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Are You Relying Too Much on RH Readings?

Craig DeWitt

I spend a lot of my time looking at issues with buildings, and in the process I see a lot of wood floors. The floors are great sources of information, maybe not quite in the style of tree-ring information, but they still provide solid information. Since wood acclimates to the environment rather slowly, wood gives a better picture of long-term average conditions than an instantaneous measurement.

In a class I just taught in Las Vegas, I showed that indoor relative humidity (RH) can vary from pretty high in the morning to pretty low in the afternoon. So when you inspect a floor can influence your results. If you want the indoor environment to be outside some particular range, choose your inspection time accordingly. To get a better picture of long-term average conditions, measure the moisture content (MC) of some wood objects in the house. Cabinets and furniture are good items to measure. They are exposed to the indoor environment and can provide relatively long-term data. If your floors measure different, you still have long-term information, but you also have an indication of something else influencing the floor.

For example, this is a great time of year for low power bills. With the mild outdoor temperatures, in many homes neither heat or AC is running. That means indoor humidity levels can creep up. If you are inspecting a cupped floor situation, and you measure 63% RH (like my house was running today), can you conclude that the high RH caused the cupping? You could, but you may be wrong. The high RH may be temporary, and may be due to the mild temperatures and lack of heat or AC. Check again tomorrow when this cold front comes through, and you may find low RH with cupped floors. The MC of the floors and other wood objects will give you a better idea of what happened to cause the cupping.

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