Renovation & Design

Relative humidity plays major role in comfort of home

CP/This undated illustration provided by Kaz, Inc. shows the warm mist Vicks Germ Free Humidifier. (AP Photo/Kaz, Inc.)

QUESTION: My 977-square-foot home, built in 1962, was retrofitted with a high efficiency furnace a couple of years ago. The usual story, we now have a humidity issue in the winter. The house never had a fresh-air duct and is tightly sealed, as tested by Manitoba Hydro. The hot-water tank was changed over to an electric unit. The chimney was capped and the flu pipe is no longer used. I am considering installing an energy recovery ventilator (ERV) to reduce the humidity. Or, perhaps a damper controlled fresh-air intake would be sufficient. Since we have an unused flu pipe running from the basement to the roof, can this be used for either an intake or exhaust for ventilation? What would you recommend?

Thanks and regards, Dave Laurie.

We have a problem with moisture leaking into our attic from a disconnected bathroom fan vent that has resulted in the development of frost. This discovery led to a discussion regarding the appropriate relative humidity inside our house. One source indicated once the outside temperature drops to 0 C, the indoor humidity should be at 40 per cent, but when it drops below -12 C, it should drop to 30 per cent, and even lower at -24 C, at 20 per cent. Other sources have told us that indoor relative humidity should stay between 30 to 50 per cent, otherwise structural issues such as cracking drywall and separating hardwood floors can arise, as well as health issues. We purchased a hygrometer and it is consistently reading between 34 and 39 per cent. Is there a definitive answer to the safe measure of indoor relative humidity?

Thanks for your time, Dianna and Al Sveinson.

Answer: While your two questions appear to be quite different on the surface, the underlying theme of moisture control is consistent. We will look at the reasons for moisture issues in the heating season and provide guidelines for minimizing this common problem.

Yes, it is that time of year again where the outside temperatures are beginning to creep below the magic freezing point of 0 C, especially at night. Because of this, our homes are now mostly sealed against the cold with the heating systems running regularly. When we start this yearly heating cycle, we are trapping warm air inside our comfortable abodes. Contained in this heated air is a certain amount of moisture, dissolved as invisible water vapour. If this level of moisture, typically referred to as relative humidity (RH), is allowed to rise too high, problems can occur. These problems can range from a small amount of condensation on our windows to major mould-growth issues on walls and ceilings. To combat this, we must try to find a balance between proper ventilation and personal comfort.

As you have experienced in the house in the first inquiry, high RH issues can often begin to occur when various energy efficiency upgrades are done.

This is because all of these upgrades tend to make the building envelope, or the interface between outside and inside, less permeable to air movement. Unfortunately, the former openings in these locations, such as the old furnace chimney, acted as natural ventilation to let out damp air while old windows and doors let in fresh, dry air. Now that these pathways are gone, we have to strive for other ways to increase incoming fresh air and vent out humid air, all without wasting too much energy.

A heat recovery ventilation (HRV) or ERV unit may be an excellent way of accomplishing this goal, but is a moderately expensive solution and may have limited effect in a retrofit situation. Before going to the effort and expense of putting this device in your home, try using existing components. The simplest method of reducing high RH is to use bathroom and kitchen exhaust fans more often. These should be installed in most homes and are easily turned on with a simple switch. To make them even more efficient, timers can be installed in the place of the ordinary electrical switches to allow longer, yet more controlled use. Running a bathroom fan for an additional 10 to 15 minutes after taking a shower may vent enough air to prevent excessive humidity. Of course, this will only work if the home has enough makeup fresh air coming in to replace the vented air. So, installation of a fresh-air intake in the basement may be a good idea, but using the old chimney for this purpose is not. Installing an insulated duct, connected to a vent hood installed through a foundation wall, is the proper way to go. A duct with a damper will give you more control over the amount of air, but may be unnecessary as the air pressure drop when an exhaust fan or dryer is in use may self-regulate the amount of air drawn in.

Now comes the hard part, trying to find the right balance and deciding on the appropriate relative humidity. The second question is an excellent one on the specific levels of RH, but there is indeed much debate on this topic. Some building-envelope specialists say you can never have too low RH, but they may have never lived through a Manitoba winter in an old house, where the RH can drop to 15 per cent or lower in the middle of winter. That low a level, at room temperature, can leave homeowners with dry, sore throats and cracking skin on their hands. Alternatively, the experts who suggest keeping the RH above 30 per cent all winter are dead wrong, as that level will surely cause excess condensation on windows and other areas that can cause more damage than a few minor plaster cracks, which may result from the dry air.

Ensuring a proper level of winter ventilation, while maintaining a proper RH, should minimize any moisture problems to your homes and be comfortable for the occupants. From my experience, levels between 25 and 40 per cent, at normal room temperature, should achieve this goal, with the RH at the lower end of this scale during the coldest winter days.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors -- Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at


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