A Humidifier Is a Bandaid — The Problem Is Infiltration

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air leakage infiltration humidifier hvac duct bandaid

Does your heating system have a humidifier attached to it? If so, it's most likely there to treat a symptom while leaving the underlying cause of that symptom alone. The symptom is dry air in winter. Unless you're in an extremely dry or cold climate, the cause is not dry air. The real cause is lack of air-sealing.

First, do you know if your home has a humidifier? If you look at the furnace/air handler and see a smaller attachment that has connections for water, electricity, and drainage, that's most likely a humidifier. The photo here shows what one model looks like. What they do is introduce water vapor into your duct system so that the heated air in wintertime carries it into your house.

Ideally, you want the relative humidity (RH) in your home to be in the range of about 30% to 50%. For a lot of us, the summertime problem is keeping the RH low enough, but it's the opposite in wintertime, even in humid climates. The air in homes often gets too dry. You'll notice it because you get the static electricity shocks every once in a while, or you have to keep putting lotion on your dry skin.

Why is your home's humidity too low?

The company trying to sell you that humidifier says your home's humidity is too low because there's not enough water vapor in the indoor air. Yeah, yeah, that's true, but it's like saying you're naked because you don't have any clothes on. You're just repeating the same thing in different words. What we really want to know is why you don't have any clothes on...uh, I mean why the humidity in your home is so low.

The answer to that question is: Cold air is dry air. Understand that and you'll know what I mean when I say a humidifier is a bandaid, not a real solution. The chart below, called a psychrometric chart, has the explanation.

psychrometric chart humidity temperature 450

I won't give a full lesson on psychrometrics here, but let's focus on two points on the chart, point A and point B. Point A corresponds to outdoor air when it's moderately cold, 32° F. Let's say it's also raining and the relative humidity is 100%. Among other things, the psychrometric chart shows how temperature and relative humidity change. For our purposes today, here's what you need to know about the chart:

  • When the temperature changes, you're moving left (lower temperature) or right (higher temperature).
  • Relative humidity is shown as the curves, which are all bunched up together at the lower left. They spread out and move upward as the temperature increases.

When outside air at 32° F, 100% RH leaks into the house, it gets warmed up. Your heating system has to do extra work and you pay extra money to heat up that outside air, but we're interested in what happens to the humidity right now. As that air heats up, it moves to the right. If you keep your thermostat at 70° F, the conditions of that outside air that leaked in eventually land at point B.

If you look carefully, you'll se that we crossed 8 RH curves to get to point B, each covering a range of 10% RH. That means we dropped from 100% RH when the air was still outside to 20% RH after it leaked in and warmed up. Wow! That's a big drop in relative humidity.

Of course, the 20% RH applies only to the volume of air that leaked in, not all the air in your home. The 20% RH air mixes with whatever you have in the house already, and the final number will be the RH of the combined air masses. The key to your low humidity, though, is that the more infiltration you have, the lower your indoor air's relative humidity will be.

That brings us back to where we started, so we can see now that when the relative humidity in your home is too low, a humidifier is treating the symptom. It's a bandaid, not a real solution. If you really want to solve the problem, seal the air leaks. It's a lot less expensive, too, because there's no operating cost for air-sealing.


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D Gough

Dry homes are not always leaky homes. There are some cases where the homeowner's activities just don't produce enough humidity to stay in the "zone". I have seen it happen about 4-5 times in the last 20 years. 
I've seen substantially airtight homes with airtight ducts, with dry crawl space or basements and/or the homeowner(s) apparently didn't cook much, perhaps didn't breathe much, had very few house plants, no wood storage indoors, no pets, little to no mopping and maybe one shower a week whether they needed it our not. Plus, there were bath exhaust fans that really worked. The winter RH stayed consistently below 30%. Stuff would shrink and the homeowner complained about static electricity yada yada... A steam humidifier is the only thing that would add sufficient moisture to solve the problem.  
Plus, with unconfessed sin in the home, I also can see supply air only ventilation being contributory to dry interiors.

Allison Bailes

Dale S.: There's just no telling what code officials are thinking sometimes, but overall, I think codes are improving. There's going to be a session on this topic at this year's RESNET conference called Do Building Codes Really Incorporate Building Science? 
D Gough: Yes, I'll admit that infiltration isn't going to be the culprit in every case. I mentioned extremely dry or cold climates as possible exceptions in the article, but yes, tight homes could have problems, too, if they don't generate much moisture inside. Overventilation can cause problems as well, and I have firsthand experience with that one in the green home that I built
Darrel T.: You could go back to the house (if it still exists) and do a blower door test. ;~) 

John Poole

Darrel's comment reminded of those decorative cast iron "humidifiers" that you sometimes see advertised for use with woodburning stoves. 
You fill them with water and place them on your stove top, and they humidify the surrounding air. I have no direct experience with this, but I'd imagine homes that rely heavily on wood or pellet stoves, especially cookstoves, would have pretty severe dry air problems in the winter.

