I thought that the purpose of these blocks was to insulate, saving you from doing a double wall construction or coming to the idea after a few years that maybe you should insulate. 15cm would be the minimum that I would use if I was going to the trouble of using these blocks.

here everything is a single wall 7cm construction..

Sure In Europe we used at least 20 cm + much easier to build with the thicker blocks, channel out for electric and water pipes.. which is not possible on a single 7 cm block or brick..

Guess it is all about cheap......... a ready built 3 bedroom single house here out of Town cost around 2 - 2.5 million, same sort of house size in UK would cost many times more than that......... not many Thais would buy a standard house for 50 - 80 + million

Well, everything built by Thais is 7cm construction, plenty of Farangs know about insulation and radiation, no point in insulating the roof if the walls aren't insulated either. When the sun shines on the wall where my office is it begins to radiate heat into the room until well after the sun has gone down. I will be building a double wall here, and insulating the space as well. I think the extra cost of using a 15cm block (or 20cm) would be well recompensed in terms of comfort and electricity bills.

You can easily build a house for ฿500 000.- out here, whether you would feel comfortable in it or not is another question.

Insulation of walls and roof is fine but I reckon most of the heat come through the windows anyway. I have seen mention of reflective film on House window glass. Is this the best option?

Insulation of walls and roof is fine but I reckon most of the heat come through the windows anyway. I have seen mention of reflective film on House window glass. Is this the best option?

Depends, 9 years ago now I put reflective film on all my House window glass, only problem is I never have Air Con switched on so most of the windows ore open 24/7....... saying that 2 spare bedrooms the windows are closed, both rooms are cooler than the landing is..

So guess thae answer is yes as long as you keep the windows and doors closed