To my knowledge, only Therm-a-Rest and Sierra Designs properly rate the temperature ranges of their quilts using the EN standard. Originally designed for sleeping bags, the standard test assumes that there is a hood involved, which quilts obviously do not have. TaR and SD circumvent this problem using the simplest approach possible: just throw a hood on the thermal mannequin and be done with it.
In a previous post on buying a winter quilt, I used data from TaR and SD to put together an equation that could estimate the EN comfort rating of any quilt given the amount of “fluff”, or the product of fill power and ounces of down. This assumed that the quilts were all more or less the same size, but now that I’m looking into half-quilts I need something a bit more robust. In order to make the equation size-agnostic we must swap fluff for fluff-per-area. Fortunately, almost all quilt manufacturers specify the length, shoulder width, and footbox circumference of their various models. If we convert the footbox circumference into a diameter we can then calculate the “area” of the quilt as if it were a trapezoid:
area = length × (shoulder width + (footbox circumference ÷ π)) ÷ 2
After incorporating this year’s latest offerings from Therm-a-Rest, bringing the total number of data points to twelve, we now have the following chart:
This yields the following equation:
EN comfort ℉ = -7.32 × fluff-per-square-inch + 64
The R2 value is 0.892, meaning that about 90% of the quilt’s temperature rating is explained by this equation. It isn’t the 98% slam-dunk that we had before but that’s to be expected since we’ve gone from 5 to 12 data points. In any case, this should allow us to estimate the EN comfort rating of quilts of any size, or at the very least determine whether the manufacturer’s estimated rating is comfort or limit.
For the gory details, see my quilt spreadsheet.