Comparing Infrared and Capacitance Moisture Inspections

Two of the most used non-destructive inspection technologies are infrared thermography and capacitance.  There are pros and cons to using either.

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How each technology works.

Roof surfaces emit infrared energy which is viewable using an imaging radiometer (infrared camera).  When the inside/outside temperature of a roof assembly is sufficiently different, areas of the assembly  that contain wet insulation will conduct more energy to the surface and the surface will radiate more energy, allowing the thermographer to locate patterns in real time that distinguish  between the wet and dry areas to be marked on the roof surface.

Capacitance meters use two pads a few inches apart to read the electrical characteristics of the roof assembly below the meter.  Dry material below a dry membrane yields a zero reading while wet materials will yield a positive reading. Readings are taken on a grid pattern.  Wet readings are grouped together and marked as a wet area on the roof surface.


Infrared requires the surface to be clear and viewable.  Areas that are under equipment or covered in water, loose equipment or debris are not viewable.  The roof has to be the same construction and under the same energy influence.  Radiated energy from walls or equipment and convection energy from roof units or wall louvers will affect the surface temperatures.

Capacitance meters only read the location below the meter.  A good meter is sensitive enough to detect trace moisture.  The reading can be considered to represent a larger area depending on the type of insulation and roof construction.


Infrared provides an image of the wet area which may help identify the source.  Infrared may locate a smaller area that a capacitance grid could miss or wet insulation under a dry layer that a capacitance meter would not detect.

Capacitance can be used during the daytime and require less trips or time on site.  Capacitance can detect trace moisture that may not affect the surface temperature enough for infrared to see a pattern.


Infrared thermography requires a dry surface at the time of the inspection and for a period of time before the inspection.  There must be an inside/outside temperature difference of the roof assembly in order to provide the energy to conduct to and radiate from the surface.  Wet areas may be masked by sufficient energy from walls, roof units, or anything that can influence the surface temperature.

Capacitance requires a dry surface at the time of the inspection.  The readings are spot and taken on a grid.  A small wet area in between grid points may be missed.  The meter cannot distinguish lighter moisture near the surface from heavier moisture in the top layer of insulation.

A Real Life Comparison

Recently we had the chance to compare the two head to head on a small modified roof.  We performed the capacitance inspection first by walking every other run of the membrane taking a reading every 5-6 feet.  We located and defined 3 wet areas by taking additional readings around the positive readings until we encountered negative readings in each direction.  After the roof cooled, the same areas were located and no additional areas were discovered.  The left photo is after locating and marking one of the wet areas using the capacitance meter with the dots indicating a wet reading and the No Symbols indicating a dry reading.  The thermogram is of the same area after the roof had cooled.  Looking close at the thermogram, one can see the marks from the capacitance inspection.

Either technology would have worked and found all there was to find on this roof under the conditions and at the time of the inspections.

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Which is best for your roof?

It depends.  Generally, if one layer of Perlite or wood fiber, a capacitance inspection may give the best bang for the buck. Multiple layers or where you want to find where the core of Polyisocyanurate has wet, infrared can provide more information. When the entire condition of the roof is desired a combination of infrared and capacitance will provide information about the plies, the top layer of insulation and lower layers.  And for some roofs a nuclear moisture inspection may be a better fit than either, but that’s another article.

This article is written primarily considering low-slope roofing.  The same applies to other similar constructions such as insulated wall assemblies or insulated steep-slope roofing.

Trace Leaks to the Source

IBD can use several non-destructive ways to trace leaks to their source.  See below or download Trace Leaks to the Source.


Leak source located at vent pipe using infrared .
Two leaks located on ballasted EPDM roof using infrared
Area of wet insulation as seen from below a metal deck on an EPDM roof.
Locating where leak reaches the metal deck using infrared.
Active leaks at fasteners on a metal roof using infrared.
Leak traced to vent pipe using a capacitance roof moisture meter.


Thermography Brief – Moisture in Ballasted Roofs

Infrared thermography can be used to locate wet insulation in a ballasted single-ply roof system. The ballast stone does increase the challenge and may require waiting longer after sunset to start an inspection. A higher vantage point can help to pinpoint areas for closer inspection. The standards call for a large inside/outside temperature difference, but this is not always necessary. The main condition is that the ballast stone is spread evenly and has a uniform depth.


The wet area is around the vent and extends diagonal toward the lower left of the thermogram. See next image for a more defined view from above.  Ballasted EPDM / EPS insulation / concrete deck. Outside temperature was 73F+/-.


The above image shows the same wet area of the first image, but taken from a the vantage point of a higher roof level.  Ballasted EPDM / EPS insulation / concrete deck. Outside temperature was 73F+/-.


EPS insulation wets (absorbs moisture) very slowly and a small wet area like this one only 1-2 feet wide will appear amorphous.  Ballasted EPDM / EPS insulation / concrete deck. Outside temperature was 73F+/-.


A distinct board pattern extending from below a raised walkway on the right side. Ballasted EPDM / Polyisocyanurate insulation. Outside temperature was 34F+/-.

Download the PDF version of this Thermography Brief.

