The effectiveness of Double Glazings depend on the air/ inertgas gap. The wider the gap, the more effective the double glazings will be in combating heat transfer. Swedish standards reccomend gaps upto 20 mms between the two glazings. The maximum gap my UPVC window farme supplier could accomodate in his frames was a 10 mm airgap sandwitched by two panes of 5 mm each. That brings the total thickness of the sandwitch to 21 mm as 1 mm is occupied by the epoxies that are used for gluing the two panes. Gaps of 5 or 6 mm are a waste of money.
However, please keep the window sizes as small as asthetically possible as windows are the biggest source of heat transfer in a building despite using double or tripple glazings. Double glazings facing the sun for more than two hours during the day will ultimately make the room unusable. The smaller they are, less heat gain / loss.
UPVC window frames would now cost something like Rs 750/ sq ft. Double glazings with imported Low E Glass, both sides tempered, sandwitched on an automatic plant would be another Rs 400 per sq ft. On the other hand, simple local made float glass double glazing manually done would be Rs 130/ sq ft. It all depends what is your budget and how effective you want the system to be. In a one kanal house the average total openings for windows runs from 800 to 1000 sq ft. But you dont need to have all of them double glazed. Double glazings make sense in rooms where you are going to have air-conditioning / heating 24 hours a day.
Double walls with Jumbolon sandwitched is a effective way to stop heat transfer. Again it all depends how effective you want your system to be. The more money you wisely put into constructing your home, the lesser are your energy bills. During the last three years, my maximum electricity bill has been Rs 14,000 with one Inverter AC operating as long as the power is there. My maximum central heating costs for running a gas boiler for water radiators has not exceeded Rs 6000 per month. This is based on the boiler running 24 hours a day through out the winters which constantly maintains the temp at 22 Degrees. Just try to operate heating and air conditioning in an uninsulated house and you know what you will end up paying.
Use as much jumbolon insualtion as you can afford now. Just Two inches did not work for me. 6 inches did. Depending on your heating /cooling requirements, it will pay you back in the first two or three years.