In our a/c design work at Energy Vanguard, we sometimes attempt to talk people out of things. I’ve discussed this in regard to makeup-air systems for variety hoods, but today let’s take a look at a different one: humidifiers. We do not include this in our scope of work. If you believe the indoor humidity is too low, you could put a humidifier in an airtight house, however you should beware in doing so. It might produce more problems than it fixes.
What’s the ideal indoor humidity?
I covered this question a while back, but let me recap it here. There is no magic number for “the very best” indoor relative humidity. Nor is there one for humidity, which is usually the best metric to use. As I wrote in the other article, it depends upon a number of factors.
That’s why you’ll normally see a recommended range in which to keep the relative humidity. But even that range isn’t set. Trips outside the range are okay as long as they do not last too long.
A great variety for indoor relative humidity in winter season then would be 30 to 45 percent. But you ‘d much better have a truly good structure enclosure with triple-pane windows if you want to attempt 45 percent in a cold climate.
What do specialists state and do?
In the 1980s, a research paper claimed that the very best variety for indoor relative humidity is 30 to 60 percent. You may well have actually seen a version of the graph they consisted of. It’s called the Sterling chart, and the one I had actually produced my book is listed below. (For more background on this, see my article on that paper.)
The Sterling chart relating health factors to relative humidity Stephanie Taylor, a medical doctor with a degree in architecture, says that information from health centers show that 40 percent needs to be the lower bound of the variety, not 30 percent. I heard her say on a podcast that she utilizes a humidifier in her Vermont farmhouse to keep a minimum of 40 percent relative humidity. That appears a bit scary to me, however she didn’t state what she’s done to improve the enclosure.
Gary Nelson, founder of The Energy Conservatory, lives in Minneapolis. You may recall that I wrote about how he heats his home with a heatpump that has no auxiliary heat. I called him this week, and he told me he never ever utilizes a humidifier in his home. Nevertheless, his relative humidity stays between 30 and 35 percent in the coldest parts of winter. He’s got a great structure enclosure with triple-pane windows and very little air leakage, so he can stay at 30 percent or above without a humidifier.
Then I put the question to Lew Harriman, who made a career of examining and writing about wetness problems. He stated that he ‘d never say never to putting a humidifier in an airtight house. If you’ve got a structure enclosure that can handle it and you do not try to keep the indoor air too damp, it can work.
Why might the humidity be too low?
Periodically a client of ours will come back to us after your house is occupied and inquire about including a humidifier. We follow up with these concerns:
- How airtight is your home?
- What ventilation rate are you using?
- Are you using an energy-recovery ventilator (ERV) or a heat-recovery ventilator (HRV)?
Airtightness is extremely essential since every cubic meter of cold outside air that infiltrates your house dries the indoor air. Cold air is dry air, you understand. The more airtight your house, the less you need to worry about humidification.
The ventilation rate matters a lot too. If it’s too high, it also can dry out your house. I talked with a client in Seattle just recently who was complaining of low humidity. He needs to have been running his ERVs at 180 cubic feet per minute (cfm) in total but instead had them maxed out at about 400 cfm. When he turned them down as I suggested, the indoor humidity was great.
Then there’s the problem of the kind of whole-house ventilation system. As I’ve written, most houses must have an ERV, not an HRV. An HRV will dry the air as much as seepage due to the fact that it does no moisture recovery.
So before you leap to the conclusion that you need a humidifier in an airtight house, have a look at those things.
3 kinds of humidifiers
When I consulted with Lew Harriman, the problem of humidifier type came up. He has a preference, which I’ll tell you below. First, here are the 3 types:
- Evaporative humidifier– Warm air blows over a wetted pad, vaporizing water into the airstream. This type can get a bit nasty inside, especially if it doesn’t get cleaned regularly.
- Steam humidifier– Include heat and boil water to humidify the air. It takes more energy than the evaporative type.
- Ultrasonic humidifier– Ultrasonic vibrations develop a fine mist to increase humidity. But if you don’t utilize pure water, whatever else is in the water ends up in the air too, so you can get a boost in particle matter too.
Harriman says he would advise the steam humidifier because it remains cleaner than the evaporative type and does not add to the particulates the method an ultrasonic humidifier can.
A simple way to identify the best indoor humidity level
OK, let’s boil this down to a simple test you can do. If you’re considering putting a humidifier in an airtight home, you need to begin by determining the indoor relative humidity. Thirty percent or higher is good. More than 45 percent is most likely too expensive.
This would be too much condensation on
windows in winter season. Now here are those real-world tests:
- The humidity should be high enough that you do not have a fixed electrical power issue.
- The humidity should be low enough that you do not have more than a little bit of condensation on the windows, and it doesn’t begin leaking down the method you see in the picture above.
And again, trips outside those bounds can be okay if they do not last long. If you have a constant problem with condensation on the windows, you might remain in threat of decomposing the structure. Harvard found that out the tough method. (Yeah, I wrote about that too.)
Lagniappe
Considering that I have actually mentioned fixed electricity, you also ought to know that there is a National Static Electricity Day.
Allison A. Bailes III, PhD is a speaker, author, building science expert, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a successful book on building science. He also writes the Energy Lead Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.
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