Inflatable down insulated sleeping pad

[Colin Krusor]

This pad was an experiment. It weighs 15.85 ounces, it is 64" inches in length (for my 62" long girlfriend), and it contains seven ounces of 800fp down. When inflated, it is 3.75" thick over the middle five baffles and the outer two are 4.25" thick to prevent rolling off.

It is something between a "balloon bed" (http://www.balloonbed.com/) and a Kookabay pad. Like a balloon bed, it has a thin, elastic bladder inside a fabric envelope. Like a Kookabay pad, it is meant to be durable and the internal volume is contiguous (it has a single inflation valve).

To make the inner balloon I used a hobby iron to make seven tubes from a tough, stretchy heat-sealable urethane vacuum bagging film called Stretchlon (http://www.airtechonline.com/whatsnew/index2.htm). The film weighs about 1.2 oz/square yard. I then joined the tubes at one end and made holes for air to pass between them. I then nicked each tube, added one ounce of 800 fp down to each, and resealed them. The balloon is bigger than the fabric envelope, so the film is not being stretched when it is inflated. The fabric bears the tensile load; the film is just an internal air barrier. The valve is a 9mm kitesurfing kite inflation valve.

The fabric envelope is "coyote brown" Momentum 90 from thruhiker. The baffle walls are made from 0.93 oz/yard spinnaker and are 3 inches high. There is a hole for the valve to protrude, and openings (with snap closures) at each end for easy removal of the inner balloon. The balloon is loaded with down for shoulder season alpine and winter use, but it could be replaced in summer with an unfilled inner balloon.

I think, with a lighter fabric like SevenD, a 72" long, 3-4" thick pad with 6-8 ounces of down could come under 16 ounces using this design. I don't have any guess about the R-value, but I don't know of any other pad in this weight range that uses this amount of down. I'm pleased with the result and I expect my girlfriend to be much warmer than me (on my Synmat UL7) this winter.

Any ideas or feedback are welcome.

[William Zila]

that's AWESOME I am very intrigued

[Michael Pappas]

This is really interesting. I guess the advantage of this over just having the tubes inflated but not filled with down is that the down creates even more small areas to trap heat than if it was one large volume. How well does it pack down?

[Warren Greer]

Nice way to build a pad and great weight as well. All that down should just kill heat transferance. Can't wait to see where you go from here. Keep experimenting.

[Adrian MITCHELL]

sexytastic.

[Clint Hewitt]

Wow that is impressive!

Few questions

What was the total cost for you to make this?

What was the total time?

How wide is it?

Can we get a picture of it packed?

[j lan]

Incredible weight for that much down! o_O

Keeping lady friends warm is one of the most noble of all causes in the camping world!

[Steven Evans]

Colin, that thing is a work of art. Amazing.

Question: How does the air get from one tube to the other? It is tough to see from the pics, but it appears they are all seperate tubes yet you mention that it has one valve.

[Link .]

I then joined the tubes at one end and made holes for air to pass between them.

[Colin Krusor]

Clint, it would take a bit of calculation for me to estimate the total cost, because I had some of the materials on hand already (M90, spinnaker), and I bought more of the Stretchlon film than I needed. I think, if you made no mistakes, used low-cost fabric, and bought just enough material for the project, it could be done for $50-$70, I think.

The time investment was large. It only took about five hours to make the outer envelope from the raw fabric, but fabricating the balloon was a long process. I tried several balloon designs and I made quite a few mistakes. It would have also been much faster if I had an impulse sealer instead of a little hobby iron.

The width is about 24.5" flat and 19" inflated. The photo below shows the size all rolled up.

Steve, I joined the tubes at one end while they were all still open at that end. I made a circular "weld" between each pair of tubes by pulling the two layers of film down over the bottom of a teacup and heating the circular surface with the iron. Then, with a metal tube heated over a propane flame, I melted a hole in the center of each circular weld so air could pass between them. There is no screen to prevent down from flowing between them in small amounts during inflation and deflation. Then I sealed the tubes, tested them for airtightness, and added the down (by nicking and resealing).

[Franco Darioli]

What prevents down exiting the valve during deflation ?
Franco

[Colin Krusor]

Franco, I used contact cement to attatch a little lump of polyester fiberfill against the inside of the valve. I had tried a screen in the past, in one of the heat-sealable nylon (Kookabay-style) pads I made, but the down seemed to clog it pretty easily and deflation was very slow. The fiberfill wad seems to work better, but the valve itself has a very small aperture. I ordered a large "dump" valve to facilitate deflation and I think I'm going to install it on the other corner.

[Franco Darioli]

Ah, thanks for that.
Clogging those valves seems to be the major problem using down.
I think that Exped uses open cell foam
Franco

[Steven Evans]

Thanks Colin, I pride myself in thinking I read the posts carefully, but perhaps I was too excited! Your description painted a great picture for me and it all makes sense now. I have to say, that is a very well thought out design. Again, well done…I'm jealous :)

[Steven Evans]

One more question: Do you know what the inflatable tubes and the fabric envelope weigh separately? You probably know where I am going with this…cuben fiber envelope?!?!

Also, big thanks for sharing the fabric (stretchlon) source.

