Humidity and Wood Floors (With a Vented Crawl Space)


We’ve been in our home for a couple of years. We’ve had various issues (this seems to be a common theme for new home owners regardless of the age of the house). The home was built in the 1920s so almost everything is wooden.

This year we’ve noticed a change in the wood floor in many areas of the home. Specifically, more boards creak and some even seem to give more than they used to. Part of this may be caused by the shifting of several sets of jacks used to provide additional support to the floor joists. Until I go back under the house I won’t know if the jacks have shifted – this is very possible if they weren’t set on a solid base such as concrete or blocks.

Regardless, I believe I’ve narrowed down one other cause. The amusing aspect of this is that it turned out to be the opposite of what I expected.

About a week ago I picked up an inexpensive temperature and humidity sensor from Walmart for about $7. I suspected humidity was a contributing factor. After all, our house has a vented crawl space. Frankly, it could use some more work, too. There doesn’t seem to be plastic everywhere beneath the house nor is there any kind of insulation beneath the floor (yet).

Initially, I suspected that a high humidity level was causing the changes in the floor. Based on some quick Internet research it seems that 60% humidity is considered the high-end of indoor humidity levels, especially for homes with wood floors.

For about the first week after purchasing the sensor I noted that the humidity never went that high. For the most part, it was between 30%-45% on average. It started to creep up into the high 50% late last week.

It wasn’t until the indoor humidity levels reached 60% and higher that I realized that the boards weren’t popping or creaking as much as they had the previous week. Once I realized this I started to laugh to myself because it became obvious what was actually going on and why things seemed to have changed more since spring.

The first couple of summers we tried to save money by programming the thermostats to allow the house to warm up during the day, while we were at work. There were times I had it set to let the temperature reach 85 degrees, though later, and last year, I would lower it to 80 and 76. This actually didn’t seem to help much in our home, which probably has much to do with the fact that the floor and walls are not insulated and the attic has a minimum level of insulation (somewhere around R11 with several areas lacking any insulation).

At those settings the house was warm (or even hot) when we came home and it took the house several hours to cool to a comfortable level. For example, it might reach 76 by 10 pm on some nights. That may sound comfortable but we have a large, old house and most zones do not match the temperature at the thermostat.

This year I set both thermostats to 74 degrees when we’re not at home, which seems to be the ideal temperature for keeping our home comfortable without needing the A/C units to run constantly (they actually ran more frequently when the thermostat was set higher).

This week we’ve had some rain. In addition, the temperature has been much cooler. As a result the A/C units haven’t been running as much and, naturally, the indoor humidity has increased.

So, what’s going on and why did I find this amusing? Letting the A/C cool the house to 74 (or close to it) reduced the average indoor humidity level. Doing this consistently, across an entire summer, has apparently reduced the amount of moisture in the boards themselves.

To sum up a very long post, running the A/C at a near-constant 74 degrees during the summer caused the boards to shrink due to lower humidity. When the boards shrunk, the floor changed.

What’s the solution? Well, this isn’t an area I know very much about but I think we need to have plastic installed over all of the ground in the crawl space, which may make a minor difference (there’s plastic over much of the ground, but not all of it). I think the biggest difference will occur when we have insulation added beneath the floor, which should reduce the temperature difference between the top and bottom of the boards and also halt air flow through the wood floor, which we do know does occur in some spots and probably through the house in undetectable drafts. It’s very probable that we may find a significant difference in our energy consumption, once we add insulation.

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