What is “Ghosting?”

I recently performed an inspection on a day that the “Delta T” was perfect for Thermal Imaging. Oh wait, I should back up… I know what you’re thinking, “What the heck is he talking about?” “And what the hell is a Delta T?” Ok, the term Delta T is a term used in Thermal Imaging and the HVAC industry as a measure of the difference in temperature. HVAC technicians use this to measure air temperature before being either heated or cooled and after. The difference in the temperatures is referred to as the Delta T. It is a way to determine if the system is operating correctly or not. In Thermal Imaging a 15 + or – degree delta T (the difference between indoor and outdoor temperature in this case) is necessary to allow the camera to show issues with insulation, moisture intrusion, etc. On this particular New Glarus day, the delta T was somewhere around 52 degrees.

As I usually do, I scan the entire home with my infrared camera as part of my standard home inspection. Even when conditions aren’t like they were on this day, the thermal camera regularly shows me where each framing member is. With the naked eye you couldn’t tell in this home but sometimes you can, more on that later. On this day each framing member was clearly visible through the lens, indicating that the attic insulation was not covering the bottom chord of the trusses. In the past, fiberglass batt insulation was often laid between the trusses exposing the attic side of the framing members. Fiberglass and cellulose insulation had an R-value exceeding 3 per inch of thickness. Wood members R-value is somewhere around 1 to 1.5 per inch so these locations would be much cooler on the living side of the home and will allow warm air into the attic during the winter. This creates a condition that will cause extensive condensation of cold surfaces. This is also one of the most common causes of extensive mold formation.

In this home a mold remediation had been done in the past but the contractor did not increase the insulation levels. Guess what? Yup, the mold returned. At first glance I thought it was a ventilation issue as this is the most common defect when it comes to mold in attics but the ventilation system was corrected during the most recent roof replacement. Oh, and by the way, if you are in the process of getting quotes for a roof replacement and the roofer does not have anything in the bid regarding the ventilation it may be best to move on to someone else. Every time the roof is replaced the ventilation system should be addressed or at least verified that it is designed and operating correctly. Also, the roof vents should also be replaced on each re-roof. On this particular home both the mold remediation contractor and roofer neglected to address the real issue with this attic which was inadequate insulation levels and air sealing.


Ghosting is actually a slang term for shadowing that forms on ceilings and walls at the framing member locations. Technically it is called “Thermal Bridging”. If your ceiling has dark shadows at each framing member it is caused by condensation forming during periods of cold weather, dust particles stick to these locations which forms the “shadow”. Most commonly these show up on the ceiling near the exterior wall where the truss meets the wall. Often times it is difficult to insulate these areas well and it results in cold spots. On some homes they run all the way across the ceiling. I usually know what I’ll find in the attic in this case. The insulation Must cover the ceiling joist or bottom chord of the truss.

In conclusion, there are many causes moisture damage in the attic, not all of it comes from the exterior, as a matter of fact I would say a good portion of attic moisture issues I find actually come from the interior due to condensation damage caused by poor air sealing and insulation levels.

I hope you found this post informative, as always if you have a home related question, email me at the address below and I will help as much as I can.



Aaron M. Zuehlke is the owner and inspector at Zuehlke Inspection Service, LLC, a full-service home inspection company serving Southern Wisconsin. Specializing in Home inspection, Radon Testing, Mold Testing/inspection, Residential Thermal Imaging, and Manufactured Home Foundation Certifications. He also manages several rental properties through Zuehlke Properties, LLC. He can be reached by email at Aaron@Zuehlkeinspections.com or by calling the office at 608-931-7485.

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