Humidifiers, threat or menace?

Based on my inspections the humidifier may be the most neglected and unloved appliance in your home. Probably nine out of ten humidifiers I inspect are a mess.

Humidifiers are used in winter time to add moisture to dry air during heating season.  A humidifier mounted on a furnace uses water from a water line to add moisture to the air heated by the furnace, which is then distributed throughout the house using the hot air ducts. There are three basic types, evaporative, drum and flow through. The simplest is the evaporative type, a hole is cut in the hot air plenum of the furnace and the humidifier, consisting of a water reservoir and some metal plates, is installed in the hole.

Evaporative humidifiers
Evaporative humidifiers

Heated air absorbs the water as it passes by. They are simple, often have no controls other than a float or valve that maintains the water level. They are shut off by cutting off the water supply (which will work for any other humidifier as well).

The drum type has a hollow cylinder covered with a water absorbing foam driven by a small motor, the cylinder or drum rotates through a water reservoir with a float to control water level.

Drum type

There are automatic controls to turn the motor on and off, often there is a manual damper to close off the air passing through the humidifier, and the water shut off valve. The humidifier is mounted on either the hot air plenum or the return (cold air) plenum, there will be a short length of duct connecting the humidifier to the plenum that the humidifier is not mounted on. When the furnace fan is on air moves through the connecting duct from the supply to the return, passing through the humidifier and the rotating drum, picking up moisture as it passes through the humidifier, which then goes back through the furnace from the return side to the supply air to the house.

Flow through

A flow through humidifier is installed the same way as a drum type, on either plenum and with a duct to the other plenum but there is no drum or reservoir. A rectangular pad containing 40 mm thick metallic mesh is fed water from a valve above the pad, water is either picked up by the warm air passing through it or it flows back out through a plastic condensate line into a floor drain. Water flow is controlled by an automatic control called a humidistat.



The humidistat is an automatic control that is activated by sensing humidity, only releasing water onto the pad when it is needed.  There should also be a manual damper to shut off air flow through the humidifier.

When I inspect humidifiers, this is mostly what I see,


calcified evaportive type




Eeew! what is that? Calcified drum type
Calcified pad from flow through type
Rust on furnaces caused by overflowing humidifiers

When I find a badly maintained humidifier my recommendation in most cases is to remove or shut down the humidifier. Most homes in Edmonton don’t need one, for non Edmontonians, you probably don’t either, but don’t take my word for it ūüôā Edmonton has a humid continental climate, the outside design temperature is -32 C (the projected coldest two weeks of the year). What this means is that no one needs a humidifier in summer, and when the weather is cold inside humidity needs to be kept low to prevent condensation inside, which can lead to damaged building materials and mold.

So who needs a humidifier?

You have valuable, make that priceless, museum quality antiques made of wood, paper or textiles that will be damaged if they dry out. (yup you need a humidifier)

You have hardwood floors. (maybe) The bad news is that your floors will dry out even if you have a humidifier, if the floors are old hardwood, no need, they are already dried out, what you see is what you get, new floors will dry out too, that is what hardwood floors do, they will shrink as they dry out.

You have health problems related to dry air. (If your doctor says you need a humidifier, you need a humidifier)

You feel cold but the thermostat is set to a high temperature. ¬†(Dry air will make you feel colder than humid air, you need to wear a sweater, you don’t need a humidifier)

You have a hygrometer, it is accurate, it reads less than 20%. (Your house may be too dry, extremely dry air can cause health problems, you may need a humidifier.)

You have an accurate hygrometer you are heating your house and the readings in your home are 40% or higher, condensation is forming on your windows. (You DO NOT need a humidifier, you need to turn it off if you have one, you may need a de-humidifier if you do not have a humidifier, or you have turned your humidifier off)

Turning off your humidifier

Turn off the water, turnoffwater
Close the damper, turnoffdamper

Turn off the humidistat turnoffhumidistat


If you must have a humidifier, the only kind I recommend are the flow through types, because they only release water when needed, when the furnace is running and the humidistat calls for more humidity. Flow through humidifier pads have to be checked regularly and replaced when they become calcified, usually once a year. The damper should be closed for the summer (the ‘off position may be labelled ‘summer’), the humidistat set to zero or off in summer, the water does not necessarily need to be turned off for a flow through, but it won’t hurt to do so.

