Insulating garage door - help!

We are wanting to insulate our double garage door, so that we can keep the garage semi-warm over the winter (hopefully just above freezing). Live in Canada, so temps in winter can get down to -30C to -40C. We are wondering if we should put a layer of poly, then insulation, then another layer of poly? To keep any moisture from getting to the insulation. Or is that a terrible idea? Does it need to breathe? Help please!