[Colin Krusor]

Steve, the weight came in very near the theoretical total given the parts (valve, snaps, etc.) and materials:

1. Momentum 90: one roughly 50"x65" piece = about 2.3 square yards (due to the rounded corners)= 2.53 ounces

2. Spinnaker (0.93 oz): Six 3.5" x 53" strips to make 3" high baffles = 0.78 square yards = 0.72 ounces

3. Urethane film (Stretchlon 200): five 11"x68" tubes and two 13"x68" tubes = 4.25 square yards of film = 5.1 ounces

So the total theoretical weight for the balloon and the fabric envelope together is about 8.35 ounces. The actual weight, according to my scale, is 8.85 ounces (due to the valve and the snaps).

So a version with an empty balloon (no down) for summer use would weigh less than nine ounces total (for the 3.75" thick, 64"x19" size). If you wanted a length of 72" and a thickness of 2.5", you could keep the total weight (unfilled) under nine ounces by using SevenD in the outer envelope instead of M90. With M90 it would be about 9.1 oz.

I actually considered making an outer envelope out of 0.51 oz cuben, but it would reduce the total weight of my down-filled version by less than one ounce (from 15.85 ounces to roughly 14.93). Most of the weight of the envelope is the Stretchlon film. The soft fabric surface of the M90 is much nicer to lay on than cuben, and it is probably more puncture and abrasion resistant, too. Also, from an aesthetic standpoint, the M90 rounds out nicely to make a very smooth, clean-looking inflated pad because it stretches a little bit. Because the cuben has no stretch, every bit of imprecision during cutting and assembly (no matter how small) would appear on the surface of the inflated pad as a crease or wrinkle, and the corners would be a crumply mess. For these reasons I decided that a cuben version didn't really make sense.

[Colin Krusor]

I put in a dump valve on the corner opposite the inflation valve, because deflation through the first valve was very slow. The placement of the dump valve seemed perfectly rational at the time.

It's clear now that this may become an annoyance. The junctions between the tubes, at the head end, have no screens or other means to prevent down movement between them. I just thought it wouldn't be necessary (most DAMs don't have any barriers between baffles). But the down is all flowing into one baffle because the inflation and deflation valves are on opposite corners. Inflation tends to encourage the down to drift away from the inflation valve (into the tubes nearer the deflation valve). Deflation, too, draws down out of the tubes nearest the inflation valve and causes it to accumulate around the deflation valve. After a few inflations and deflations, most of the down is now in the two tubes nearest the deflation valve.

The holes between the tubes are too small to effectively redistribute the down manually by pushing it around. I might have to occasionally fill and empty it a few times in reverse (in the deflate and out the inflate) to redistribute the down. If I make any similar pads in the future, I'll put the two valves close to eachother or put barriers between the baffles. Hopefully this observation will be helpful to others attempting MYOG DAMs.

[Steven Adeff]

now just sew a quilt onto the top and you've got an UL all in one deep winter sleeping solution!

[Todd Stough]

I'm curious why you need individual tubes instead of one large balloon? Since they are all connected the air flows between them now, with one balloon you wouldn't need to worry about down shifting. What am I missing?

[Michael Duke]

A balloon inflates until it pops. Separate tubes control height.

[Colin Krusor]

Todd, yes, as Michael says, having parallel tubes is necessary for basically the same reasons that baffles are necessary in a quilt or sleeping bag. Without them, some parts would flatten out while others bulged up. Try making a mattress-sized balloon out of painters plastic (or anything airtight) and laying on it. You'll be laying directly on the floor and the uncompressed parts of the balloon will bulge up around you.

[Ryan Smith]

"A balloon inflates until it pops. Separate tubes control height."

Wouldn't baffles control the height like a normal air mat?

Ryan

[Colin Krusor]

Ryan, I guess maybe I misunderstood your original question. I don't see how one could have one big balloon (instead of joined tubes) and still have baffle walls.

The baffle walls could be inside the balloon, but then the balloon bears the tensile load when inflated. This would require thicker film, which wouldn't need the fabric shell. This would result in a heavier pad, like a Big Agnes Clearview. The idea with my design is that a lighter pad can be achieved by using a very thin balloon film and light nylon fabric to bear the load.

Incidentally, Kookabay-style pads also use a urethane barrier film and load-bearing nylon fabric. My design is only lighter than the Kookabay design because of a supply problem. The lightest heat-sealable fabrics (that I know of) use 30d yarns. The balloon-in-shell design allows the fabric to be 20d or lighter. There is also an argument that the balloon-in-shell design might be more resistant to certain kinds of puncture risks (like thorns), because the tough, flexible balloon film can tent over a sharp point that comes through the fabric (the laminated film in a Kookabay-style pad can't tent), but I can also think of plausible arguments against this, and I don't have time to produce sacrificial prototypes to test it.

[Ryan Smith]

Ahhh, I see what you're saying Colin. Some of the large mfg'ers are using 20d nylon now which is probably in the 2.5oz sq/yd range. So your method should still be lighter overall.

Ryan

[Todd Stough]

I still don't understand why a single air bladder inside the nylon envelope won't work. I'm picturing my platypus water bladder. If you fill it partially and apply even pressure on it it flattens out nice. Why wouldn't it do the same thing if filled with air and down? The shell would still prevent the balloon from over stretching. Has anyone tried it to know it won't work?

I suppose what I don't get is why it is any different from multiple linked tubes, there is nothing preventing the air from moving around now.

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