Relative humidity (RH) is a term that is often referred to when dealing with hardwood floors and it is often the cause for environmentally-based flooring issues. Yet few people understand what RH really means, which is not surprising because it is a bit of an abstract concept. I hope this explanation will help bring it into focus.
Most people understand that humidity is the amount of moisture vapor in the air, but in the case of RH, what is it relative to? The simple answer is temperature. Warm air is able to hold more moisture vapor than cold air. So when we measure humidity, we have to account for how warm the ambient atmosphere is, and state the humidity in relation to that – got it?
You can liken it to a sponge. If you squeeze a sponge in your hand and immerse it in water, it will absorb a certain amount of liquid. However, when you release your grip and let the sponge decompress then immerse it again, it will hold significantly more. The same mass has different capacities for holding moisture, depending on the prevailing physical conditions. The same is true for cold and hot air. Cold air will only hold a small amount of moisture vapor and hot air can hold a lot more.
RH measures the ratio of how much moisture there is in the air compared with how much moisture it could hold at a given temperature. A relative humidity of 50% means that the air contains half of the moisture vapor that the air could hold at that temperature. For example: air at a temperature of 60oF/15oC has a capacity to hold x amount of vapor. If it is currently holding only half of x, then it has a relative humidity of 50%, that is, the air is holding half the volume of moisture that it could at its current temperature.
And here’s the thing: heating or cooling of an air mass doesn’t change the amount of vapor in the air; it changes the capacity of vapor that air mass can hold. Just like a released sponge, warmed air has the capacity to hold more vapor than it did in its cooler state.
A good way to visualise RH in action is to picture a calm lake on a cool summer morning. A layer of fine mist is hovering just above the surface of the water. Where the mist has formed, the air is cool and has absorbed as much moisture as it can – its RH is 100% - but there is actually more moisture than the atmosphere can absorb, and the excess is seen as mist – tiny droplets of water suspended in the air.
The same amount of moisture exists down at the surface of the lake where there is no mist to be seen. That’s because the warm lake water (it’s summer, remember) is heating the air directly above it. This warmer air has more capacity to hold moisture vapor than the cooler layer above it, so the relative humidity directly above the surface of the lake is lower – 75 or 80%, say - even though the same amount of moisture is in the air. And because the warmer air has the capacity to absorb all the moisture, there is no excess to form a mist. The only difference between the two layers of air is the temperature, but that has a significant impact on how much moisture they can absorb. Same air mass + same humidity level + different temperatures = different RH.
So how does all this relativity relate to the hardwood flooring in our homes? As we all know, wood flooring has a certain inherent moisture content, and can exchange moisture with its ambient surroundings, releasing moisture in a dry environment or drawing it in from a moist one.
And because temperature has such a direct relationship to the moisture in the environment, moving wood from a cold space to a warm one can have a significant impact on the wood, because even though the absolute humidity is the same in both rooms, the relative humidity is very different. As with our lake scenario, a cold space with 100% RH will not take on any more water; but the same space warmed up may have RH that’s a mere 25% and will have plenty of capacity to draw moisture from the wood flooring. That is why hardwood installation and maintenance instructions always stress that both temperature and relative humidity be maintained at recommended levels, to provide a stable environment.
Here's a couple of concrete examples: if you have an air mass of 35˚F/2˚C with 100%RH and you warm up that mass to 75˚F/24˚C - with no change to the volume of moisture - the RH would now be 25%, well below the recommended minimum for hardwood flooring. In other words, if you have a cold empty house and you suddenly crank up the heat without also allowing more humidity into the space, the RH will effectively drop. Flooring subjected to this kind of environmental shift would lose moisture and contract, causing gaps between the boards.
The flip side is to have a very warm space - 95˚F/35˚C with 25% RH – and cool it down to 60˚F/15˚C, again without any other influences to increase or decrease the amount of moisture vapor in the air. The cooler air will now have a much higher RH, above what is recommended for wood flooring. The boards will then take on moisture and expand, causing a cupped appearance.
All of this is good reason to recommend customers purchase and install a hygrometer to monitor the RH in their homes. It will help keep them and their hardwood floors healthy and happy.