If you are still using an evaporative or drum type, the water should be turned off for summer and the float and reservoir taken out and cleaned, and put back dry. Drums or the drum media should be replaced when calcified. You should also be measuring the relative humidity in your home in winter, never allow it to reach more than 30%. If you see condensation on windows, especially bedroom and living rooms windows, your inside humidity is too high, and you need to turn down or turn off your humidifier.

If you are installing a humidifier, the best place to mount it is on the return air duct, it can be mounted on either return or supply, except evaporative types which must always be mounted on the supply or hot air plenum. Mounting the humidifier on the return plenum will minimize (but not prevent) damage to the furnace should the water line leak or the humidifier overflow, so check your humidifier often.

Help! The ceiling leaks but the roof doesn’t?


It was¬†very cold, -20-30 C (or well below zero F) for a few weeks, but the deep freeze has ended,¬†overnight it warmed up to well above freezing,¬†everything is melting.¬†¬†And there is this huge new water stain on the ceiling… ¬†If this is your house, keep reading.

Here are some clues.  Your house has a humidifier and you use it, you do a lot of cooking, there are more than three people living in the house, you have a lot of house plants, someone, or maybe more than one someone, likes their long, hot, morning showers, and maybe forgets to use the bathroom fan, or maybe there is no bathroom fan.  Maybe you live in a older house with new insulation, or maybe your house has a cathedral ceiling.

Frost inside attic hatch


The picture above was taken on a very cold day notice the ice (frost) built up inside the attic hatch opening.

The next picture, from another house was taken a few days after a big melt. This was an older house with upgraded insulation stuffed into the eaves, blocking ventilation.  A brand new rotary ventilator was spinning merrily on top of two year old shingles.  Insulation and rafters were very wet, and there were indications of mold growth in the attic.  Water was leaking everywhere.



What is happening?
Air from inside the house leaked into the attic.  Warm inside air holds more moisture (humidity) than cold air. When the inside air enters the freezing attic the moisture condenses as thick frost. Because the frost does not evaporate readily, it builds up like the snow in your yard. The more inside air leaking into the attic, and the longer and greater the cold spell, the greater the build up of ice. If it warms up suddenly, the ice melts, there is too much water to evaporate, so it runs down, if it can collect somewhere, or it finds an opening, you have your leak, and a water stain.  Water may even run down the back (inside) of sloped roof sheathing and rafters and trickle into the walls.

If the air inside your house had been dryer, the moisture would have been absorbed by the dry air inside the attic before it could condense. If the attic was warmer it would also be drier because the air in your attic came from outdoors and able to hold more moisture, the leaked humid inside air might have dried or been absorbed  before there was enough of it to leak. If the ceiling and vapor barrier were intact the inside air would have stayed inside.

Your indoor humidity is too high if you can see this


If you have this problem, there are a number of things you can do to prevent it. The first thing is to reduce the humidity inside the house. In all the cases above, the humidity indoors was over 30%. Even though 30% is recommended for comfort and even more for fine wood furniture or hardwood floors, it is too high for very cold winters. If you can see condensation on your windows, your humidity is too high.

Use your exhaust fans, you should have a kitchen exhaust to outdoors if you do a lot of cooking, you should have a bathroom fan in every bathroom that is used for showering. If you have no fans, you should open a door or window (this may be tough Рbe strong :-)) If these methods do not get indoor humidity below 30% you may need a de-humidifier.

Eaves are blocked by insulation


Check the¬†attic for adequate ventilation. ¬† A well ventilated attic is a healthy attic. ¬†If your house has upgraded insulation, it may not have been properly installed. ¬†Air needs to be able to flow in from the eaves and out the ventilators on the roof. ¬†Mixing static and rotary (turbine) ventilators is a bad idea, use one or the other. ¬†A rotary ventilator can ‘short circuit’ attic ventilation by drawing air out of a nearby static ventilator instead of from¬†the eaves. ¬†Many pros only¬†recommend static ventilators, rotary ventilators sometimes¬†work too well, drawing air into the attic from inside the house.

