When it's negative 8 Degrees in Canberra, how much heat is lost through windows?
Last weekend in Canberra was freezing! The weekend minimums reached -8 degrees in some areas of the ACT and many residents would have noticed the cold spell. But, if you’re heating your house, do you know how much heat is lost through your windows when it’s that cold outside? It is going to vary greatly if you have single glazed, double glazed or triple glazed windows.
Let’s start with a simple formula for calculating heat loss through a wall, floor, window or door:
Q = A x Tdiff x U
- Q = Heat Loss (rate of heat flow) through the window assembly, given in Watts (or Joules per second)
- A is the area of the element being investigated – in our case the window assembly, in square meters
- Tdiff is the temperature difference between outside and inside of the window assembly, in Kelvin
- U is the heat transfer coefficient of the window assembly (U-value), given in Watts per square meter Kelvin
This means that heat loss reduces if:
- the area of an element is smaller,
- the temperature difference between inside and outside becomes smaller
- the conductivity of the material is reduced (ie the U-value becomes smaller)
In order to do the maths we need to make some assumptions.
Let’s assume that the occupants in Canberra like to keep their house at a cosy 20 degrees inside, and the area of all of their glass windows in their house is 50m2.
Therefore from the forumla above,
- Area, A = 50 m2
- Temperature Difference, Tdiff = 28 degrees (20 degrees inside, minus minus 8 outside equals a difference of 28 degrees)
Lastly we need to know the U values for single, double and triple glazing. Fortunately we can reference two of these from a reputable Australian source yourhome.gov.au, and the third from our German triple glazed window manufacturer.
Using Yourhome’s specifications we have the following U Values.
- Single Glazed Aluminium windows: U-value = 7
- Double Glazed UPVC windows: U-value = 3
- Triple Glazed Timber Windows: U-value = 1
The formula we will use for each example is: Q = A x Tdiff x U
Where A = 50, and T = 28, therefore Q = 50 x 28 x U Value
Heat lost through single glazed windows on a very cold day
For single Glazing, U = 7
Q = 50 x 28 x 7 = 9,800W or 9.8kW
If the inside and outside temperature stayed stable for one hour, it means that 9800 Watt-Hours will be lost through the windows, better known as 9.8kWh. Thats almost $1.96 lost per hour with electricity prices at 20c/kWh. Now imagine if it was this cold for 2 hours every day, for a whole month. It would cost $117.60 for only two hours of heating each day!
Heat lost through double glazed windows on a very cold day
For Double Glazing, U = 3
Q = 50 x 28 x 3 = 4,200 W or 4.2kW
This is clearly a great improvement from single glazing, with less than half the heat loss. With electricity prices of 20 cents per kWh this would cost about $0.84/hr to maintain temperatures of 20 degrees inside. This is about equivalent to two large bar heaters running at high capacity. if it was this cold for 2 hours every day, for a whole month. It would cost $50.40 for only two hours of heating each day!
Now lets look at what the heat loss would be with high performance triple glazing
Heat lost through triple glazed windows on a very cold day
For Triple Glazing, U = 1
Q = 50 x 28 x 1 = 1,400W or 1.4kW
With electricity prices of 20 cents per kWh, this would cost about 28 cents per hour, a huge improvement from single glazing, and a significant cost saving over double glazing. If it was this cold for 2 hours every day, for a whole month. It would cost only $16.80 for two hours of heating each day!
This formula shows just how much heat is lost through single glazing, and how much heat loss can be improved with double and triple glazing.
So now that you know how much heat you’ve lost through your windows on that cold day of July, why not look to do something about that, and install some German Triple Glazed windows.
You might not be able to afford not to.
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