Space blankets have a mylar coating which reflects heat. Wouldn't making it breathable make it less warm? It would seem like the heat would just escape instead of being refleced and trapped in by the mylar. But I honestly don't know a lot about this subject.
I don't know how well mylar really keeps you warm as a blanket, but if you have ever put a space blanket on the opposite side of a fire with a tarp shelter to your back then you will know.... it's like being in an oven.