Fill Density

Dave,

For garments and sleeping bags the rule of thumb is 10-20% overfill. In a sleeping pad you do not have to rely on the down to loft the shell material. This means you can get away with very little down; it will still fully loft and provide useful insulation value. The amount of down used versus R value is roughly linear until you start getting into extreme over stuffing. For a ballpark figure using a standard 72×20 pad I would give 5oz of down an R value of 5 and so on. I say ballpark because design and many other factors do play a role.

I know it can be a tricky topic to research because most of the available information is related to garments, sleeping bags and quilts. I would love to see lab testing done specifically for down mats.

Good luck on the project Dave. Keep us updated and be sure to post some pictures!