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Last weekend A friend and myself took down a 30 year old pergola. Already I am feeling an improvement.
A pergola produces benefits in the Summer when you want to shade windows from the sun. In a Canberra Summer, you want as little as possible sun hitting the windows and heating an already warm house. In a Canberra Winter, you want as much sunshine as possible to hit the windows and warm a cold house. My pergola was not very good at either. It was designed to have a vine of some sort grow on it to provide Summer shade and that did work well. Unfortunately the vines gradually prised the joints apart making it start to collapse.
As well as that the construction of the pergola meant that there have been large shadows across the windows in Winter. A cross beam cast a shadow of 1.2 m2 and other beams and poles produced shadows of another 1 m2 . Effectively this amounts to 1.4 kW of shading when taking into account the angle of the sun in Winter and a few other factors. Removing the pergola has therefore produced the equivalent of a 1.4 kW heater for 5 hours a day (on average considering fog and cloud). At current electricity prices that is worth 7x$.21 or about $1.50 a day.
What does it mean in reality? Well, on Tuesday, the indoor temperature at 5 PM was 22.9 degrees. On a cloudier Wednesday it was 21.8. On an overcast Thursday it was 20.1 and on Friday it was 22.3 with light cloud and haze. Saturday was hard to compare because I turned heating on until 11 when the sun came out. It was 22.5 degrees then, with a lot of cloud but warm patches. Minimum temperatures varied from 16.2 to 18.1 over the week, depending mostly on how cold it was outside.
This morning with no fog and clear skies, it is warm enough before midday with a temperature of 22.3 degrees. It should be higher than that soon. Comparing with two weeks ago when the weather pattern was similar, there is a difference of 0.8 degrees. Not very scientific but it is a fair comparison.
Overall, there will be a warming of the house by somewhere between 0.5 and 1 degree. Presumably this reduces the amount of heating required in the evening and saves a substantial amount of energy.
Compared to this time last year the temperature in the house is just over 1.0 degree warmer at 5 PM. It is hard to tell how much of this is due to paving (reflecting more light through the windows) and how much is due to the reduced shading of the windows. It could be just due to more sunlight and higher average temperatures. However, the comparisons do not show that much variation and, in terms of minimum temperatures, this year has been a bit lower.
Next things to do are:
- Replacement pergola that will be completely redesigned and provide good shade in Summer while casting no shadows in Winter. It will effectively give the house eaves on the North and use solar PV panels as the “roofing material”. More about that soon.
- Organise for two or three additional adjustable shade blinds over each of the windows. This will allow me to make real time adjustments in October-November and March-April which are months that have highly variable temperatures. I can let more sun in or exclude it to adjust temperature. Also, in Summer, they provide an extended shadow to reduce reflective light (you can see how reflective the paving is in the photo)
- See what I can do to increase the thermal mass and wall/slab insulation to retain more heat or cool within the house. There are several options to consider.