When I was groing up in rural Ontario I used to marvel at the frost patterns that were made on our bedroom window on those cold February nights.  It wasn’t until years later that the terms Dew point, relative humidity, saturation temperature, enthalpy, wet bilb, dry bulb, sensible heat, latent heat or evpourative rate meant anything at all.  To-day all of these terms are found when studying psychometrics.  And this all come down to Humidity.

R.H. can be defined easiest by stating that it is the amount of moisture, as a percentage, that is currently in the air compared to what it could hold at a specific temperature. That is to say 20% RH @ 20C  is far less air bourne moisture than 20% RH at 30C. 

So, at what point does moisture collect on your windows in your house in the middle of the winter? Mmmm, that depends on the Dew Point.  Getting confused?

Let’s go at it like this; if your house is 24C and the outdoor temperature is -12C the glass in your windows could reach -6C, at that point the moisture in the air that is being held in suspension will condense against the cold glass as it cools. If a shower is turned on or food is being washed, boiled and prepared, then the moisture content of the air will increase (rise in RH) and more moisture will collect on the window. Soon the window becomes so wet that the condensation runs across the sill and down the wall to the floor. And the question becomes, How do you cope with it & control it?

With “Ventilation” ….. in any form, by any method available to you. The easiest way is to open a window about 1/2″ (1.26cm) as far away from an exhaust fan as possible and allow fresh cool air in. As the air warms it will expand and it’s RH will decrease allowing it to absorb more moisure as it makes its’ way across the house and out the exhaust fan to the outdoors.  And as new air comes in it carries out more moisture etc,etc and eventually your windows stop sweating and the RH in the entire house drops.

How much does this add to your fuel bill ?  30 x 1.08 x  77 x “fuel cost” (because nothing is free.) Can you minamize that expense? Yes.

With an HRV the calculated cost can be reduced as much as 80% or more.  And you can take that to the bank!