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The Humidity Factor
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The Humidity Factor


0Indoor pools are wonderful as both design challenges and family recreation centers. But as Paolo Benedetti discusses here, they’ll stand the test of time only if you take care of moving the moisture they generate away from the indoor space — no shortcuts allowed!

More and more frequently these days, I see photographs of beautiful indoor pools – and always have one big question: How did the designer and builder deal with the moisture in the air?

It’s more than the usual curiosity I have about other folks’ watershapes, because I know from experience that humidity is the largest obstacle to overcome in designing or building an indoor pool. In fact, the failure to address the evacuation of humidity from the indoor space can have devastating structural, safety and health repercussions.

I know, of course, that dehumidification units designed to remove moisture from the air of enclosed, moisture-rich spaces are expensive to purchase, install and operate; that they are the only accepted means of controlling moisture in the air; and that there are lots of variables that go into specifying and sizing the correct unit – so many that bringing in an expert is almost always advisable.

I also know, unfortunately, that many designers and builders ignore those realities and try to work their ways around this requirement – acts of foolhardiness this article is meant to discourage.


It’s simple: When you work on an indoor pool project, all you need to do to avoid headaches that commonly lead to disastrous results is to reach out to companies that manufacture humidifiers. Most of them will put experts at your disposal to evaluate needs, select the right system for the task at hand and even guide you to local contractors who will take care of matters on site.

The key here is recognizing that it’s virtually impossible to avoid that kind of contact by “brainstorming” your way around what can be a distinctly expensive proposition. Rest assured that pool and mechanical contractors have been wrestling with this challenge far longer than you’ve been alive – and that your hair-brained, boot-strap, cost-avoiding methods and ideas – an open ceiling? sliding walls? cross-ventilation through numerous windows? matching water and air temperatures? – have all been tried by someone before you, and all those attempts have failed.


If the area occupied by an indoor watershape is open to other parts of the structure, it’s particularly important to install a dehumidification system that will remove moisture from inside areas and release it harmlessly outdoors.

Yes, an automatic pool cover is a partial solution and typically reduces a pool room’s humidity. But reduction is not elimination, so the installation of a good cover only allows for the downsizing of the dehumidifier, not its omission.

So please don’t even bother trying: You’re not going to come up with any bright ideas that haven’t already failed to measure up. As an industry, we’ve already been there and done that.

To understand this long line of inadequate “solutions,” it may help you (as it has me) to grasp the fact that humidity is formed in the air from the evaporation and splashing that occurs in an indoor pool. This happens because Mother Nature always seeks balance. When you start with a pool of water that is 100-perecent moist, it’s pretty easy to wrap your head around the fact that addressing the imbalance between the watershapes and the air that surrounds it is no small task, no matter your climate zone.

The issue is that humidity in the air can lead to the growth of black mold on a structure’s walls, light fixtures, windows and wall coverings. Nothing is immune, not drywall, not glass, not wood or paint. And black mold can get nasty: Breathing the spores has been linked to respiratory infections and pneumonia and can even lead to death in some instances. One familiar manifestation of this is “lifeguard lung,” an ailment linked almost exclusively to safety and maintenance people who work around indoor pools.

That’s bad for people, and beyond mold, humidity can lead to all sorts of additional problems for buildings. The moist, invasive and persistent vapor can penetrate and permeate structural members, walls, ceilings and floors, leading to corrosion in places well out of sight and adequate visual diagnosis. Wiring can be affected – anything metal, in fact. This can lead to shorted circuits, system failures and, heaven forbid, fires or electrocutions.


As the last points above underline, much of the damage to be done by high humidity in enclosed spaces occurs where you can’t see it – on the inside of walls. But it’s also an issue where you can see it on wall surfaces, glass windows, tile finishes, painted areas and skylights. Under certain conditions, that water will even collect as droplets when the temperature of these surfaces falls below the room’s dew point.

Whether you like it or not, you’re dealing with a complex microclimate with its own “weather,” and the best way to do so is to include an adequate dehumidification system. No ifs, ands or buts.

You simply have to accept the fact that you cannot heat the glass, walls, floors, steel I-beams or tile in ways that will keep them warmer than the air within the room. And if a project is in a part of the country where winters are harsh, you are guaranteed to have condensation and water droplets unless the air is being dehumidified. Even if the moisture content of the indoor air is low (say, with a covered pool or in a desert climate), less water vapor will condense on these cooler surfaces – but it will appear just the same.


Indoor pools can be a wonderful addition to a home, but there’s a lot at stake in how they come together and how you work with the various surfaces and materials and, for the long haul, keep them from being damaged by humid air.

And it can get worse: In the right conditions, this condensation will gather together and form run-off – an appearance of water that involves its own set of destructive capabilities. Think about standing inside your living room and misting the room with a garden hose – all day, every day. Would you guess that you’d start to observe negative effects to the room’s walls, floors and furnishings?

Dehumidification systems combat these situations with air turns and air pressure, while a proper overall construction process involves the inclusion of appropriate vapor barriers.

[ ] Air turns are, by definition, the number of times the air in the room is exchanged – similar in concept to turnover rates in swimming pool circulation systems. To keep the air fresh and to allow the dehumidifiers to operate properly, most indoor pools will feel like a bit like wind tunnels when their systems are operating.

[ ] Air pressure is a different animal: To help keep water vapor from migrating into other rooms and undesirable places, the air pressure of the enclosed space is controlled in such a way that it is lower than the surrounding area, thereby drawing in moist air that might otherwise flow into other spaces and keeping it within the room where the dehumidifier does its job. This sort of pressure differential is critical to a successful design and effectively keeps moisture from permeating the entirety of the building.

[ ] Vapor barriers are also a crucial design/construction component. These should be applied behind the drywall and should envelop the entire space as added protection that helps to keep incidental water from penetrating through the walls and into the rest of the structure. Helpfully, special “breathable” membranes have been developed just for this purpose: These materials are installed by specialists and require the taping or gluing of seams as well as extra attention at penetrations.


If your clients are dead set on having an indoor swimming pool or spa, then there’s no way around it: Sizing, selecting and installing an effective dehumidification system is a must – no compromises, no self-derived “solutions,” no excuses. It’s an expensive part of the package, but it’s required. And it’s best to take the extra step of having it function automatically in response to each day’s situation outside and within the space.

In this case as in many others, please don’t assume that you’re wiser than everyone who came before you: There is no cheap or easy way out. And there’s always an option if the expense of a dehumidification system is a potential deal-killer: How about suggesting a nice outdoor pool?

Paolo Benedetti is principal at Aquatic Technology Pool & Spa, a design/build firm based in Morgan Hill, Calif. He may be reached at [email protected].

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