This is a serious problem that may result from insulation and air leakage problems in wall cavities. The implications are damage to structural members and possible loss of structural integrity over the long term. Damage to interior finishes and indoor air quality problems are also possible.
Detection is going to rely mostly on indirect evidence. Unless there is considerable damage, it's going to be hard to identify this problem without removing interior or exterior finishes.
Damage is usually concentrated at the bottom of walls. Condensation tends to run down wall studs and get caught on bottom plates. The end grain of the studs sitting on the bottom plates wicks moisture up and, as a result, the bottoms of studs and sill plates show rot first. In some cases, there's evidence of staining or rusting nail heads on the inner face of the wall near floor level. Damaged subflooring may be visible. In severe cases, there may also be evidence on the inside of the foundation wall below a wood frame wall, for example.
You should be able to get a sense of the
of the wall. A wall with a vented rain screen is more likely to dry quickly than a face-sealed system. You may be able to see evidence of moisture in walls around electrical boxes. In some cases, you can see staining or rusting if the boxes are metal. You may also see mold and mildew in the box or on the surfaces around them. If you have found a missing air/vapor barrier and lots of air leakage paths into the wall, you should suspect concealed damage. This risk is heightened if the exterior siding is stucco or another face sealed system.
You may also get some clues from the condition of the living spaces. If there is evidence of condensation on walls (especially below windows) or on ceilings, you may be more suspicious of condensation in the wall cavities.
On interior surfaces, look for mold and mildew on upper floor levels, particularly in corners or areas behind furniture or clothing in closets. Areas that are cooler, have lower airflow and are dark are more prone to mold and mildew. If you are in an area with prevailing winds, the windward walls will be colder. Look for mold and mildew on wall and ceiling surfaces on the windward side. The same is not true of damage inside wall cavities. Air leakage from the house into the walls is more likely on the leeward side than the windward side of the house. If one were going to cut an investigative hole to look for interstitial damage from condensation, you might do it on the leeward side of the home on an upper floor level. Air leakage into walls tends to be greater on the upper floor levels than on lower floors, because of stack effect.