Mesa AC Repairman

Out in Arizona we have a big problem with houses getting too dry all year round. While we dont have to use heaters very often they are still used a few month of year and during that time there is a lot of dry air problems that come about. But it is like that most of the year so we really have to dig deep to get to the root of the problems homeowners have with dry air.

John Proctor

One other problem with humidifiers is that many of them bypass air from the supply side to the return side creating a bypass loop that detracts from both cooling and heating efficiency. Most of the bypass humidifiers have a manual damper to shut off the recirculation in the summer, but the homeowner usually doesn't do anything with it.

Mary Beth

While we're on the subject of humidification, any thoughts about hanging wet clothes in the house to dry -- and how bad this action might be for adding moisture to the home from the inside-out. The dryer-hater in me hates to waste the propane. We inside-dwellers don't mind having a little extra humidity in the dry winter. Usually I dry things half-way, then hang them up to dry the rest of the way. Please tell me how bad this is!?

Allison Bailes

John P: That depends on how big the stove is and also how efficient. I once had a woodstove with a catalytic converter, and once we got the fire going and closed the damper so that the exhaust went through the converter, it drew very little air.  
Mesa AC: Indeed you're right. I mentioned that my remarks didn't apply to extremely dry climates because you may be too dry anyway. Of course, good air-sealing is still important because then the humidity you add is more likely to stay in the home. 
John P.: Great point. I've seen those bypass humidifiers here in the Southeast, and you're right - homeowners won't close a damper because they don't even know it's there. And most certainly won't go down into the crawl space twice a year to open and close it if they do know. 
Mary Beth: What you're doing should be fine in winter, when the air in your home is probably dryer anyway. It could cause problems in the summer, though, unless you live in a dry climate like Colorado or Arizona. I'd suggest buying a digital thermo-hygrometer (which measures temperature and relative humidity) and keep it near your drying rack. If the RH gets up towards 60% or above, you'll want to do something differently.

Matthew Cooper

If cooking and cleaning activities include the proper use of exhaust fans and infiltration is controlled, a properly sized heating system will still reduce humidity below levels considered comfortable in low-humidity winter climates in my experience. Properly controlled steam humidifiers such as those manufactured by Norco can effectively work and in many cases are necessary. For new construction homes if the humidity isn't properly maintained and there are hardwood floors, the flooring manufacturer/installer will not warranty the flooring. 
We also have designed and installed many systems where a homeowner needs elevated humidity for either health issues or expensive art collections. Controls are a key component along with effective ADPI for proper air mixing. Devices such as cold-snap indicators on windows tied into DDC systems can overcome the inherent challenges posed by the introduction of moisture into the properly controlled environment.

Jaap Weel

This is very interesting to know. I always wondered how heating a house could somehow cause the humidity to go down and where the heck the water ended up. Now I understand a bit better. 
For now, I'm living in an apartment where I cannot do much about air leakage. The landlord clearly has bigger fish to fry, for now, than sealing the building; there's some real deferred maintenance the prior owner has pushed off onto the current, and it looks like they're investing considerable money just getting the building back into its original 1960s condition, let alone upgrading it to 21st-century standards. 
Besides, us tenants are paying the heating bill, and the housing market in my area is heating up, so they're having no trouble at the moment filling every vacancy at rapidly increasing prices. 
So for now, I'll keep running my little drugstore humidifiers. But when (hopefully soon) I have a home of my own, I'll consider improved air sealing first.

Amy A

As a homeowner, and researcher I like to review information based on studies. This blog spoke to the science part of my interest. Great Job! 
Here is my question: I get the point to SEAL, but what I do not know is who is considered a professional in sealing?

Allison Bailes

Amy A: I wish there were an easy answer to your question. For existing homes, the best answer is to see if you can find a company with BPI certified professionals or a RESNET Energy Smart contractor. There are plenty of companies that do good air-sealing work without having those, however, but that makes it harder for the homeowners who don't know the details to know if you're being hoodwinked or not.  
Also, I'd suggest getting a full energy audit or home performance assessment. Here's an article I wrote about that: How to Choose a Company to Do a Home Energy Audit.

Dirk Muir

I am heating an 80year old home near Thunder Bay, NW Ontario (Canada) with an indoor woodfurnace. Night time temperatures are frequently in the -20 to -40'F range. Not only do we have static sparks and dry skin & lips, but I frequently wake with my mouth and throat completely dry. My doctor has suggested that a persistent cough and throat irritation is due to low humidity in our house. This is despite running 20-30 litres a day through two humidifiers. Through your postings, I now understand that the woodfurnace is driving infiltration of air, that once warmed in the house, has very low relative humidity that the humidifiers can't compensate for. Other than sealing the house (a major refit), is there anything else one can do to buffer humidity levels? What about feeding exterior air direct to the furnace to reduce the infiltration gradient?

John Proctor

Anything you can do to reduce air infiltration will help. Supplying combustion air straight to the furnace is a help as well as any amount of air sealing (generally starting high in the house.