The insulating properties of fabric
The hypothesis that Thinsulate fabric retains heat and keeps water in the flask warm for the longest period of time, is proven to be true.
Thinsulate is a thermal insulating synthetic fiber that is used in making warm clothing. It is a trademark of 3M Corporation and has been sold since 1979. Thinsulate is made using very fine fibers and therefore has a very high density. This makes Thinsulate very effective in trapping heat and keeping us warm.
Try to repeat the experiment using different materials like fur, wool, silk and leather.
The experiment can also be repeated by increasing the layers of fabric and comparing the results.
Clothing - http://en.wikipedia.org/wiki/Clothing
12 tips to help you choose warn clothes - http://www.websiterepairguy.com/articles/household_tips/stay_warm.html
Thinsulate - http://en.wikipedia.org/wiki/Thinsulate
Handle hot water carefully. Adult assistance is required.