Moisture Inspections

Moisture Inspections

Opportunities to Improve Building Performance and Lower Cost

  • Roof inspections
  • Wall inspections

Roof Inspections

Quality assurance inspections

Performed during or shortly after the completion of a new roof or replacement roof project.  The purpose of the inspection is to show any of the hidden details that are viewable with the infrared. What is viewable depends on the type of roof, insulation and installation.  This type inspection should be planned in advance of the project.

Fastener patterns due to the minor amount of heat transmitted thru the fasteners.

Preventative roof moisture surveys

A cost effective use of infrared in low-slope roofing.  Inspections are scheduled annual or biennial depending on the age of the roof and are started well before the expected decline of the roof system.  The first inspection serves as a baseline.  Minor problems are located and marked for repair, sometimes even before they leak into the structure.

This small wet area was only 2 feet across, representing a minor inexpensive repair on this TPO roof system.

This large wet area contained roughly 350 square feet, representing a major and expensive repair.

Inspections to pinpoint hard to locate leaks

Most leaks can be located and stopped without the use of an infrared camera.  There are some leaks though, that using infrared could be beneficial, even considering the cost. Those leaks are the ones that keep coming back, or maybe never leave.  The ones that several have looked at and it still leaks.

Looking up at a steel deck, the cool (darkest) spot is where the leak first hit the decking. The rib that shows cooler than the decking is the one where the water is running

Pre-restoration/replacement roof moisture surveys

The most common type of roof moisture survey accomplished with an infrared camera.  The survey is performed before a roof is restored or replaced to determine how much, if any, of the insulation will need to be replaced.

A clear distinction can be seen between the the wet insulation boards to the right and the dry to the left.

Open flashing around this pipe was found to be the apparent cause of this wet area located on a MB roof system.

Wall Inspections

Locate moisture in EIFS

EIFS is everywhere and everywhere EIFS is moisture is also.  Most everything behind the surface is adversely affected by moisture.  It is important to stop moisture before it can damage the insulations or structure.  An inexpensive non-destructive infrared inspection once each year can help protect the investment of a entire building shell.

Darker areas to the right contain moisture under the surface of the EIFS.

Moisture appears to be penetrating along a crack in the surface.

Locate moisture in masonry

Masonry units that are sealed by paint may take on moisture through pinholes in the paint.  The moisture affects the surface temperature which is visible to the infrared camera.

Moisture in this masonry wall shows as darker.

Trace leaks behind metal panel, vinyl and other thin sidings

Moisture on the back side of a thin panel will change the surface temperature and may be viewable by the thermographer.

Thermography Brief – Moisture in Metal Roof Systems

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Large cool (dark) area indicates moisture in the vinyl faced roof insulation. Green arrow points to apparent leak entry along rake edge of roof.  Red arrow points to single dripping location approximately 30’ down and 12’ over from entry point.

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Red arrow points to single dripping location from this large wet area. Cooler (darker) pattern indicates moisture in the insulation.

Additional Notes

  • This 20,000 square foot building had 10 wet areas totaling an estimated 1,255 square feet.
  • 7 of the areas were wetting the insulation but had not started to leak to the interior.
  • Best time to inspect and trace leaks to entry point is during or shortly after a rain.
  • Some insulations dry faster and patterns may disappear within a few hours after a rain.
  • Patterns often indicate the path and/or entry point of the leak.

Download PDF of Thermography Brief – Moisture in Metal Roofs

A Reminder to Cut the Roof

Stuart L. Raney Level III Certified Infrared Thermographer

It is a typical roof inspection using an infrared imager to locate hidden moisture. The roof is in pretty good shape and no exceptions have been located on the first two sections. Walking across the third roof section, the first exception is spotted. It is a small one, roughly 2’ x 2’, and appears to be half of a 2’ x 4’ Perlite board.

Stepping in the middle of the exception reveals the softness created when board type insulation becomes wet. With a small exception like this, it is tempting to mark it and move on to the next, but first let’s check it with our capacitance meter. Sure enough, the meter pegs the needle, but to make sure we whip out the pin-type moisture meter. Inserted into the center of the area, it also pegs the needle. So now we have a footstep and three advanced pieces of technology that all agree the roof is wet, or do they?

The footstep only tells us the roof was slightly softer in that area. The infrared imager only reports that the radiated energy was slightly higher. The capacitance meter only reports that the electrical impedance of the area is different from the area around it. The pin-type meter only reports that it encountered a different electrical resistance.

In order to confirm the presence of moisture we take a core sample of the roof. What we found was a piece of sheet metal laid below the membrane, apparently to cover the opening left by an old vent pipe that had been removed. The metal changed the radiated energy seen by the imager, the impedance seen by the capacitance meter, the resistance seen by the pin-type meter and small hole in the deck changed the firmness felt by the footstep. All these were good reasons to suspect a wet area but none good enough to verify one, even when all four agreed.

This is an old tip, but one worth revisiting. This exception was actually encountered on a recent inspection and could have been misinterpreted had the roof not been cored to confirm or deny the other results.

Perhaps a good way to understand the importance of core cuts is to realize that the visible evidence of a core is the only method of investigation that determines if a roof is wet or dry. Infrared imagers, nuclear gauges, capacitance meters and even pin-type resistance moisture meters can only be used to narrow down areas of the roof and limit the number of cores that must be taken. So if you are in the business of roof moisture surveys, your primary tool is a core cutter. You just use the fancy equipment to tell you where to do the real work.

First published as an Tip of the Week for August 31, 2009