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.
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
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.
OUTDOOR TEMPERATURE MAXIMUM INDOOR RELATIVE HUMIDITY
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