Thermodynamically, it's rather marginal where you place the foam insulation. With one on top and one on bottom, the only heat loss out of the BA Clearview will be through the sidewalls which is a rather small surface area to lose heat out of.

Both on top, you'll lose from the sides and the bottom, however the temp of air in the clearview is going to be much lower (because it's more insulated from your body) which results in little, if any, difference from the above.

Ultimately, if heat loss through the sidewalls were a non-issue. It would make absolutely zero difference where you placed the foam pads. Total heat flux depends on total delta T and total thermal resistance (order of resistors makes no difference). However, the sidewall issues muddy this a little, but not much as their total surface area is relatively low.

All this talk of self-inflatables being heavenly is making me want the Thermo 6. I would actually save two ounces going with the Thermo over my REI Lite-Core (18oz). REI dividend checks are coming...

The 1/8" Thinlight weighs 2oz full sized. What if you cut that in half, as well as trimmed the edges down to 16" or so (to cover where your body is actually putting a lot of pressure on the pad). You could put one on top of the pad and one on the bottom. The 1/8" with the sides trimmed could maybe get down to 1.6oz?

Then bring a slab of 1/4" Thinlight - about 2oz worth, to put under your legs and feet.

I've tried various things like this with my Lite-Core, and the only thing I don't like is bringing all these foam pads, cut at different lengths, arranging them, sometimes re-arranging them in the night because they have shifted, etc. You get the picture. It feels simpler and nicer to have one pad, and one slab of Thinlight.

Just my two cents.

I hear you about the hassle of multiple foam pads. I remember snow camping once with two Torsolights, and I woke up every hour or so with a cold bottom whenever the two pads slid apart from one another. This still occasionally happens when I use a thinlight folded up for my legs.

I have, however, found that a foam pad stays put if placed on an inflatable. In fact, putting the Thinlight under my inflatables really keeps them in place, and acts as a sort of glue adhering the inflatable to the groundsheet. Placing the thinlight above the pad also sticks remarkably well; I can shift from my back to my side without movement, as the friction between them really keeps things in place. I really like how grippy too GG foam is! But it seems to be much more grippy with the full weight of my torso, while it still slides around under my legs (or maybe I just kick a lot in my sleep!)

I think I would try to find away to attach the 1/8" and 1/4" pieces so that it makes one long full-length pad, to which I place the torso pad on top. Of course, if I were truly thinking about a viable option if the pad bursts, then I should be thinking about carrying a full-length 1/4" pad. But that sounds so bulky! At least the inflatable would pack smaller without the insulation.

Something else I just realized--the slightest Clearview pad is 11oz, but it's 60" long! So I could get a longer pad (aka more comfort) for lighter weight than my 47" pads (including the Thermo 2/3). Since the GG pads are all 60", these would pair quire well. Now it's just making the decision between two 1/8" Thinlights or one 1/4" Thinlight. one advantage of two 1/8" pads: they would pack easier; two rolled up side by side could fit in a side pocket. I've never liked wrapping my pads around my gear in my bag, because then I can't pull the pad out for my lunch break without completely unpacking my bag!

I usually don't need a pad for my feet anyway, as I just double up my socks (I bring spares anyway), which adds an equivalent amount of insulation. And if it does get cold, it's not to hard to curl up a bit on a 60" pad.

I also just noticed that the non-insulated Ether Thermo 2/3 is 3oz lighter than the insulated version. If I decide the Clearview is not sturdy enough, I could compromise with the non-insulated Ether and still save a few ounces. Still, 60" and even lighter makes the Clearview quite tempting!

Edited by jcarter1 on 02/06/2008 10:27:55 MST.

> Thermodynamically, it's rather marginal where you place the foam insulation.

