Not totally. The heatsheet is just a sheet; the heatsheet bivy is the same stuff as bivy. I carry a hs bivy and a tarp in my ditch kit, one probably will keep you warm, the other will probably keep the wind and rain at bay. You could make a bivy into a sheet, but not the other way around, not with much success; besides, why pay extra for the bivy if you plan on slitting anyway? :P
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-IronRaven
When a man dare not speak without malice for fear of giving insult, that is when truth starts to die. Truth is the truest freedom.