if you have the money - yes.
up here we paid 12k for 1 window and 1 x 4 panel sliding door.
If you care about energy ratings, heating & cooling efficiency of your house, you should get double glaze. Bit surprised this is not a standard inclusion from your builder.
So, welcome to the wonderful world of NatHERS! (National house energy rating scheme).
I went through this recently with my build. Basically what happens is that the builder gets an energy consultant to re-create the whole house in software such as FirstRate5 which performs analysis based on your location to determine the heating and cooling requirements. I also did this myself, it is free to download and use, although it is all quite complicated. As you are in Melbourne, the heating requirements will be much higher than cooling. The biggest factor for heat loss is firstly through the ceiling and walls and then windows, offset by sunlight warmth coming in through the north (and to a lesser extent east and west) windows in winter and sun on the roof.
I started off with a Nathers rating of 5.4 stars for my Sydney location (heating 39.2 MJ/m2, cooling 20.7 MJ/m2) and ended up with a 7.1 star rating (heating 25.7 MJ/m2, cooling 12.4 MJ/m2) by increasing the r-rating of roof insulation to R4.0 and outside wall insulation to R2.5, increasing the size of all north facing windows and reducing the size of south facing windows (top priority) and then east/west facing windows. The FirstRate5 software also tells you the energy requirements for each zone (room) and so I could see that the south facing rooms had the highest heating requirements. As such I only needed to put double glazing on two south facing sliding doors. Increasing roof insulation from R3.0 to R4.0 made a big difference, increasing from R4.0 to R5.0 made very little difference.
I would start by minimising or eliminating south facing windows (eg as per
oklouise excellent plan to move the family room window from the south to the east). Ensure that the builder is using the right insulation, put double glazing on the remaining south facing windows ONLY and then see how the Nathers rating looks. It’s very easy for the energy assessor to then change the other windows to double glazed in the software to assess the impact, which may not be that much. This would be the most cost efficient path forward, $20,000 certainly is a lot extra to spend.
where is the North?
As OzSurferAI has mentioned, double glazing alone won't be as efficient as complex approach to house insulation.
In Melbourne I would start probably start with insulation of the slab perimeter and finish up with combined R5.0 roof + ceiling insulation.
Also, I would be very closely checking how well is wall insulation is installed and once roof and windows are in, checking the whole build with the thermal camera for air leakages.
Double glazing some but not all windows might be an issue as the profiles will be certainly different, so you might loose consistency between looks.
If you double glaze all windows, consider Low-E glass for western facing windows too to prevent overheating during summer.
I would say double glazing for southern facing windows/doors is a must.
After all, it is not about rating, but about your and your family comfort, the question to be asked if you enjoy waking up in the cold house in the mornings?
$12k is not a lot of money after all when comparing to the overall all-in cost of double story house build in Australia.
WBR,
Alex
Note: to get the cooling load down I changed most windows to awning type as they have a 90% airflow rate in NatHERS compared to 45% for sliding windows. But for upper level windows you will need to have child safety screens specified and included in the pricing otherwise the windows will be made with restricted openings (12.5 cm max) and have only 10% airflow in NatHERS. Also having fans in living areas makes a big difference. It’s all very complicated!
Hmmm, I don’t know where I got $20k from. $12k is more reasonable and
alexp79 does have some good points.