RH (Relative Humidity) is the measure of water vapour in the air relative to its temperature - or it's ability to hold that water vapour. 

At 100% the air  can no longer hold any additional moisture - called dewpoint

To decrease the RH you can do 2 things

1. Increase the temperature
2. Dilute the air by adding dryer air ( from outside)


Average interior environment of a home 
21 C (Degrees Celcius) with 30 % RH

The Dewpoint is 2.78 C and any surface colder than 2.78 C will have condensation

Mould will grow at a minimum of 70% RH

If your house is 21 C with 30 % RH and the outdoor temperature is -25 C, the temperature next to some exterior walls such as inside closets, at windows, etc will have very cold surfaces due to limited insulation values. At 7.87 C, the RH will be 70%, and from 7.87 C to 2.78 C the RH will be between 70-100 % or enough moisture to grow mould on surfaces that have a food source  (drywall, dust on windows, etc)

Lowering the temperature overnight to 16 C, will cause the RH to be 42 %, or lower the air's ability to hold moisture. So what will the inside temperature of a closet or window drop to if the outdoor temperature is very cold, say -20 C?

Here are some Do's and DON'T's

  • Don't block heat vents
  • Don't place clothes or other items against exterior walls 
  • Do leave closet doors open when outside is really cold
  • Do open blinds and curtains to let heat access the windows
  • Do TURN ON your Principal Exhaust Fan when outside is really cold
  • Better yet - upgrade to an HRV if your buying new or renovating 
  • Do buy a hygrometer and keep your interior humidity at an appropriate level to prevent condensation

If your windows are sweating (have condensation forming) then likely you have too much humidity in your house. 

Many people find that when they buy a  new home or upgrade their old windows to new ones they get more condensation - That is true. Why?

Why new homes have more window condensation . . .

Most new homes are built very energy efficient. That means they have a sealed combustion furnace and are built very air tight. That also means minimal fresh air from outside coming in to dilute the humidity in the house. 

If the house is built energy efficient and air tight you also won't run the furnace as much which is how you get fresh air into the home.  Unless you switch on the principal exhaust fan (located in the hallway near the thermostat) you won't be able to reduce the humidity.

HRV (Heat Recovery Ventilator) 

If you have an HRV, that unit will run all the time and bring fresh air in and exhaust stale air out. It keeps the humidity steady and indoor air quality high. Most new homes SHOULD have one, but most homebuyers are not aware of these units and are more interested in granite counter tops than fresh air until the first winter when the windows start sweating.

Average Moisture Sources per Week 
SourceAdded Moisture –                           Liters
Average Family of 4                                         63
2 Pets                                                               30                
Drying Clothes                                                 12
Gas Range                                                        9
Refrigerator                                                       9
3 Large Houseplants                                         5
Hand Dishwashing                                            3
Bathing – 8X                                                     3
Showers – 8X                                                   25
Wet Basement                                                 14
New Construction Materials (first 36 months)   5
Seasonal Moisture storage in materials            8
Here are the maximum humidity levels you can expect to run your home before the windows sweat.

                                                      Dual Glaze windows         Triple Glaze Windows
-30 Celsius  and Colder                   16% at 21 Celsius               32% at 21 Celsius
-29 Celsius to -24 Celsius                22% at 21 Celsius               38% at 21 Celsius
-23 Celsius to -18 Celsius                24% at 21 Celsius               45% at 21 Celsius
-17 Celsius to -13 Celsius                28% at 21 Celsius               50% at 21 Celsius
-12 Celsius to -6 Celsius                 33% at 21 Celsius                62% at 21 Celsius
-5 Celsius to +2 Celsius                  40% at 21 Celsius                68% at 21 Celsius

** For window condensation only - Maximum interior RH most houses can handle is only 30-35% and maximum for the Alberta Building Code is 35%

NOTE 1 - Minor sweating may still occur along bottom edge of window glass depending upon the spacer insulation and window material
NOTE 2 – Wind will have a dramatic effect on the glass temperature and decrease the indoor maximum relative humidity
NOTE 3 – air circulation across the window glass will decrease condensation on the glass as it warms the glass surface 
NOTE 4 – Always leave curtains open, leave blinds up 10-12”, do not place furniture in front of windows blocking floor vents

This table was compiled by comparing Canadian Window manufacturers data 

In simple terms, when air cannot hold anymore humidity (vapour) it begins to turn to liquid, or sometimes referred to as "sweating"

Condensation on windows is an indicator of high humidity in a home. There are many sources of humidity that can cause your windows to sweat..

When air from the house leaks into the attic, it condenses and freezes

That could be caused by

  • High interior humidity
  • High interior air pressure from stack or mechanical pressure
  • Inadequate air sealing techniques 
  • Disconnected Vents - Fans, Dryer, Kitchen Hood, Etc
  • Exterior high humidity entering the attic space through soffiting - IE Front door under large vented soffit area

To stop the build-up you need to prevent the humid air from entering.

The Video on the right is a result of a false plumbing wall creating a dead space that has no seal to prevent air leakage - See Picture below top left

Most common sources

  • Wood windows are lower R- value than Vinyl

  • Smaller windows are lower R-value than large

  • Acrylic and glass block are lower R-value

  • Window glass closer to the interior performs better 

  • The tighter the window seal the more it condenses  ( less air to dry it )

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