garage on a common wall with the conditioned living space.  The panels are installed inside the 2×6 wall cavity with what appears to be 2 inches of extruded polystyrene (XPS) insulation between the back of the panel and the drywall of the conditioned space of the home. 

The normal heat source inside the garage is electric in-floor heating cables.  A floor drain is present, and at the time of my first visit, a dehumidifier was operating.  The in-floor heating system has not been in operation since March 9th of 2022 and two small electric space heaters were supplying heat to the garage.  The homeowners indicated they have seen garage humidity levels as high as 70%.  The temperature inside the garage is typically maintained at 50°F.

The photos below show the corrosion and rust in the panels.

I installed three sets of SensorPush HT.w temperature and humidity monitors around the home.  The first set was placed in the garage, a second inside one of the electrical panels with the cover installed, a third was placed inside the home. 

Results of the testing showed the humidity inside the garage holding mostly between 42% and 48% with a high reading of 50%.  Temperatures inside the garage were between 50°F and 55°F with a low temp of 33°F.  My assumption with the low temperature was that the garage door was open for a period of time.

The sensor inside the service panel showed an average temperature of 55°F and humidity level of 35%-40%.  A low temperature reading of 44°F was registered during the same time that the garage temperature dropped to 33°F.

The average temperature inside the home was 72°F and humidity levels ranged from 20%-22%.

There was evidence of accumulating road salt (sodium chloride) on the garage floor.  The salt is brought into the garage during the winter months on the two vehicles that are stored inside the heated garage.  As the vehicles warm, the ice and snow mixed with the road salt melt and accumulate on the floor.  The liquid water eventually evaporates, leaving salt crystals behind.  During my initial visit, I could smell salt in the air inside the garage, indicating that some is becoming airborne.

The first set of electrical service panels lasted several decades before corrosion and rust damaged the equipment.  The newer equipment lasted only a year.  I believe the in-floor heating system not working is the main reason for the rapid deterioration of the panels.  Radiant heat systems warm surfaces, which in turn, warms the air.  The heated floor warmed the framing and drywall directly in contact with the electrical service panels, which caused the temperatures of the metal electrical components to remain above the point where condensation can form, at least most of the time.

The humidity level inside the garage during testing averaged around 45% with a high reading of 50%.  I was told the humidity has been as high as 70%.  The temperatures inside the garage are maintained around 50°F.  At those readings, the dew point temperature is a little more than 40°F.  There were several times during the testing that air temperatures inside the garage were below 40°F.  I believe this occurs when the garage doors open and cold vehicles enter the garage.  The metal cases and components of the electrical panels become cold, condensing moisture out of the air.  The condition of the panel indicates this happens often, and in some cases, the moisture accumulation must be substantial.  There is evidence of rusty water stains under the electrical panels seen in the top photo.

A second component of the corrosion I believe is the sodium chloride accumulation inside the garage.  The second photograph shows some accumulation of white film on the main breaker of the service panel.  I believe this is salt dust, which is accelerating the corrosion.

There may be one additional reason for the corrosion, and that is by way of air movement.  With the electrical service panels being installed in the wall cavity between the living space and garage space, there could be some air movement inside the wall cavity.  When the home was originally constructed, air sealing wire chases were not common.  If there are holes present low, near the floor (possibly entering the conditioned basement) and high in the wall (into the vented attic space), the wall cavity can act like a chimney, moving air through the cavity.  This air movement might be able to draw moisture and salt into the cabinet from the garage space.  We are now required to seal all drilled holes and other penetrations that leave the conditioned space of the home. 


I have four suggestions that I believe will greatly reduce the corrosion of the electrical service panels.  One is to reduce the interior humidity level in the garage.  The second is to repair the in-floor heating system.  The third is to dilute the concentration of salt dust in the air.  Lastly, seal any wire penetrations that lead into the unconditioned attic spaced and into the conditioned basement directly above and below the electrical panels. 

The first and third suggestions can be accomplished by installing an air exchange unit, specifically an HRV or heat recovery ventilator.  The CFM rate does not need to be large; I would start at around 25 CFM of continuous air exchange during the heating season and then monitor the garage humidity level.  I would like to see the humidity level maintained at 30%.  The rate of air exchange could be adjusted so that humidity level is maintained.  The advantage of using an HRV is that the constant exchange of outside air with inside air will naturally dilute the salt dust.

The HRV model I would consider is the Broan AI series 110 CFM HRV (model B110H65RS).  This is the smallest HRV Broan makes in the AI series with a low CFM rate of 35.  A little more than is needed, but in the ballpark.  The AI series is self-balancing meaning it is constantly measuring the exhaust air with the incoming air and adjusting to maintain a balanced air flow.  This will eliminate the risk of pressurizing or depressurizing the garage, which could cause other issues.  I also recommend following the manufacturer’s instructions on maintaining the unit, which will be required every six months to one year.

Repairing the in-floor heat system should keep the metal equipment of the electrical service panels warmer, reducing the risk of the equipment reaching the dew point temperature.  A side note, HRV systems do require a slightly warmer environment to operate correctly, the temperature of the garage may need to be raised slightly to around 60°F.

The last suggestion I have is to seal any electrical penetrations above and below the electrical service panels.  This would be accomplished by using canned spray foam both in the attic directly above the panels and in the basement.

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