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Neoair vs. Space Blanket


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  • #1603030
    Andrew Dolman
    Spectator

    @andydolman

    So did the prototype without the radiant barrier still have a non-radiant barrier suspended between the two layers of baffles? If so I stand corrected, and surprised, at the 25%.

    #1603111
    bill smith
    Member

    @speedemon105

    I think alot of people here are using anecdotal evidence and dismissing space blankets altogether. While they are not ideal, and they don't stop other forms of heat loss, the amount of heat they can conserve is not insignificant. Depending on the difference between skin temperature and the outside temperature, you can be losing several hundred watts of energy through IR radiation.

    http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/bodrad.html
    Play around with it some.

    #1603162
    James Klein
    BPL Member

    @jnklein21

    Locale: Southeast

    Actually, most of the anecdotal evidence I have noticed seems to support space blankets. Most of the misinformation I have seen comes from inappropraite application of theory (ie it appears to be "knowledge" based not experience based).
    I few points for those still following this:
    >As Bill's link shows – heat loss via radiation xfer IS significant, even at 3season temperatures.
    >Space blankets don't need an air gap to be effective at blocking IR radiation. Distance btw surfaces has NO effect on radiation heat xfer.
    >Assigning an eqivalent R-value to a space blanket is almost useless as it will only apply to the temperatures it was tested at — the same applies to assigning a % of insulating value.

    #1603324
    Roger Caffin
    BPL Member

    @rcaffin

    Locale: Wollemi & Kosciusko NPs, Europe

    Guys

    We have a review of the Neo-Air on the site. If you go through it you will find out more about the use of radiant barriers. They do something, but under many conditions 'not very much'.

    A Space Blanket does most of its 'good' by limiting wind chill and stopping evaporative loss. The radiant barrier in it is often rather irrelevant. Try using a very large trash bag instead.

    Cheers

    #1603341
    James Klein
    BPL Member

    @jnklein21

    Locale: Southeast

    Roger, for some reason I've never notived that acticle. Pretty good review though I would like to know where you guys came up with the radiation claims.

    >>"The amount of heat radiated from the body is small compared to typical convection losses, especially at colder temperatures"
    –while I could come up with some scenarios where this is true, I don't think it is typical. Also, the colder it gets the more significant IR heat xfer becomes relative to other modes of heat loss (its a fcn of T^4).

    >>"…and the pad only intercepts a fraction of this radiated heat"
    –A small fraction is "intercepted" (ie absorbed) and a larger fraction would be reflected back to you, though I don't think thats what you meant when you used intercepted.

    >>"any intervening clothing and fabric … will already block some of this infrared radiation"
    –This is true so long as "some" is emphasized. Most all fabric is nearly black to infrared radiation (emissivity ~ .8-.9). So only a small portion of you heat is reflected back by these

    >>"Third, placing a radiation shield within the pad will only halve the amount of radiative heat loss"
    I guess what you mean here is only half b/c its only below you?. If so that similiar to saying the down in a DAM is gimmicky b/c it only blocks some of the convection losses.

    Finally in responce to your post in this thread:
    Space blankets do help with wind chill and evaporative losses but they (unlike a garbage bag) will also significantly reduce radiative heat transfer. In MANY conditions this can be the a very significant heat loss mechanism.

    looking forward to discussion,
    James

    #1603368
    bill smith
    Member

    @speedemon105

    "A Space Blanket does most of its 'good' by limiting wind chill and stopping evaporative loss. The radiant barrier in it is often rather irrelevant. Try using a very large trash bag instead."

    Why use a trash bag instead, when a space blanket will do more? Im sorry, but I don't see stopping the loss of hundreds of watts of heat as irrelevant.
    Your attitude is my point exactly. Because it isn't a do-all insulation, it should be discarded as worthless.

    #1603397
    Jason Elsworth
    Spectator

    @jephoto

    Locale: New Zealand

    Almost 30 years ago now I had a Mountain Equipment synthetic bag with a silver radiant barrier in it. The concept didn't seem to catch on widely, but I did find this online http://www.livefortheoutdoors.com/Gear-Reviews/Search-Results/Sleeping-bags/Mountain-Equipment-Mithril-II-/. They claim it adds 3 degrees to the bag.

    Going back to pad R values and the Neoair. Eddy Meechan wrote an article in TGO magazine a few months ago where he suggested that the Neoair's R value would to some extent be dependent on the amount of air you have in it. The more air the higher the R value. Most people seem to use their Neoairs some way from fully inflated. Whereas the cynic in me says that the lab test to determine the Neoairs R value may have been done at full inflation :).

