Last Friday, our company performed its tightest blower door test ever: 0.28 ACH50. That’s not a typo – this house was twice as tight as the Passive House requirement. I’ve tested duct systems (probably also this week) that weren’t as tight as this house. It’s great that we’re making homes tighter, but there are also steps that builders of these homes need to take to prevent winter-time humidity problems from happening.
The first thing you need to know is that wintertime humidity problems are not the same as summertime humidity problems, and usually the source of the problem is quite different. Doesn’t that seem sort of obvious? But more than once, I’ve been consulted on an apartment building that was growing mold in February, and a so-called “building scientist” had already told the owner that oversized air conditioners were causing the problem. “Over-sized air conditioners cause humidity problems” has become such a mantra in the building science community that many people will jump to that even when the air conditioner hasn’t been on in 6 months. What’s more amazing is that people get paid to give this advice. I’ve heard all sorts of crazy explanations, like water evaporating out of the condensate drain, or magically escaping from the furnace. It’s got to be the HVAC guy’s fault, right?
To get some perspective it helps to think about where humidity comes from in the winter. It comes from indoors. Warm air can hold a lot more humidity than cold air can. So when it’s cold outdoors, the outdoor air is very dry. Indoor humidity in the winter is generated inside from things like showering, cooking, and breathing. Or a humidifier if you have one – which if you have a new, airtight house and you live anywhere near me, you should not. But we’ll get to that.
The first sign of a wintertime humidity problem is usually condensation on the inside of the windows. This is a great time to remember that windows are the “weak link” in your building envelope. They’re not very well insulated, which means they’re the coldest exterior surface of a home. The frame is the worst part of the window, so condensation starts at the edges of the glass. If you take action to remedy the problem when you start to see condensation on the edges of windows, you can usually avoid problems elsewhere. Not to ignore all the cool stuff that happens inside the walls, which I’ll cover in a future blog.
In a leaky home (most older homes and probably a lot of code-built new ones), the indoor air gets dry in the winter because the house is constantly being flushed with infiltration of cold, dry outdoor air. This happens less as your building envelope gets tighter and interior-generated moisture can build up. Contaminants like VOCs can also build up, which is why you need a whole-house ventilation system. A ventilation system brings in outside air and will help dry out the house. But a ventilation system is not a dehumidifier and the type of system matters.
An energy recovery ventilator (ERV) actually recovers some of the humidity from the exiting air and keeps it in the house. That’s great if your house tends to run dry. But for the home that I just tested below the Passive house standard, you want to get rid of this moisture. For that house a heat recovery ventilator (HRV) is a better choice: it recovers sensible energy, but allows all the moisture to leave the house. In case you were wondering, when you factor in differences in efficiency between the heat transfer media and latent effects, the energy impact of the choice between ERV and HRV is about a wash in our climate. So we really can make this choice based on how much you need it to dry out your house in the winter. Another benefit of using the HRV is that you can pull air out of the bathrooms, replacing the bathroom fan. As a rough rule of thumb, I would prefer to see an HRV for any local home that tests below 2 ACH50.
The much-maligned exhaust-only ventilation strategy has some real upsides for combating winter humidity problems. First, it forces you to install a decent bath fan that might actually pull 50 cfm. Second, it automatically runs this bath fan all the time so it will be used regardless of whether someone turns it on. Third, there’s no heat or latent recovery, so all the moisture goes outside. I’ve seen a surprising number of winter humidity problems solve themselves just by adding exhaust-only ventilation.
Sometimes, you have to take additional steps. I believe that super-tight homes can benefit from more ventilation than ASHRAE Standard 62.2 says you need. That standard actually assumes a “baseline” ventilation rate due to infiltration that super-tight homes will never reach. In my own “super tight” (0.8 ACH50) home, I run my ventilation at about 1.5 times higher than the Standard 62.2 rate. This provides a little more drying effect.
The next thing to check is that the bath and kitchen fans are actually working. Are they moving enough air? Are they actually being used by the occupants? Are they in a location (inside the shower) where they can effectively remove the moisture?
Are the occupants doing anything that might be contributing? Some people in this area have the mistaken belief that they need to maintain 50% relative humidity at all times indoors. I know one builder who had a homeowner keeping his home at 70% RH on the advice of his allergist. The vast majority of building envelopes simply cannot handle this in the winter. Also, 70% is just insane. That’s too high even for the summertime. In reality, you probably can’t even sense the difference between 50% and 25%. Also, most wood floors aren’t affected much either.
The colder it gets outside, the drier it needs to be inside to avoid condensation on windows. You can do math to estimate when window condensation is likely to occur, but you have to remember that the edges of the window are worse than the middle of the glass. In practice, I tend to start seeing condensation around the edges of new code-built windows when the indoor humidity goes above 40% on a 20 degree day. A tight home with poorly insulated windows might need to stay even drier. Some examples: retrofits where old windows are kept, passive solar homes that don’t buy insulated heat-gain glass, and homes with storefront or custom metal specialty windows. We should all be trying harder to avoid these scenarios.
A final pet peeve on winter humidity: I am tired of people telling me that “forced air makes a house too dry”. This is simply not true. Infiltration of cold, dry outside air makes your house too dry. The colder it gets outside, the more infiltration you get, and the more your heat needs to run. Say it out loud with me: Correlation is not causation. Your heating system does not make your house dry. You just happen to need heat when your house gets dry.
So, if you are building a new house and dislike bugs, drafts, cobwebs, and high energy bills, then you want to build an airtight enclosure. And if you build an airtight enclosure, you will most likely not need a humidifier. In fact, you may need to think a bit more about how to get humidity out of your house. I’ve seen malfunctioning humidifiers cause more problems than I’ve ever seen a humidifier solve. Often (but not always) the malfunction is with the person setting the humidistat. You don’t need it.
Copyright 2012 Amy Musser