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In my country the maximum temperature reaches to 50 degree Celsius. Normally it is a dry weather but only for two months, July and August very humid. On the other hand, in winter it is very cold for only two months December and January, when it goes to 2 or 4 degree Celsius.
So, What is the sustainable house insulation for such regions?
Insulation doesn't care about directions, so whatever keeps a house warm in a cold climate will help keep it cool in a hot climate. Vapor barriers are different--my understanding is that the vapor barrier should go on the side of the wall that sees the most humidity. But insulation simply slows down heat transfer in all directions, so as long as you have a way to cool off the interior, whether it's AC or a cool night breeze, any insulation will hold the day's heat at bay.
Consult a local professional about the best insulation and vapor barrier choices for your area, because humidity and local pests can influence the effectiveness and longevity of your system. Make your decision based on factors such as those, because as far as heat transfer goes, an R-value is an R-value. If your energy comes from sustainable sources, the "green" insulating materials will be clear winners for sustainability, but otherwise just get the best insulation your budget allows.
This answer gives a good overview of some of the insulating materials you might consider.
If you have questions regarding the effectiveness and durability of insulating materials, the folks at Home Improvement can probably help you out.
It depends on the exact climate. There are many hot climates... In some cases reflective insulation is a good option. But thermal insulation may also be important.
Anyway, home sustainability in hot climates depends largely on shade, natural ventilation and home design. See the shade issue, for instance: you need to shade your windows, roof and walls, and also the ground around your house, by using trees and shrubs but also devices awnings, shades and pergolas. It’s important to reduce the temperatures around the house.
For more details, see these links to House-Energy pages:
Natural Cooling Guide: http://www.house-energy.com/Cooling/Natural-Cooling.html
Alternatives to Air Conditioning: http://www.house-energy.com/Air-Conditioning/Air-Conditioning-Alternatives.htm
I recommend that you research hemp, being both insulating and moisture-absorbing, as well as non-irritating, and sustainable on all different accounts.
Here is some info from buydutchseeds.com:
"One of the very common uses of industrial hemp is for insulation in buildings…Hemp insulates better than many other materials, such as cotton. It is a composite material, in which the hemp fibers are mixed with other materials to form a strong bond. Hemp has been known to have a low U-value, that is, it allows a lower heat levels to pass through it. The lower the U-value, the better the insulator a material is. It has been proven that Hemp insulation has a U-value of .040, which compares favorably to the U-value rate of 8-inches of insulation of fiberglass…Hemp's R-value stands at about R-3.5 per inch of thickness. The more the material limits the flow of heat through it, the higher its R-value…Insulation made from hemp is a natural buffer. It helps regulate condensation levels without causing any deterioration to thermal efficiency. This eliminates mold which is a major challenge to timber frames. Timber has been known to rot and get distorted in shape when exposed to high moisture levels especially during storage…Hemp also allow buildings to “breathe”. This has the fantastic benefit of preventing structural damage and potentially serious health issues as a result of poor air circulation."
Here are a couple of links I can post per this forum's limits; other websites are hempcrete.ca, thehempbuilder, and many more:
http://www.hempcompany.ie/category-s/1823.htm
http://greenspec.buildinggreen.com/blogs/hemcrete-hemp-lime-composite-insulation-walls
Per the clarifying comment below the question, I think it's also important to note a non-insulation solution to the same problem.
In a climate that's hot, keeping sunshine out of your home helps immensely in reducing the need for cooling. Remember that a building with windows represents the original greenhouse effect (letting in visible light, but not letting out infrared radiation).
There's a couple other good solutions to this problem:
Window coverings, that you manually pull down when the outside air is less comfortable than the air inside (i.e. in the hot summer, you keep coverings closed in the day, and in the cold winter, you keep them closed at night, and vice versa). I particularly like light-colored honeycomb window shades.
Plant bushes and trees outside your home's windows, on the side(s) that face the sun. If your home is hot in the summer, and cold in the winter, then you should pick deciduous plants, that will block sun with leaves in the summer, and lose their leaves, allowing sun in during the winter.
Using this strategy, I've comfortably kept my house at 80°F or lower at all times, with zero air conditioning, even as temperatures in my area can approach 100°F on the hottest summer days.
