Feb 14, 2005 at 12:24 pm #1215895
Michael KirbyBPL Member
@strider518Locale: Whatcom County
Anyone have any experience with Big Agnes mummy style pads? I am trying to decide between the insulated and non-insulated air-core. The coldest temps I expect to encouter are about 30 degrees F. I am more interested in comfort than weight. Thanks.Feb 14, 2005 at 12:54 pm #1335712
Douglas DurhamBPL Member
The standard Air Core (without insulation) is Ok but a bit chilly at 35 F. It provides no real insulation. Good comfort over 45 F. I am an average sleeper and use a bivy and the Big Agnes Zirkel (rated to 20).
The insulated Air Core is warm in the 35-40 range, may be lower, but I have not used it below 35. Since it does not weigh all that much more than the non-fill version, if you are just buying one pad, get the insulated one.
I prefer the self-inflating mats below 35. Even though Air Core uses synthetic fill, blowing in moisture when the temps are 35 or lower creates a chance of freezing clumps of fill.
See this link for a review of pads and temps using a hammock and a Zirkel. The comfortable temp ranges if you are sleeping on the ground are 5-10 degrees lower.Feb 18, 2005 at 2:15 pm #1335794
@dking1005Locale: Olympic Peninsula
I have a rectangular Air-Core that I’ve used by itself down to about 35 degrees. It seemed to be slightly cool at that temperature, but I slept OK. I’ve used it in the snow in combination with a 3/4 length ridge rest and it was very warm and comfortable.
Another alternative is the Exped foam air mattress combination. A bit heavier and MUCH bulkier but comfortable with a very sticky surface you don’t slide off of.May 6, 2005 at 5:45 pm #1337138
Hey, can anyone comment on the Big Agnes Insulated Air Core vs. the Pacific Outdoor Equipment Insul Mat Max-Thermo?May 16, 2005 at 9:38 pm #1337308
Just picked up a Big Agnes Insulated Air Core pad. This pad has a nicer construction than the POE Insulmat Max-Thermo. I’m concerned about the slipperyness of the fabric, especially on sloping sites or on a silnylon floor. One idea to mitigate this is to use some SeamGrip polyurethane to make anti-slip lines or dots. I’ve done this with Thermarests before.
Can anyone comment on my concerns or my solution?May 17, 2005 at 4:43 am #1337309
Jim ColtenBPL Member
I don’t know about PU sealer as an anti-slip on the big agnes mattress but here’s an idea for taking the slip out of a sylnylon floor.May 17, 2005 at 12:52 pm #1337314
The BA Encampment sleeping bag is conservatively rated as a 20 degree top bag without any insulation on the bottom. I measured the top loft at 2 ½ inches of Polarguard Delta. I was not wearing any clothing in the bag and the bag was in a tent. I previously used this same bag with various inflatable sleeping pads in the same environment without any cold discomfort in 40 degree low temperature. I was unable to sleep without shivering using the BA Insulated Air Core both times that I tried it.
After this hypothermic experience I decided to measure all of my different sleeping pad’s thermal resistances to see how the Insulated Air Core compared. I tested the pads with max inflation for optimal R values, using a controlled environment and computer based sensors. The insulated air core actually tested slightly warmer at ¾ inflation R value 1.8 versus R value 1.5 at max inflation. The Blue Closed Cell foam pad was tested by me in the controlled environment but wasn’t one of the pads actually I sleep on in 40 degree weather. Every minute my test system measured and recorded the ambient temperature, the isotherm heat source temperature, and the heat that passed through the mattresses.
Sleeping Pad Vendor R Real R Min Temp
Blue Closed Cell Foam (3/8) 1.36 1.36 75
BA Insulated Air Core (2.5”) 15 F 1.8 69
Insulmat Max-Lite (1”) 3-4 3.5 50
Thermarest LE (2”) 4.1 4.2 43
Thermarest Standard (1 ½”) 5.2 5.2 32
There are a large number of conflicting standards for determining what R value equates to what lower temperature comfort. My Min Temp analysis is based on the EN 13537 Lower Limit standard which best matches my personal experience wearing no clothing when I sleep.May 17, 2005 at 6:00 pm #1337316
Wow Richard, I’m not sure I understood what you just said about ‘r’ values, but here is my experience. I own a BA mica bag(rated @ 20 degrees) and recently purchased an insulated air core pad. I like the comfort for my back, but I think I ‘sleep cold’ as I toss and turn frequently and although the temp at night in NH has been in the low 30s to mid 40s, I seem to need something for my torso by early morning. My legs and feet are fine. Is this the bag or pad?, I am not sure. I think it is the bag, but I will put a 1/4″ reflectix pad below the air core tonight and see what difference there is and report back. Overnight tonight should be high 30s. I have no experience with any other commercially produced pad but before the air core I used 2 pieces of 1/4″ reflectix as the insert in my BA Mica, I was just looking for more comfort on the ground. My tent is either a Eureka Spitfire or my new Tarptent Cloudburst, which I absolutely love.
