Capacitance

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.

IR_90841a  IMG_3852

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.

Coverage

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.

Advantages

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.

Limitations

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.