Certified Moisture testing Blog

Interior vs. Exterior Probe Testing

There has long been a dispute as to whether moisture probe testing the external walls of a structure should be performed from the interior or from the exterior.

There has long been a dispute as to whether moisture testing the external walls of a structure should be performed from the interior or from the exterior. The nationally accepted practices favor exterior probe testing because water typically winds up on the exterior side of the wall cavity once it works its way into the system. However, there are many factors to consider when determining which testing approach to use on a given home.

Why do we want to do probe testing for moisture?

For starters, why test at all? Moisture in the walls of homes continues to be a nationwide problem. The causes are many but the effects are the same: a potentially rotted home with tens of thousands of dollars in damage. Bulk water – rain or snow – enters the wall through a breach or juncture, typically unsealed window joints, roof terminations, decks, porches, etc. When water finds its way past the first line of defense – the cladding on the home – the next obstacle you hope it will encounter is some sort of building paper that serves as a water-resistant barrier. If this isn’t present in the system, the water will have reached the sheathing of the home. When water gets into the system and isn’t channeled out, it will permeate the porous sheathing and eventually degrade the material.

The most common argument against exterior testing is that it harms the home because the test sites create an opportunity for water to enter through the water-resistant barrier. What is often not understood is that the water-resistant barrier already has thousands of breaches from every nail and staple used to attach the cladding, trim or metal lathe. When performing exterior probe testing, trained inspectors properly seal the small inspection holes.

Exterior testing also allows the inspector to get to every area that needs to be inspected, something architectural design and large furniture can prevent inspectors from being able to do from the interior. For example, window corners are a high-risk area where we often find problems. Typical construction practices create a stud framework around the window with a stack of wood studs at each corner supporting different parts of the building above. As a result of these stud blocks, it is not possible to accurately test a window corner from the interior. Also, most homes have limited areas that can be accessed from the interior – cabinetry, wall tiles, plush wood paneling, etc. all limit where interior testing can be done.

What kind of tools do we want to use for testing moisture?

Another thing to consider is the type of tools that can be used for assessment and where their use is most effective and accurate. Currently, there are no non-invasive testing tools or techniques that will conclusively identify moisture levels or damage in the walls, whether tested from the exterior or the interior. Probe testing is the only method that measures moisture and allows for damage assessment. When a probe is pushed or tapped from the exterior, the sheathing should feel firm and solid. If the sheathing is deteriorated, the probe will easily push through the sheathing into the open wall cavity. When testing from the interior, deteriorated sheathing can feel firm because it is tight to and supported by the cladding. The sheathing is then unable to give when pushed, making an accurate assessment of damage more difficult from the interior.

Because intrusive testing is a necessary part of accurately assessing moisture, there is also the matter of restoring aesthetics after holes are drilled. This is a far easier task to perform from the outside because monotone claddings and textured materials help blend and hide the test sites. Interior colors and wall materials vary from room to room, making it difficult, sometimes impossible, to disguise test sites. However, when the cladding is impenetrable, such as with brick, metal and true stone, the test sites cannot be adequately repaired. In these cases, interior testing would then be the preferred method.

Exterior probe testing is almost a requirement

Testing the exterior of the home has become a very common, almost a required, inspection for buyers, especially on homes clad with stucco and stone. National relocation companies, schools, and engineers all recommend testing from the exterior because bulk water enters from the outside. None of these organizations teach interior testing as part of their testing protocols simply because the findings tend to be inconclusive. Taking all these factors into account, there are times when interior testing is necessary, but an accurate assessment is best achieved from the exterior.


If you’re looking to do testing on Interior walls, exterior or both request an inspection

If you need an inspection done on a home or business, we offer a handful of services that can help put your mind at ease. When it comes to moisture detection, mold issues or even air quality testing we are your certified pros. We’ll help solve the issue, just click to contact us, send us an email or give us a call at 651-272-5552. We’ll contact you to schedule the inspection!