Jim EllsMay 17, 2005 at 6:04 pm #1337317
To comment on Richard Sullivan’s concerns, I have never slid around with the air core in the sleeve insert of my BA mica bag. I think that is the point of having the sleeve, to keep you under the pad.May 17, 2005 at 6:09 pm #1337318
The amount of insulation needed under the body depends greatly on the surface you are sleeping on. If you are sleeping on dry leaves or pine needles, then you probably don’t need any pad or other insulation under you, even for below freezing temparatures. If you are sleeping on snow or there is freezing water running under your tent floor, then you need a very thick pad. Bare rock (a slab of granite, for example) is somewhere in between.
I routinely sleep in near freezing tempartures with nothing but a 1/8″ pad underneath me. The only times I remember my underside feeling cold is when cold rain water once ran under my groundsheet–that was very cold feeling.
(BTW my 1/8″ pad is a ridgerest. 90% of the pad is more than 1/8″ thick, but the hip and shoulder areas–the places that count–become severely compressed after about a week of usage.)May 17, 2005 at 10:28 pm #1337320
Jim Ellis said, “I will put a 1/4″ reflectix pad below the air core tonight and see what difference there is and report back.” Your body looses heat 3 ways simultaneously, 1)radiation, 2)convection, and 3) conduction. Reflectix is primarily designed to stop radiation loss if there is an air space on both sides. The air bubbles between the foil layers provides only a slight conduction/convection benefit. Use a 3/8″ or thicker foam pad under the Insulated Air Core, to reduce conductive heat loss, and you will find the problem is not your bag.May 17, 2005 at 10:31 pm #1337321
Frank is correct that the material you sleep on will make a difference. My experience is that damp ground is also a major factor. The water in the ground conducts heat away 24 times faster than the air spaces in dry dirt.
Frank is obviously better conditioned for the cold than I am. A sleeping man generates between 75 and 100 Watts of heat. For an average built man, this translates into a heat output per square meter of surface area of between 47 to 55 W/m2. However metabolic heat output is a very complex issue involving body fat index, age and sex. In general, young well nourished men produce more body heat than older men and much more than women. The sensation of cold is also linked to age and conditioning. Novices feel the cold more than experienced mountaineers. People who work outside feel more comfortable in cold environments than people who work in offices. After the age of 25, it is generally true to say that heavy people feel the cold less than slim people.
In summary, the relative insulation, based on R-values, is valid. The temperature that you will feel comfortable may not be.May 18, 2005 at 2:02 am #1337324
Excellent posts. Very informative. Always appreciate learning something.
My thanks to all,
pjMay 18, 2005 at 2:45 am #1337326
Mr. Nisley (or anyone else who feels qualified to speak somewhat authoritatively on this subject),
Please quantify, or qualify, the exact, or approx., percentages of heat loss due to the three aforementioned methods of heat transfer.
Since these percentages may vary depending upon conditions, particulary conduction (involving coeff. of heat xfr), please give an example of the conditions under which the percentage “breakdown” for the three methods of heat transfer apply. Also, please describe, in your opinion, the best ways, for the camper, to minimize heat energy loss for each of the three methods of heat xfr you prev. mentioned.
Since warm air rises above the sleeper’s body, and also since as the night progresses the outside air may end up being colder than the ground, after one’s body has been warming it for a while, how does this affect one’s use of insulating mat’ls? Please comment on both initially when the ground may be as cold as the air (but due to its composition has a diff. coeff of heat xfr than the air) & then later when it has been warmed and the delta-T b/t the sleeper’s body & the gnd had changed relative to the delta-T b/t the sleeper’s body & the air above & outside the sleeping bag?
Estimates, or even “guesstimates” would be helpful to at least illustrate how heat transfer occurs while one is sleeping.
Any thoughts about publishing, perhaps even as a purchasable/downloadable .PDF, a treatise on this subject on the BPL website? There is precedence for this method of distribution on the subject of “Stove Theory”. You’d have one sale that I know of!!!
I really appreciate your comments on this subject.May 18, 2005 at 6:21 am #1337331
Mr. Nisley, I did not mean to imply that reflectix was the solution, you may have inferred that in error. I was trying to respond to the previous posts, with my minor experience with the BA insulated air core pad, and address concerns over the efficacy of the pad to do its stated job, one of which is insulating, and to what degree one could expect results to be favorable. I’m sorry if I misled you.
I did use the reflectix in the sleeve under my air core pad last night. The low temp in SW NH was 38 degrees and I did not need to add clothing at all, so maybe the pad is really only good to the high 40’s or more. I have had chilly times by early morning on previous occasions when the temp was higher than 38, using just the air core pad. For what it is worth, I am 51, 5’7″, 170lbs. and normally toss and turn.
So, if the original poser of the question about how good the pad really works is satisfied, I will bow out of the discussion.
JimMay 18, 2005 at 8:10 am #1337336
Jim, please don’t bow out. Your personal experience is of significant value to me and I expect the others monitoring this forum.
