After doing some research, I haven't found any milk bottle insulation which has good reviews. My guess is that these were not meant for long period of insulation. Since I have the Closer to Nature bottles, I decided to find out if the kept-warmth period of this product is sufficient for our use. After doing various combinations of different temperature of hot water and checking the time when the water got too cold for making milk, I found that (as a rough guide) if you boil the water, allow about 15 minutes for the the water to cool before you pour into the milk bottle, then put this bottle into the insulated bag, this water will still be warm enough for making baby milk after (roughly & maximum) 4 hours. Obviously if you want to use it before 4 hours, you will have to either 1) cool the water for longer before you put it into the insulated bag or 2) cool the milk bottle when you took it out of the insulated bag, before you make the milk. If you want the water to keep hot for longer than 4 hours, I think its best to use thermal flask.