Hmmm... I haven't tried sandwiching an air mattress between two foam pads, but I have tried it with foam on the bottom and other times with the foam on the top. It subjectively seemed like the foam on the top was warmer. Maybe because the foam insulation results in a lower temp delta, so convection is slower? Any physicists in the house? :)

As far as the thinlight 1/8 or 1/4. Yes, 2 1/8 is more packable than the 1/4... but I found the 1/4 more durable. It's possible to accidentally tear the 1/8. Hasn't happened with the 1/4. The other issue with the 1/8 is that since it has almost no rigidity, that when it's on top of something slippery, and you are moving on top of it, it can end up getting scrunched down leaving cold spots. I experienced this in a bad way when I tried to use one in a hammock.

--mark

Nice points Mark... with the foam on top, you're moving more of the total delta T / heat flux away from your body and located into the pad. Er... I'm not sure if I'm explaining that right... you felt cooler because the actual temp touching your body would have been lower, but net heat loss would have been roughly the same (we're talking where the heat generated is in the core of your body and the ultimate transfer of said heat to the ground).

Basically, the foam next to your body moved the primary delta t away from the surface of your body, so you didn't FEEL it as much.

The difference between 1/4" on top or 1/8" 'sandwich' shouldn't, theoretically, be noticeable (as again you're shifting the primary delta T away from the surface of your body). However, you make a really good point about the rigidity issues, that could cause a problem...

There's also probably a difference using an uninsulated air pad in a bivy versus out of a bivy/no bivy. Using cc foam on top or below still leaves the sides open to expel heat, but I would think a bivy would help trap and recycle that heat back into the sleep system. It would at least create a warm layer of air between the outside of the sides of the air pad and the outside of the bivy, reducing heat loss.

It would probably help even more to wrap one's sleeping quilt around the bottom of the pad; the bag would provide the insulation to the sides of the pad. This may even allow one to place the cc foam under the pad with no more heat loss than placed above the pad. It would take a little longer to warm up since you first have to heat the air in the pad, but it should already be warm from having just blown it up. Once warm, though, the air in the uninsulated air pad should be well insulated with the foam below and the quilt along the sides (and a bivy outside of this). The problem with this, of course, is the dead air space created by wrapping one's bag under the pad (versus around one's body).

I can confirm Mark's impressions that a 1/8" closed cell foam pad seems warmer when placed on top of a Big Agnes Insulated Air Core inflatable mattress. I experimented on a Mt Adams last June in sub freezing temps and much preferred the cc pad to be on top as well.

BA rates this pad as adequate down to 15 degrees. This seems a bit on the optimistic side, especially considering that the insulation is quite sparse in these mattresses.

The cc pad help a great deal eliminating the coolness of the sleeping surface which, in my Marmot Helium, is noticeable.

Edited by kirt on 02/14/2008 02:08:57 MST.

Yep, closed cell on top. I have no scientific reason for it other then it is warmer. If I use a MB 90 with a 3/8 CC pad underneath, I struggle below 0 degrees C, if I put the 3/8 CC pad on top, no problems all the way down to -20C... huge difference for me.
I thought it had more to do with spreading out the load applied on the inflatable mattress. If the inflatable is on top, things like hips/elbows/shoulders can compress the pad to near nothing and use only the closed cell underneath for insulation.
With the CC on top, your hip/etc. pressure is displaced over a larger area and therefore doesn't compress it as much.
...just my conspiracy theories...

>>I thought it had more to do with spreading out the load applied on the inflatable mattress. If the inflatable is on top, things like hips/elbows/shoulders can compress the pad to near nothing and use only the closed cell underneath for insulation.
With the CC on top, your hip/etc. pressure is displaced over a larger area and therefore doesn't compress it as much.
...just my conspiracy theories...


That was my impression of what was happening. I bring a 3/8" Thinlite with a Montbell 90 when I expect to be sleeping on snow, the 1/4" otherwise. Closed cell on top of open is definitely warmer. It also feels more comfortable for my hips, for the same reason you describe--the pressure is displaced over a larger area. That said, if there is any possibility of puncture, such as in desert areas, the closed cell goes on the bottom.

John,

Yes, it would as the air matress would be losing heat ot the temp on the inside of the bivy rather than the temp of outside air.

You'd be amazed the insulative properties of a single layer of fabric when it traps a film of air...