Whole house humidifiers can cause serious problems with houses, especially in Minnesota. Sometimes they destroy houses. This topic gets a lot of people whipped up, and I always hear the same arguments for having a humidifier: they prevent hardwood floors from cracking and splitting, they make the house more comfortable, you'll get nosebleeds if you don't... the list goes on. I admit, those are all valid arguments. Nevertheless, the damage caused by an improperly used humidifier can far outweigh the benefits, and humidifiers are almost never used properly. From all the houses that I inspect in Minneapolis and the surrounding areas, I know this.
Whole house humidifiers are installed in the furnace ductwork, adding moisture to the air that gets distributed throughout the house. The amount of moisture that's added is manually controlled with a dial that is usually located next to the thermostat or the furnace. To maintain the proper level of humidity, the owner needs to know what the temperature is going to be, and then adjust the settings on the humidifier accordingly. This is why I have such a problem with whole house humidifiers! Here's an excerpt from an owner's manual:
"it is important to anticipate a drop in outdoor temperature and reduce the setting accordingly to avoid excessive condensation. For example, with an outdoor temperature of 20°F the correct setting will be 35% RH. If the temperature is expected to fall to 0°F that evening, then merely reduce the setting to 25% several hours prior to the temperature change."
"Merely?" Get out! Nobody does this! Ok, one person does. A buyer's father attending an inspection got defensive about humidifiers when I told his son that I don't recommend them. When I explained that most people don't use them properly, he said he adjusts his humidifier twice a day. My jaw dropped and I told him I was impressed. This is definitely the exception, and if you're one of the few that use a humidifier properly, give yourself a pat on the back.
The biggest problem with whole-house humidifiers is that too much moisture gets added to the air, and the moisture condenses on the windows, walls, and especially in the attic. Excessive moisture in the home causes mold growth. Excessive moisture on the windows stains the finish and damages the wood. Excessive moisture in the attic causes condensation, which can rot the roof boards. I've inspected many attics with rotted or delaminating roof boards, and in every case there was a whole-house humidifier present. If you've read my previous blog about attic bypasses, you know how moisture gets in to the attic.
My recommendation is usually to not use whole house humidifiers unless you're an extremely ‘type A' homeowner, which means you'll check the weather forecast and adjust your humidifier all the time. I do have a bit of good news about humidifiers though- the newest ones come with automatic humidity controls, so you don't need to adjust the settings all the time. I learned about these at Aprilaire's web site, and I think they're a great idea that will probably save a lot of homes from damage. If you must use use a whole-house humidifier, use one of these.
Reuben Saltzman, Structure Tech Home Inspections