I am thinking of modifying one of Big Agnes’ AXL Air Pads (by cutting it down say to 5’4″) and adding in approximately 4 ounces of Hyper Dry Down. This should give me a pad which weighs around 360 grams is 4″ thick yet has an R-rating of around 6.0 – so suitable for say -20C. The Hyper-Dry down should be water resistant enough it should not matter if I blow it up directly by mouth.so long as I hang it up to dry from time to time with the valve pinned open in some arm sunlight eg next to my study window.
The down can be purchased eg from eg Ripstop By The Roll for US$25 (800 fill) to $38 (850 fill) per three ounces (plus delivery). The AXL Air pad starts at US$139.95 (July 2018). Note it also comes in a 25″ width configuration. In this dimension you could have a down filled pad that you can really luxuriate on at under 480 grams.
I do love my AXL Air pad – it is the most comfortable night’s sleep i have ever had in the back country – but it is not quite warm enough for winter. I figure this mod will ‘winterise’ it.
PS: If I can wait for it I may be better using this new Climasmart insulation from Downlite (as they seem to have ‘solved’ the moisture retention problem: http://www.textileworld.com/textile-world/knitting-apparel/2018/06/downlite-introduces-new-climasmart-temperature-regulating-performance-fills/
PPS: As you can see (below) a reader (Belle) has alerted me to an Exped Synmat HL Winter M (http://www.exped.com/australia/en/product-category/mats/synmat-hl-winter-m which weighs 430 grams and has an R-rating of 5.0 at 183 cm long 52 wide (35 cm at the foot) and 9 thick. You do need a 60 gram bag to blow it up, but this also doubles as a pillow. This is an awesome pad.
She also has a point that some down would escape the AXL valve when you deflate it. I was wondering how to attach some mesh on the inside of the valve (possible, though a bit messy). Also technically I would only need 3 ounces of down to fill it (85 grams) and I could cut the pad down with a taper (such as the Synmat has) probably making the whole thing considerably lighter, possibly even less than 315 grams if you are short like me and can fit on a 5′ mat. I still think this might be an interesting option especially for a dedicated gram counter who already owns the AXL Air pad.
The Synmat is available from Summit Gear for A$ 219 (July 2018):