I've made an attempt to make a comparison of the two options questioned by the original poster. To calculate heat transfer through a wall, the following formula can be used (

http://utwired.engr.utexas.edu/conserva ... g/cool.cfm):

Q = U * A * deltaT

Q = heat transfer through a wall in BTU per hour.

U = 1/R where the R value (also known as thermal resistivity) of the wall materials in the wall will each have an R value and they are combined to arrive at a final R value.

A = area of the wall in square feet.

deltaT = temperature difference between inside and outside in degrees F.

To make the comparison, I've calculated the heat transfer in a room through 3 walls. This assumes the 4th wall is comprised of windows and the ceiling is insulated so they will be constant in all comparisons. For a room that has 3 walls that are 8.2' (2.5m) high and 16.5' (5m) long this gives a total area of 406 square feet. For the temperature difference, I've used an inside temp of 77 deg F (25 Deg C) and an outside temp of 95 deg F (35 deg C) giving a deltaT of 18 deg F.

Qcon block 20 cm:

U = 1/R (where R = 8.4 for a 20cm block) = 0.12

A = 406

deltaT = 18

Q = 0.12 * 406 * 18 = 877 btu/hr = 257 watt/hr = 1.16 baht per hour at 4.5 baht per kilowatt-hr.

Double 10 cm Block Cement w/ 1" air gap:

U = 1/R (where R = 2.6 for the two blocks and a 1" air gap) = 0.38

A = 406

deltaT = 18

Q = 0.38 * 406 * 18 = 2777 btu/hr = 814 watt/hr = 3.67 baht per hour

I also made a comparison with a single 8" cement block with air holes:

U = 1/R (where R= 1.11) = 0.90

A = 406

deltaT = 18

Q= 0.9 * 406 * 18 = 6577 btu/hr = 1927 watt/hr = 8.7 baht per hour

So it's obvious from this comparison that a Qcon wall is more efficient. This is all fine and reasonably straightforward, however to make a cost comparison between the two you have to take into account your planned air con usage i.e. how many rooms and for how many hours per day, and the temperature variations where you live. This is far more difficult to calculate as it varies tremendously for each person/family. For myself and my wife, it's rather simple, we only use air con in one room of the house (the master bedroom) for about 8 hours in a 24 hour period. In Phuket at 10 PM the outside air temp averages about 29 degrees and we set our air con at 25 degrees. By morning the average outside temp is about 25 degrees so the average during the night is let's say 27.5 This results in the following cooling cost comparison over one month:

Qcon: 77 baht per month

Double Cement Block: 225 baht per month

The difference of 148 baht per month between the Qcon and double cement block would not be enough for me to select one over the other on heat transfer basis alone. I also did this calculation for an average European winter climate where I used a deltaT of 45 deg F (25 deg C) and the cost difference then becomes 1562 baht per month for only 8 hours per day. In reality, the average would probably be closer to 14-16 hours per day or a cost difference of 2,928 baht per month, certainly significant enough for most people to consider. And that's just for a single room not the entire house. Other factors such as ease of use come into play as well and ask any electrical or mechanical contractor in Thailand what they prefer and they will always tell you Qcon or Superblock; they are far easier to cut through and work with.

Obviously cooling systems have to overcome many other factors such as gaps between windows and doors, heat given off by surrounding concrete terraces that have absorbed heat during the day and then release it at night, air con units that give off heat right outside the room they are trying to cool (why not capture this heat to heat your water?). These can be significant contributors to cooling costs and will likely account for the majority of night time cooling costs along with heat transfer through the ceiling and slab. Pay attention to the type of roof and insulation you use. Money spent here is money well spent. I currently rent a house that has no insulation in the roof or walls and the roof has dark brown tiles with no ventilation. The inside temperature of the house is always 3-4 degrees C higher than the outside temperature even at 6 AM. The only time it approaches the outside temperature is when it's been overcast and raining for several days in a row. Imagine the additional load this places on the A/C system when it's being used. This is a prime example of how not to build a house (and it was built by a British guy in his mid 30s who claims he was a builder in England before moving here).

I hope this is useful. I've gone through the calculations more than once so hopefully they are free of errors but no guarantees.