These days, almost every sleeping bag sold is given a thermal insulation numeric "rating", either the USA version or the more accurate European version. But vintage bags did not have this industry standard.
Is there any way to get a good idea of the insulating ability of a vintage bag other than getting your body inside it for a night of sleep?
The main problem with any ratings is that the insulation degrades pretty quickly, at least the synthetic one. Down holds its insulation properties longer, but it’s not perfect either. So the only reliable way to find out the quality of an older bag is spending a night in it.
The primary rating that should be on the bag is the pounds of insulation.
It is a somewhat imperfect measure, but a summer bag might well be 2 or 2.5 pounds. A three-season might be 4, but that would not likely be enough for actual outdoor winter camping - but OK for a semi-heated cabin space.
An older full-winter bag might be six pounds or more, with associated both heavier bag and fuller/fluffier/thicker walls.