    In his article he also points out that there is no standardised R value test that pad makers use, so comapring the R values between the pads of different makers may be a bit suspect. He also argues that R values in the lab and those in the field may vary cosniderably. He concludes that R values are probably of most use in comparing mats of the same construction type by the same manufacture.

    #1603428
    George Geist
    BPL Member

    @geist

    Locale: Smoky Mountains

    > Depending on the difference between skin temperature
    > and the outside temperature, you can be losing several
    > hundred watts of energy through IR radiation.

    Indeed a Space Blanket's IR barrier can cut a significant loss of heat, especially when placed over the body between your 98 degree body and the near absolute zero degree night sky. But even in less extreme temperature differences the T^4 in the IR radiation loss equation can account for hundreds of watts lost just as Bill says.

    #1603435
    Stuart Allie
    Member

    @stuart-allie

    Locale: Australia

    Um, "the near absolute zero degree night sky?" I don't think so. If there was no atmosphere between you and the sky, you'd be right, but then you'd have bigger issues :)

    Don't forget that the atmosphere is also radiating in all directions based on its temperature. It's the difference between your body temperature and the ambient air temp that matters; the temperature of outer space really doesn't come into it.

    #1603440
    Franco Darioli
    Spectator

    @franco

    Locale: Gauche, CU.

    "Going back to pad R values and the Neoair. Eddy Meechan wrote an article in TGO magazine a few months ago where he suggested that the Neoair's R value would to some extent be dependent on the amount of air you have in it"

    Oddly the review at Backpackinglight (….) came to the same conclusion.
    http://www.backpackinglight.com/cgi-bin/backpackinglight/thermarest_neoair_review.html
    Franco

    #1603443
    Jason Elsworth
    Spectator

    @jephoto

    Locale: New Zealand

    Oddly the review at Backpackinglight (….) came to the same conclusion.

    This could go some way to explaining why some users seem to sleep very cold on a Neoair and some seem to do fine? Obviously many other factors will come into play. However, I would suggest that the amount of air in the mat may have less of an impact in the traditional foam and air pads than with the air only pads like the Neoair.

    #1603446
    Franco Darioli
    Spectator

    @franco

    Locale: Gauche, CU.

    My impression is that "perception" comes very much into it.
    For example I have a mate that happens to be very experienced in tenting in cold weather,( and I mean -30-40f air temperature), yet he firmly believes that sleeping naked he is warmer than wearing a base layer(not a compression layer).
    Now since he is "sure" of that, it works for him, does not work for me though.
    Franco

    #1603452
    Stephen Klassen
    Member

    @steveyk

    Hi Adan,

    My experience with sleeping on snow (air temps from +12C during an inversion down to -23C) is as follows:

    R3.5 left me cold. Not at the beginning of the evening, but early in the morning when metabolism slows. (Montbell pad or Prolite 3 with GG 3/8" thinlight.)

    R5.3 and I slept warm the full night. (Thermarest Toughskin).

    My bag was a Marmot Aiguille (O F degree rating) and I used either a sylnylon VBL from FF, or a WM Hotsac (which has a reflective coating on the inside).

    With regards to reflective coatings, I found that I could not use the WM Hotsac – too much moisture buildup in the VBL. I have used my FF silnylon VBL extensively and never had such moisture buildup. I only use the VBL at below freezing temps (except one time when I used the Hotsac as an emergency blanket in the low 40's F- very warm, and very wet).

    When we snowcamp, we always lay a reflective space blanket under the tent. It only covers about 2/3 the floor area, and we always know exactly where it has been once we pack everything up – the compacted snow outside the area of the space blanket is very icy, while the snow that was under the space blanket is merely compacted, not icy. Needless to say, I always try to get in the middle of the (3-person) tent.

    Anyay, back to conduction and radiation. I was getting cold feet (literally) and decided to augment my booties. They came with 3/8" of closed cell foam (approx R1.3?). I added cutouts that were about 5/8" (additional R1.7ish? for total of R3ish?). Feet were no longer cold.

    Then, one trip my skiing partner had cold feet. His booties had seen extensive use, and the synthetic insulation had long since collapsed. He didn't have the extra layer of foam. So we started to swap booty parts. We tried my lofty booties with and without the extra foam, and compared them to his saggy booties with and without the extra foam. What we found was that his saggy booty with extra foam was warmer than my lofty booty without the extra foam, and we really couldn't tell the difference between the booties if they both had extra foam.

    To make a long story short, after that night I decided to ditch the R3.5 pad/-10F bag system and went with the R5.3pad/+20F synthetic bag and clothes for winter camping. And now I don't wake up cold in the early hours.

