Brady
May I ask exactly what you intend to pad? As far as shoulder straps, using a piece of foam for insulation would weigh how much? an ounce, maybe two. Consider the extra design and construction work making a pack with a slot and or zippered shoulder strap that you can stuff a rag into? (I suppose you could carry food in your shoulder straps). What else? The waist band. Do you want a comfortable waist band that fits, or do you want a big lumpy waist band that completely disappears when you have to wear those clothes? A very high quality fully padded large waist band weighs 10 oz. You can probably get an ok one for light camping that weighs between 2 and 6 oz. So we are talking about a 4 to 10 ounce savings - maybe - for using clothing items to pad your pack.

One thing you can do is use unpadded shoulder straps and put your folded socks under them, and if you're going under say 15-20 pounds, you don't need a padded waist band anyway, but every time you stop the socks fall out.

Why do you suppose that other UL members of this group do not use their clothes to pad their shoulders or waist? But most of us put soft folded items next to our backs. Do not flaunt conventional wisdom completely when designing, without understanding why other people, most other people, do it differently. If your idea was new and not one already sort of rejected, then you might say that you are inovative, however yours is not an innovative idea, no offense. smile
Jim
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These are my own opinions based on wisdom earned through many wrong decisions. Your mileage may vary.