Demystifying Relative Humidity

April 27, 2014

Posted in macstaff

When it comes to flooring, hardwood is one of the most enduring choices you can make, both in terms of style and durability.  But, like anything of quality, it takes a bit of tender loving care to keep it looking its best.  We have discussed in another post the basics of how wood responds to water, and you know that too much water will cause wood to expand in damaging ways.  It is also true that if wood is allowed to get too dry it will contract significantly, creating unsightly gaps and sometimes cracks.  These conditions, though, are extremes.  In many cases, just maintaining your home at a temperature and humidity level that is comfortable and healthy for you and your family will create the right conditions for your hardwood.

Modern homes are designed to keep the air inside the house comfortable and healthy.  These homes are relatively air tight, meaning that the moisture content inside the house will fluctuate far less than that of the outside air.   Plus, newer homes have central heating and air conditioning units that will also help to keep the relative humidity at a comfortable level.  If your home is older, it may be prone to “leakiness”, meaning more outside air comes in around windows, fire places, doors, etc.  These leaks can often be minimized, however, and there are a number of ways to compensate for the reduced stability.  Keep reading!

What is the right amount of moisture for the air in my home?

The Environmental Protection Agency recommends that relative humidity for wood floors be kept below 60% in the summer to keep away mildew, rot, and physical discomfort that can come from humidity levels that are too high.  In the summer, the agency suggests levels between 25% and 40% to avoid air that is too dry, which can lead to sinus infections and other illnesses.  Now if this seems like a pretty wide range, that’s because it is.  These are the outside limits of healthy relative humidity within your home and, especially if your hardwood is engineered, it should be ok within this range if you don’t push the limits.  However, you and your family, and your wood floors, furniture, doors, etc., are all going to be more comfortable in an environment with closer to 35%-45% relative humidity.

So what is relative humidity and how do I know if mine is good?

At any given temperature, air is able to hold a certain amount of moisture without starting to give some up in the form of precipitation, dew, or condensation.  Relative humidity (RH) is the amount of moisture in your air relative to the amount of moisture it can hold at its current temperature.  The closer RH gets to 100%, the more saturated the air and the more moisture remains in your skin, wood, walls, etc., preventing sweat from evaporating to cool you off, causing undesirable expansion in your floors and doors, and inviting mold and mildew.  The closer RH gets to 0%, the more moisture the air is able to draw out, leading to dry skin, cracked wood, etc.

Warmer air is able to hold more moisture than cooler air.  For example, air at 95°F can hold twice as much moisture as air at 75°F.  As air cool and its capacity for moisture decreases, it eventually reaches the “dew point”, where it becomes so saturated that it can’t hold the moisture already in it, causing the water vapor to become liquid, forming dew or condensation.  Of course, well-maintained homes will be airtight enough to prevent this from happening indoors.

To prevent air from reaching critical levels of dryness or saturation before you start to see cracks in your furniture or water running down your walls, you can use a hygrometer to measure the relative humidity in a given room.  A hygrometer is a small, inexpensive instrument with a display that tells you what the RH level is in the room where it is located.  They range from a simple single device that you can move from room to room (about $10-$25) to systems that monitor several rooms continuously ($50-$150).

If the RH in my home is too high, what should I do?

If you find that there is too much moisture in the air in your home, it is fairly simple to return it to ideal levels.  If it is winter time, it may be as simple as turning on the heat.  Remember that higher temperature air can hold more moisture, so this is often an effective way of reducing your relative humidity.  In the summer, centralized air conditioning units can remove some moisture from the air altogether.

It may also be worthwhile to purchase a simple dehumidifier.  These can be acquired at any home improvement and most home goods stores and will cost generally from $40 for a simple portable model to $200 or more for a larger, more sophisticated model.  Depending on the severity of the problem, you may want to have a dehumidifier in each room of the house, but often one or two portable units will be sufficient to balance out those rooms where, for whatever reason, the RH tends to get a little high.

If the RH in my home is too low, what should I do?

Your house and its inhabitants and their activities—people, plants, pets, bathing, cleaning, cooking—all add some moisture to the interior of your home, providing a little bit of buffer to excessive dryness.  If, however, conditions in your home fall below ideal RH levels, you can purchase a humidifier for the areas of concern.  Like dehumidifiers, humidifiers come in several sizes and levels of sophistication and range in price from around $50 to about $200.

Is there something I can do so I don’t have to think about RH?

While you should always be mindful of the environment in your home (after all, the most advanced gadgets will fail from time to time), there are a few devices that can make maintaining ideal RH levels pretty low maintenance.  Some of the more sophisticated humidifiers and dehumidifiers have integrated hygrometers that allow them to turn on and off as needed.

It is also possible to incorporate into your home an in-line humidity system parallel to your central air conditioning and heating that will maintain ideal RH throughout.  These systems can be built in as a home is being constructed or added afterward and tend to cost $200-$400, depending on the level of sophistication.

If you are not sure whether your home will need extra RH attention, talk to one of our MacDonald Hardwoods staff.  We will be happy to share some steps you can take to help figure it out.