    #1603453
    Andrew Dolman
    Spectator

    @andydolman

    #1603469
    Roger Caffin
    BPL Member

    @rcaffin

    Locale: Wollemi & Kosciusko NPs, Europe

    > between your 98 degree body and the near absolute zero degree night sky.
    Sorry, but that is wrong.
    The night sky on a nice clear night is about -70 C from memory. Stick any high-level haze in the way and the temperature climbs.

    > in less extreme temperature differences the T^4 in the IR radiation loss equation
    > can account for hundreds of watts lost
    Bit hard to imagine, since a resting body puts out about 60 W from memory.

    Cheers

    #1603478
    Andrew Dolman
    Spectator

    @andydolman

    When the surrounding temperature is about 20C, the average sized human body loses about 140w net in radiated heat. At 0c that rises to about 320w, at -10C 395w. However, this assumes that you are lying there naked. Ordinary clothing is itself a good radiant barrier, add the 2 layers of Pertex in a sleeping bag + whatever down might be between them and your radiant heat loss gets a lot smaller. It must, otherwise even at 20c we would be feeling very cold given that at rest we only generate about 60-90w of heat.

    I'm surprise that Cascade Designs found that adding a radiant barrier layer boosted the measured R value of their pad by 25%. I'd like to see details of the experiment. I have a suspicion that the non-radiant barrier mat was lacking any barrier at all where the radiant one is – and therefore also had less baffling.

    #1603499
    Rog Tallbloke
    BPL Member

    @tallbloke

    Locale: DON'T LOOK DOWN!!

    "I have a suspicion that the non-radiant barrier mat was lacking any barrier at all where the radiant one is – and therefore also had less baffling."

    I've had three attempts at making sense of this sentence. It gets more baffling each time. :o)

    #1603540
    James Klein
    BPL Member

    @jnklein21

    Locale: Southeast

    Ordinary clothing is NOT a good radiant barrier.
    Typical material we might encounter will, in the IR spectrum, have emissivity: e~=.9. This tells you that about 90% of IR radiant energy that strikes it is absorbed (10% is reflected). Also, once steady state has been reached, the material will emit 90% of what it absorbed back out.

    Consider an example where I radiate heat thru multiple thin layers:
    e*100% of the IR I radiate to layer1 is absorbed by it, of that e*(e*100%) is radiated back out by layer 1 – split btw me and the layer2. The amount of IR radiated from me that makes it to layer2 is: (e/2)^2 *100% (actually it is slightly lower since the outward surface temp of layer 1 is lower than the inner surface). This continues on such that of the IR I emit: (e/2)^n *100% makes it thru the last layer (n=#of layers).
    With e=.9 and and 4 layers it would be ~4%. This is the % that makes from my skin past all layers.. It is important to note that the same process happens at each layer: layer1 emits it own IR radiation to me and to layer2, some of which makes it to layer 3 … such that ~9% of layer1's outward emitted IR makes it out past layer4. For layer2 ~20% makes it out, layer3 ~45% and of couse layer4 is 100%.

    #1603745
    Stuart Allie
    Member

    @stuart-allie

    Locale: Australia

    "With e=.9 and and 4 layers it would be ~4%. This is the % that makes from my skin past all layers.."

    So if only 4% of the IR from your body makes it out, doesn't that mean that with 4 layers you have a really *good* radiant barrier? Even two layers allows only 20% to escape.

    So that doesn't leave much for the space blanket to do…

    #1603962
    James Klein
    BPL Member

    @jnklein21

    Locale: Southeast

    Stuart, sorry, I guess I did a poor job of describing that…
    I only provided numbers for one scenario — four layers of clothing. I was not describing the "barrier" effect of wearin 1 layer or 2 layers or 3 layers….
    In the case of 4 layers: there are five sources of theraml radiation (myself and four layers). Yeah the four layers "bar" my releases pretty well but each layer is also emitting its own IR radiation and each successive layer's emission is more effective at exiting than the previous layer's emission. Another factor to keep in mind is: as you move outward through layers the temperature is decreasing so each layer emits less than the previous layer. Clothing is not effective as an IR barrier b/c it emits IR.
    An attempt of summary:
    1)There are 5 emitters(me, layer1, layer2, layer3, layer4)
    2)Each successive emitters radiates less total thermal energy.
    3)Each successive emitter is more effective at getting it emissions out
    4)Total IR radiated from me and my layers would be the sum of each layer's contribution.
    I hope this helps, I am really not that good at explaining things,
    James

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