To answer the original question directly: No, the rating is not accurate. That’s a fact.
The EN system is simply a CLO measurement, which is referenced to sleeping data to give a limit and comfort rating. The Limit rating is where the average sized man will be at his comfort limit, sleeping in a tucked position. The Comfort rating (often incorrectly called the Women’s rating) is where an average sized woman will sleep comfortably in a relaxed position. The two numbers are usually separated by about 10 degrees F.
The reason that quilt will not be warm down to 20 degrees is simply because it does not insulate the head. And because it is not EN rated, the 20 degree rating is simply made up by the manufacturer. It cannot be used to compare to other bags, or even be used to give you a close estimate as to the temps you can use it. But under no circumstances will it keep you warm down to 20 degrees unless you somehow do something additional to insulate the head. The EN system exposes this huge deficiency in these types of products, which is likely why you never see an EN rating for them.
It is untrue that quilts cannot be tested by EN tests. I tested the Sierra Designs Backcountry quilt, which has an EN limit rating of 27 degrees. It achieves this because it has head insulation built in. If it had been a standard quilt, with no head insulation, its rating would be have been much, much higher.
Complicating all of this is that the EN system uses a pad with an R value of about 5.0. Most pads are significantly lower than this, so very rarely will you get the warmth you expect from ANY bag, because they are often paired with pads that dramatically reduce the thermal performance, especially with quilts.
If you really want something that will work down to 20 degrees, I would suggest the following:
- Assume the EE did their best in estimating the loft required for a 20 degree EN rating, and that the bag is filled to meet that rating (huge assumption, but you will fine enough evidence in these threads that I think it is safe).
- Pair it with a pad that has an R value of about 5 (Neo Air X-therm, for example)
- Pair it with some good head insulation, or wear it with your down puffy.
- Test it.
Following the above, I think there is a very good possibility that you will be warm enough down to 20 degrees, and comfortable down to a little below freezing. All of this assumes that you can keep that quilt tucked around you and on the mat, which is a huge problem for a lot of folks, so beware.
Good luck out there!!