Moisture levels in the wood structure of your home could be determined by weighing the home before and after it's dried. The difference in weight is the percentage of moisture in the wood. Instead of weighing your home, CMT inspectors probe the wood frame of the home and measure the moisture content with electric resistive testing meters—protimeters.

The American Wood Institute says that typical construction woods (SPFs) should be dried to at least 19 percent moisture content before being used in construction. The reasons for drying wood are to:

• Prevent significant shrinkage of framing members as they continue to dry.
• Minimize warping as the wood continues to dry.
• Prevent or minimize mold growth on wet wood (see mold facts).

The woods used in construction will continue to dry after a home is framed, roofed, and cladded. Most homes that do not have water intrusion problems have moisture levels between 7 to 13 percent in the wood structure of the home. A moisture level above 13 percent may indicate that moisture is being introduced into the home. Because many factors go into determining whether a moisture reading is significant or not, detailed testing and analysis are needed to verify suspected moisture problems.

Moisture Testing Methods

Click to jump to a specific method: Interior Probe Testing, Radioscopic Scanning, Infrared Photography, Bore Scopes and Visual Inspections.

Exterior Probe Testing

Can be used to test most homes. Two small holes (3/16” in diameter and about one inch apart, like a small snake bite) are drilled through the siding only. Test probes are inserted into the sheathing, behind the siding, to measure the moisture content and check for damage. After the test is complete, the holes are sealed with color-matching caulk.

Exterior Probe Testing is the preferred method used at CMT for the following reasons:

• Most moisture found in a home is caught between the sheathing and the cladding of the home on the exterior side of the wall cavity.
• Testing the exterior of a home allows the inspector to test all areas where moisture is found without being limited by framing members, floors, ceilings, furniture, and other items that can get in the way.
• Exterior testing allows inspectors to seal the test holes with color-matching caulk that virtually conceals the test holes. The test holes DO NOT harm the home or cause new sources of moisture entry.
• Exterior testing does not breach the vapor barrier on the internal side of the wall cavity.
• Exterior testing allows the inspector to determine the condition of the sheathing, which is necessary to make a damage assessment. Damage assessment is an integral part of the moisture test and is critical to filing a claim. A damage assessment is also needed to accurately determine the amount of remediation that may be needed on the home. 

Interior Probe Testing

Used by CMT to test those homes that are not accessible for exterior testing. Two small holes (1/8” in diameter and about one inch apart, like a small snake bite) are punched through the interior sheetrock and vapor barrier. Test probes are inserted into the sheathing, through the six-inch wall depth, to measure the moisture content. After the test is complete, the probe holes are sealed with a spackle compound. Damage assessment is often difficult to conclude with interior testing as it has limits — high risk areas are often inaccessible.

Radioscopic Scanning

Used by CMT on vinyl, wood, and EIFS to indicate areas where moisture maybe present. A wet wall scanner does not measure moisture levels; instead it detects areas of potential moisture. Probe testing the indicated areas is necessary to measure moisture and assess possible damage.

A scanner works by reading the density, not moisture, of the material found in the wall cavity. A scanner may offer incorrect readings because of the presence of electrical, plumbing, HVAC systems, cementious-based products and metal lathes. Be cautious of inspectors who use scanners to moisture test on stucco applications. Scanning traditional stucco just doesn’t work because most scanners do not penetrate deeper than 1/2” and some stucco applications themselves are a half-inch in thickness, not including the sheathing. Our scanners can go as deep as 3” and are much more viable tools.

Infrared Photography

Used to show areas where a temperature difference exists in a wall, roof, ceiling, or attic area. This method is often sold as a non-invasive tool, to indicate where moisture intrusion may be. However, its use should be limited because it only shows temperature variances, not moisture levels. Our experience indicates that infrared photography has an accuracy level of 25% leaving the remaining 75% of moisture issues go unnoticed. Additionally, this method does not assess damage or dry rot. For this reason, some home sellers, builders, insurance adjusters, and realtors request infrared inspections because they don't want to find moisture problems. Even when infrared testing is used, probe testing is still necessary to measure the moisture level and assess the damage to wood in the wall cavity. Be cautious of inspectors who rely exclusively on infrared photography for moisture testing as this method should only be used as a heat loss tool.

Bore Scopes

A tool used to peek inside the wall cavity. This verification tool has a limited capacity because its viewing area is very small, making water damage assessment difficult. Again, probe testing is the only way to accurately measure moisture levels. Be cautious of inspectors who use bore scopes inserted into the wall at electrical outlet boxes because most outlets are in areas where moisture intrusion issues are not typically found.

Visual Inspection

A necessary part of every moisture test. A qualified inspector knows where roof flashing is needed to keep a home dry, where caulking should and should not be applied, and knows the signs of moisture and where moisture issues occur. A visual inspection will help identify where problems may exist; however probe testing is still required to accurately measure moisture levels.

Many times stains are present on the exterior cladding. CMT's experience has shown that approximately 50 percent of the time stains do not mean moisture intrusion is occurring. However, stains need to be probe tested because moisture intrusion in the wall cavity is not visible.

The Bottom Line

Probing the wood structure of your home is the only way to accurately measure and assess moisture levels and damage. CMT inspectors always use the least invasive method to accurately measure moisture levels and assess damages.