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.

IMG_5863a  WA 1

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.

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

Roof Inspections Don’t Cost (in the long run)

Roof inspections do not have to be major endeavor or investments. The most important inspection may be the walkover performed by the building owner, manager or the maintenance guy or gal. Okay, maybe a roof inspection cost a little for the maintenance person to walk around on the roof for a couple hours, but it doesn’t take much of a find for the inspection to pay for itself in real dollars. The modern commercial roof system will cost somewhere between a couple dollars per square foot to maybe ten to twenty times that depending on size, complexity and the materials used in the roof system. The walk-around finds one small hole in the roof that is leaking rain water into the system. The leak has not made it past the insulation layers and decking, so no one knows it is there until the inspection finds it. A call to the roofer and the hole is repaired for $200.

What if there is no inspection and no found hole? The leak may wet out a large area before it leaks into the building and prompts a call to the roofing contractor. We’ll be conservative and say it wets one 4’ x 8’ board of insulation, leaks into the building, phone call, roofer sends out two techs; they spend a 1/2 day, find the hole and repair it for $800. Further investigation determines the insulation is wet and should be removed because this location is over the something-critical area. Removal and replacement of the 4’ x 8’ area costs another $1,500 for a total of $2,300. The figures are imaginary, but the scenario is very real and happens every day.

Which is better? The maintenance person finding the small hole and the minor repair for $200+/- or waiting until the drip hits the floor and the major repair for $2,300+/-.

True Cost of Maintenance

The longer a given roof last the lower the lifecycle cost. Any given roof will last longer with maintenance. If not enough maintenance is performed, the lifecycle is decreased and lifecycle cost increases. If too much maintenance is performed, the maintenance cost exceeds the benefit of increased lifecycle and lifecycle cost increases.  There must be a balance between cost of the maintenance and benefit to the lifecycle.  Just the right amount of maintenance increases the lifecycle and reduces the lifecycle cost to its lowest possible number. The true cost of maintenance is therefore $0.00.