The relative warmth between garment types within the same family (down or synthetic) can be easily approximated. This assumes the typical case where the garments all use the same fabric and insulation areal weights (for example 100g/m2 PL1 and 15 denier ballistic nylon). The average body surface area for a parka is 52%, a jacket is 48%, and a vest is 36%. A vest will keep you 75% as warm as the comparable jacket, because 36 /48 = 75%, and compared to the parka it will keep you 69% as warm. A vest is generally the most efficient insulating item you can add to your clothing. To illustrate this BSA phenomenon, as discussed above, the Montbell Thermawrap synthetic vest provides 75% of the total insulation that the jacket provides yet, the ratio of garment weights is only 63%. This phenomenon holds for all insulation types.