A well ventilated attic is a dry attic


Check the ductwork inside the attic, in Canada the only duct work normally found in attics are exhaust ducts from the bath and kitchen fans. All ductwork in unheated attics should be insulated. Straight pipe is better than flex, and uninsulated white plastic dryer vent is not appropriate for bath exhausts. All exhausts must be vented outside through the roof, not inside the attic. Exhaust ducts should be as short as possible. It is not difficult to install a ‘gooseneck’ for an exhaust duct on a normal asphalt shingle roof, there is no need to run duct from one side of the attic to the other.

Air is leaking through the gap between the electrical octagon box and the drywall ceiling


You also need to prevent air from inside the house leaking inside your attic. The picture above shows a badly sealed light fixture in a cathedral ceiling, it has allowed air to leak into the attic space as well as  water back through the ceiling. Air may also be able to leak through the wire opening (knockouts) (circled). The area between the fixture box and ceiling should be caulked so that air cannot leak into the attic. A small amount of foam sealant at box openings will seal it. (Do not fill the box with foam, the electrical wires could overheat and cause a fire.). All openings in the ceiling between house and attic should be checked and sealed if necessary, including the exhaust fan openings.

New homes have a heavy (6 mil) plastic sheet vapor barrier between the unheated attic space and exterior walls. The sheets are taped where they are joined, and molded¬†¬†plastic¬†coverings for¬†electrical fixtures are also taped into the vapor barrier. Older homes built between the 1960’s and 1990’s used lighter plastic (3 mil), lapped joints instead of taped joints, and no covers for the fixtures. Older yet homes may have lapped building paper under the plaster, or nothing at all. Whatever you may or may not have, it is not necessary to remove your ceilings and walls to upgrade your vapor barrier. Vapor barrier paint applied to exterior walls and ceilings will work just as well or better.

Cathedral ceilings can be a problem because they may not have any ventilation and their attic, if they have one, often cannot be accessed. If you have a cathedral ceiling it is particularly important that all ceiling penetrations are well sealed, and if necessary, a vapor barrier paint is applied.

And don’t forget to weatherstrip the attic hatch!

Thermal image of air leak at the attic hatch



Dry air is good air


“Put an end to desert like air in your home” says the¬†flyer containing an ad from a well known maker of heating and ventilating products. Reading further, “The average humidity in the Sahara Desert is 25 percent. In wintertime the average humidity inside some homes can be even lower….”

Yes it can, does that mean you should rush out and buy a humidifier? Not necessarily. Some of The biggest problems in homes are the result of excess humidity. CMHC recommends that humidity levels in a home in very cold weather (ie Edmonton) never exceed 30%, pretty close to that desert like air. Desert air is not only healthy, but it also preserves things, like the building materials in your home.

Excess indoor humidity results in condensation, and condensation results in, mold growth, wet insulation, wet wood, and the destruction of all building materials that need to be kept dry.

Excessively dry air on the other hand, can shrink warp or crack  solid wood furniture or flooring, dry out paper, and some textiles, make you feel colder, dry your skin,  or even create health problems for some people.

Health experts and those who sell or install hardwood flooring will recommend humidity levels of between 35 and 60%, but those levels are too high for homes in the extremely cold winters we have in Edmonton.

To understand why the ideal of 50% relative humidity is not so ideal for cold winter homes, the dew point for air containing 50% relative humidity at 20 degrees C is 10 C. Dew point is when water vapor becomes condensation.  In other words if you keep your home at the usual indoor temperature of 20 C, and your indoor humidity at 50%, condensation will form on anything that has a temperature of 10 C or less.  Like the surface of your double glazed or double pane windows when the temperature outdoors is around -18 C.  If you see condensation or frost inside your house the humidity is too high.

Next;  Keeping it dry