For A New Home, Humidity Is a Double-Edged Sword!

Or, “You did not take me seriously when I told you to watch the humidity in your new home!”

Truss Uplift

I am really mad! In the recent month, I received 3 phone calls from my last year’s clients about their homes. The floor boards are splitting and the drywall is cracking.

On the new house? What is going on???

The physics behind these problems is pretty simple. Wood is a natural material that responds to fluctuations in its environmental moisture and temperature.

If, the adjoining air, has less moisture than the wood, the wood will dry out to balance to the air. As it dries, it shrinks. If, on the other hand, the surrounding air has more moisture than the wood, all the wooden pieces used to construct your home will expand.

When the quantity of moisture oscillates drastically, wood will expand or contract to a drastic degree. This natural behaviour of wood is responsible for most of the problems encountered in modern, well sealed houses.

It is what causes doors to close improperly, floors to squeak, gaps in wood flooring, uplift on roof trusses. It also causes kitchen cabinet doors to crack, casings and baseboards to shrink and come away from the walls, and miter joints to split.

On the other end of the scale, we all know that the excess humidity in the house causes hardwood to swell and cup. It also condenses a moisture on windows which may damage the surrounding wall. It also helps the mould to grow.

As you can see, humidity is a double-edged sword. Therefore, it is essential to control it in your new home.

Here, in southern Ontario, during spring, summer or fall the control is easy. Air is relatively moist, and as long as you keep your windows closed, your heat recovery ventilator should do the job. Where as winter air is extremely dry, it can drop the relative humidity in your house to the point where it can do damage to the finishes of your house and furniture and under extreme conditions even do structural damage.

Radiant floor heating does not help. It creates a hot dry climate at floor level, draining the moisture out of the floor structure.

For 30 years, I have been talking and discussing this problem with each and every new client. I make it a point to mention it in several occasions, especially when discussing the hardwood flooring and its warranty and again at the point of pre-delivery inspection.

I usually stress the numbers: “Please keep the humidity in your house between 35% and 60%!!!”

Why then, when we went to inspect the relative humidity from our callbacks this month, the highest number we found in those homes was 18%?. At one house relative humidity was 6%. Six percent relative humidity. We did not believe it. We did not know that it could go so low. It would definitely give you problems breathing.

Do you know who is going to carry the blame for cracked walls and split floors? The builder. That’s who. Me!

So, to avoid similar problems in the future, please take care of the relative humidity in your new home. Between 35% and 60% will be just fine.

The following is the list of ideas for keeping your humidity in check:

1) During the heating season, turn the dial on the HRV to the lowest setting available.

2) Do not constantly run bathroom and kitchens fans. They override the HRV system by creating negative pressure and thus drying supply air.

3) Watch your hygrometer carefully during the winter – maintain around 45% RH.

4) Keep indoor plants – they will improve air quality, especially during the winter.

5) Once in a while open the windows. Open windows will supply all the fresh air required.

6) If you notice that the air is on dry side, cook without turning on the fan.

7) Do not turn on the fan in bathrooms while showering.

8) After taking a bath leave bathtub water in the tub till it gets cold.

9) Get an air humidifier. There some models that integrate with your HRV system.

There is one small snag with keeping the house at that level of humidity – moisture on your windows.

As the temperature outside drops to -20 C, the inside pane of glass in a window becomes cold enough to cause condensation to form on the inside pane & start running down the window.

Similarly, the colder it gets outside, the worse the problem becomes. We suggest that you simply put up with the inconvenience of occasionally “mopping up” your windows – it’s all part of the responsibilities of home ownership.

If you find it useful please share it with your friends and groups!


For A New Home, Humidity Is a Double-Edged Sword!