There are many factors that contribute to the warmth of a sleep system. I believe the best way to know how all of these contributing factors come together is to seek feedback. To date, the feedback I have received on my sleep systems has been mostly “comfort rated for the average person”. This means that the average sleeper, who doesn’t tend to run warm or cold, will be comfortable at the temperature rating. This is the exact point I am looking for. I could aim to build systems that are comfort rated for the coldest sleeper, but then I would be forcing average sleepers to carry the extra weight of an overkill system. Instead, I suggest that the buyer adjust from an average baseline. If you run cold I suggest bump down 10 degrees. If you run really cold, bump down 20 degrees. Likewise for warm sleepers.
The cottage industry is made up of mostly small businesses and individuals building things by hand. This is really great for the UL community because this is where innovation happens, but these folks typically don’t have the means or access to having their items EN/ISO tested. Therefore it’s generally a matter of stuffing to a certain loft depth and adjusting based on use and feedback. The loft depth is often determined by a widely accepted chart that generally shows what loft depth correlates to what temperature rating.
To aid the cottage industry consumer I try to be as transparent as possible about my filling process as I can and provide every useful metric that I can.
Remember that an appropriate insulation level for you is likely available through any manufacturer or hand builder, it’s just a matter of what they call it. For instance I may call 3″ of calculated loft “comfort rated for an average sleeper at 20f degrees” but you may know that you need 3.5″ or a certain fill amount. In this case, all you need to do is ignore that rating and find the metric you want.
This can create a bit of confusion in determining if you are an cold/average/warm sleeper. Your individual perception of this will depend on your experience. Since they are not held to an ultralight weight standard, mass market items are more likely to be rated to keep the coldest person warm at the rated temperature. If your experience is mostly mass market, you may not know if you are a cold sleeper and you may be used to a system that is overkill for an average sleeper. The cottage industry mainly builds based on the general loft chart. I believe that if this loft chart was followed exactly it would produce items that would be EN/ISO rated more towards lower limit/survival/extreme. If your experience is mainly based on cottage industry products, you may be much more used to systems that are a little cold.