"Mylar blankets, which are made of aluminum and plastic, are extremely warm but are not vapor-permeable, causing moisture to accumulate on a person's skin."
Mylar may reflect some heat back, but the loss of heat from the system often negates this. Insulation is still needed.
You can see this yourself - go out on a chilly day in shorts / t-shirt, wrap up in a mylar blanket, and you will be cold.
Columbia has been adding something similar to their clothing, from baselayers to outerwear:
http://www.columbia.com/technology-omniheatreflective/
The idea being, the silver dots will reflect heat, but the gap between them will allow moisture management. I'm not sure how well it works in all scenarios but I'm sure I couldn't turn my heat off in the home and be comfortable with an omni-heat baselayer.
As for the Ag nanowires, i would worry these would become airborn and inhaled as the fabric breaks down from washing, use, etc.