Unlike the older foil space blankets, the Heatsheets polyethylene ones are not noisy, and are flexible enough to stretch before breaking. The large model is 5 by 8 feet and weighs about 1.05 ounces per square yard. Cost around $6. Breathable? I had in mind making thousands of little tiny punctures with a meat tenderizer stamp, or you could make a cheap stamp with sharp tacks.
Warmlite sells an aluminized 1.4 oz silnylon, but being sil-coated, is not breathable. Very expensive, around the price of Cuben.
Would still like to know if another oz/sq/yd of Primaloft or the like wouldn't provide more insulative value. But I digress.