What Should I Set My Humidity At When Running The Furnace?

Home Inspector with Diadem Property Inspections - Serving Southeast Michigan

What Should I Set My Humidity At When Running The Furnace?

If you live in the North, you are running your furnace full-time right now. It is snowy and cold out there!

snowy day

But quality air is just as important as warm air. Many of us see that humidistat on the furnace and wonder what to set it at. How do we make sure our air is both healthy and comfortable?



Step One: No Leaking Ductwork

But, before we get into the right humidity level for our houses, let's look at one thing that will defeat our efforts to achieve the right humidity -- or temperature: leaking ductwork.

Leaking air from the supply side of the system can cause the whole house to go into a negative pressure, which means that air is being sucked into every crack and crevice of the home, through electrical fixtures, sill plates, windows and doors. This can cause higher energy use and utility bills, as well as dust problems.

Leaking air into the return side of the system, depending on from where it is being drawn, can have the opposite effect on the home; it will place the structure in a positive pressure. When a home goes positive it forces the nice, clean warmed air out from all the same places, again costing the homeowner money. If it is a very dry climate the moisture level drops in the residence causing low humidity issues like static electricity, dry itchy skin, nose bleeds, and so on.


An Example Of This Problem

relative humidity and temperature

It has been a cold week here in the Detroit area, so let's take a look at a real life example of air infiltration problems.

Warm air can hold more moisture than cold air, so the same moisture content will result in air with much lower relative humidity in warmer temperatures than in colder temperatures.

For example, on Monday we had a 15-degree day with relative humidity of about 85%. Assume that the air infiltrates into a leaky home that has their thermostat set at 70 degrees F.


So the air comes in and is warmed to 70 degrees... as this happens, the relative humidity will drop to 11%. This is much drier than even the desert area (20% RH) around Scottsdale, AZ, where Juli sells houses.

So without the benefit of a humidifier, this level is much too low for a house.

So, Assuming No Leaky Ducts, What Humidity Level Am I Aiming For?

As outdoor temperatures drop, so must the indoor relative humidity. If the windows get cold enough they will collect moisture from the indoor air -- a definite sign you need to lower the RH.

By lowering the indoor RH as the outdoor temperatures decrease, the possibility of moisture issues is reduced.

Under most conditions, building experts recommend the humidity levels be kept between 25% - 50% for the best mix of comfort and improved air quality.

The IIAQC recommends:

RH inside for outside temperatures


Stay warm -- and comfortable!

Posted by


Jason Channell     Twitter - Home Inspection LinkedIn Twitter - Home Inspection

Diadem Property Inspections
(888) 699-8710

Learn more: michigan-indoor-air-quality.com

Learn more: HouseSleuth.com

Michigan Builder's License 2101198700
Environmental Solutions Association 3818 -- Certified Mold Inspector & Assessor, Certified Allergen Inspector
International Indoor Air Quality Commission CC1983 --  Indoor Environmental Certified Consultant



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Rebecca Gaujot, RealtorĀ®
Perry Wellington Realty, Adam Conrad, Broker - Lewisburg, WV
Lewisburg WV, the go to agent for all real estate

Jason, okay I am going to show this post to my husband and let him decide what to set the humidity at when running the furnace.

Dec 15, 2010 01:58 PM #1
Sheila Newton Team Anderson & Greenville SC
Berkshire Hathaway HomeServices - C. Dan Joyner - Anderson, SC
Selling the Upstate since 1989

Thanks for the info... I thought it was cold here.. but much colder there... I can tell how dry inside is though because i am having to water my plants more lately...

Dec 15, 2010 02:27 PM #2
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