Water vapor is not a contaminant – and ventilation systems are not dehumidifiers

If you’ve been calling water vapor a contaminant, stop it.  You’re not helping.  I’ve confess I’ve said it before myself, but I was wrong.

There was a lot of discussion at this year’s RESNET conference about ventilation, humidity in buildings, and  ventilation and humidity in buildings.  More than once, I heard someone say, “water vapor is a contaminant”.  It sounds less true every time I hear it.

Too much water vapor in the wrong place will cause mold to grow, and that’s a contaminant.  But people (and maybe wood floors) complain when there is too little water vapor in our buildings.  Formaldehyde is a contaminant.  You don’t hear about anyone upgrading to cabinets with added urea-formaldehyde because they’re afraid their house will be uncomfortable.  Cigarette smoke is a contaminant.  No one says, “I had to invite a smoker over because my sinuses were really irritated.”

Water vapor has a lot more in common with heat than it does with contaminants.  If you’ve got too little heat in your house, pipes freeze and that’s bad.  Too much heat and people start to complain.  Yes, too much water vapor will damage your house and too little can be irritating.  But water vapor is part of air and it’s going to be in your house.  It’s a parameter to be controlled – just like temperature.

What does this have to do with ventilation?  Probably less than a lot of people think.  We ventilate homes to dilute contaminants.  Contaminants are bad things in the air, and less is always better.  Outdoor air usually contains less contaminants, so we use it to dilute the air in our homes.  Sometimes it’s humid in our homes when the outside air is dry and we can dilute water vapor too.  That’s a happy coincidence.  There are other times when it’s humid inside and even more humid outside.  That’s inconvenient, but it doesn’t mean that we should stop diluting contaminants.  It might mean that we need a humidity control system.

Humidity comes from things we do indoors (showering, cooking, breathing) and from outside air that gets into our homes accidentally through infiltration and intentionally with ventilation.  Humans and buildings can tolerate a fairly wide range of relative humidity, so many homes function just fine without a dedicated system for humidification or dehumidification.  It’s kind of like how homes in cool climates don’t always need to have air conditioning.  You can get by without it because a few uncomfortable days a year aren’t worth the expense of a new system.

But if it gets too humid or too dry very often throughout the year, you’re going to want (and need) a system to control humidity.  To do that, the system needs to be able to sense humidity in the house and respond to raise or lower the humidity until it gets back into the acceptable range.  That’s pretty obvious, I know.  But it leads me to one of my personal pet peeves:  a ventilation system is not a dehumidifier.

All a ventilation system can do is introduce outdoor air on a preset schedule.  If this outdoor air contains more or less water vapor than the indoor air, it’s going to nudge the indoor humidity in one direction or another.  But it’s usually a minor effect and there are no guarantees that it will be the direction you want it to go.  We shouldn’t expect it to.  In case I didn’t mention this already, a ventilation system is not a dehumidifier.

It used to be that people in cold climates needed humidifiers in the winter, people in very humid climates needed dehumidifiers, and a bunch of us who live in the middle got to live on autopilot without either system.  I’ve come to realize that as we build tighter homes, those homes are going to hold on to humidity generated indoors more, and more of use are going to need to think about dehumidification.  That’s the bad news.  The good news is that fewer homes are going to need a humidifier. (The other bad news, though, is that it’s difficult to talk homeowners out of them.)

Programs like Energy Star have started to recognize this by requiring that bathroom and kitchen fans actually remove as much air as they’re supposed to.  It’s a difficult thing to address with a checklist, though.  In dry climates, it probably doesn’t matter.  In some climates, it might not be enough.  The design trend toward open showers is a big problem where I live. We have an open shower in our house and our bath fan pulls 60 cfm directly out of the shower.  It has an occupancy sensor and a 20 minute timer.  Our bathroom windows still fog up in the winter.  Clearly, not all showers are created equal.  I think it would be a good idea to start thinking about humidity loads in the same way that we think about heating and cooling loads.

The list of things homeowners might do is endlessly surprising.  Will they use their bath or kitchen fans?  Will they dry laundry indoors?  Run a humidifier at an insanely high setting?  Make tea 10 times a day?  I have no idea.  What I do know is that even the experts can’t quantify indoor humidity sources as well as we can indoor heat sources.  It’s a great research topic.  If I were still an academic, I’d be all over it.

Until then, the type of ventilation system is just one of many factors designers have to consider.  If high wintertime humidity is a concern (as I find to be the case in tight homes, densely occupied situations, and when the window thermal performance isn’t great), a system with no latent recovery might be helpful to the home.  If high summertime humidity is a concern, a system with latent recovery (such as an ERV) would contribute less to the moisture load in the space.  The wording here is important.  If the outside air is humid, any system will bring in humidity.  The ERV will just bring in less.  These are all great things to consider when selecting a ventilation system.  I just don’t think you should expect the ventilation system to maintain the desired humidity on its own.

Many people wonder why certain strategies that they believe are detrimental to indoor humidity levels are allowed to be installed at all.  My answer to these people is that this is where designers become important.  Humidity manifests in homes very differently depending on climate, and most practitioners are focused on a single climate.  A system may work better in one climate, but we shouldn’t assume it’s the best for every home or every climate.  It would also be a shame to become too prescriptive and lose the opportunity for people to innovate.  Or it may simply be that we’re asking the wrong question.  We might be talking about ventilation systems when we should instead be talking about humidity control.

Copyright 2014.  Amy Musser.