We have both found similar results with the BA Insulated Air Core. It is an extremely comfortable and reasonably light weight pad but the 15 degree rating is not valid for the following class of users. Those of us: who are not as thermally conditioned as Frank (I am 62); those who use sleeping bags without insulation on the bottom or quilts; and those who prefer not to sleep in clothing; additional sleeping pad insulation is required in the 40’s — not just below 15 F.
Your Reflectix experiment was a valuable data point… it solved your current thermal problem. I think that if you replace the Reflectix with a 3/8″ blue foam pad you will have an even larger margin of thermal comfort for the same weight and bulk.
RichardMay 18, 2005 at 10:24 am #1337342
I want to preface this response by stating that I place more importance on “real world” experience such as reported by Jim, Frank, and others than I do on the theory. That said, I will attempt to provide a high level summary of Jim Ellis’ thermal sleep comfort problem.
When your body burns its food fuel, it is only about 25% efficient. The remaining 75% is released as heat. When sleeping, the average male generates at least 75 watts (metric) or 240 BTUs (English) of heat. To feel thermally comfortable you want to lose 240 BTUs of heat and no more or like Jim Ellis, you are going to be cold. To put this 240 BTUs number in perspective, when we are very active we can generate up to 8 times that amount of energy and waste heat. This is why you sleeping bag insulation has to be so much thicker than what you wear when you are hiking. Heat always moves from a hot object to a cold object. Our objective is figure out how to not loose any more than 240 BTUs of heat regardless of the conditions that we are camping in.
Lets look at Jim’s size, his BA Mica sleeping bag, and his BA Insulated Air Core pad in a 40 degree environment as an example.
His body produces at least 240 BTU/hour (English) or 75 watts (metric). Average (BMR) basal metabolism = 1 kcal/min * 60 min/hr = 60kcal/hour * 4 BTU/kcal = 240 BTU/hr.
His bag is a 20 degree BA Mica. US manufactures typically rate their bags using the ISO TR11079 standard. For a 20 degree rated bag, the top loft would have a .75 m2K/V (Metric) and an R value of 4.26 (English). His BA Insulated Air Core pad has .317 m2K/V (Metric) and an R value of 1.8 (English).
His body size results in an area of about 1.7 meters2 (Metric) or 18.29 ft2 (English). The top half of his body is insulated by the sleeping bag and the lower ½ his body is insulated only by the sleeping pad. He is sleeping in 40 degree weather.
Heat Loss in BTU/hour = Area in ft2 * (Tin-tout) / R value
Heat Loss in BTU/hour from the bag =9.15*(95-40)/4.26 = 118.12 BTU
Heat Loss in BTU/hour from the pad =9.15*(95-40)/1.8 = 279.56 BTU
Total heat loss by (convection/conduction/radiation) = 397.69 BTU
Total heat generation =240 BTU ( He will be cold and the cause will be the pad)
Now lets assume he put a high quality 3/8” foam pad (R=1.72) under his BA Insulated Air Core. The heat loss from the pad would now be calculated as
Heat Loss in BTU/hour from the pad =9.15*(95-40)/(1.8+1.72) = 142.95 BTU
Total heat loss by all mechanisms (convection/conduction/radiation) = 261.08 BTU. By adding a light shirt he would now be in comfortable thermal equilibrium with the 240 BTU he is generating and the 240 BTU he is loosing.
If Jim was a young and highly conditioned mountaineer his BMR could be as high as 100 watts (Metric) or 341.21 BTUs (English). By adding a light shirt he would have been comfortable with just the BA Insulated Air Core.
The above simplified BTU calculations incorporate all of the heat loss mechanisms, assuming you are sheltered from the wind. There are three mechanisms for heat loss which I will next discuss because Paul asked but it is not really important to understand why Jim Ellis was cold.
The activity level of the electrons in air becomes more active as they are heated. When the electrons change orbit they generate photons which is known as radiant heat. This type of heat transfer works like light does. If you can’t see light through your insulation, you probably won’t get radiation loss through it. In a sleeping bag that is more than ¾ to 1” thick, all of the radiation is absorbed by the fibers and heats the bag for you. If you don’t have ¾ to 1” of conventional insulation then an air gap and a foil material will reflect this type of heat back to you. This is the situation in which emergency space blankets are designed to address.
Convection is the actual movement of the heated air molecule because of wind or thermal buoyancy. The wind (forced convection) is primarily prevented by your bivy, tent, or tarp. Natural convection is the upward movement of heated air, its cooling, and its subsequent falling to create a current that moves heat. If you trap air in very small areas, then the viscous forces in air prevent the material from circulating. The sleeping bag insulation breaks up the air into tiny little pockets to minimize this circulation.
Conduction is one molecule vibrating with heat and then randomly bumping into an adjacent molecule to transfer its heat energy. Both the sleeping bag and the pad are designed to solve this heat loss. The pad is also designed to provide cushioning as a secondary consideration. Each material, including the air spaces in the insulation, has a known thermal resistance that is measured in a consistent fashion. Organizations such as NIST, and others, publish data bases of these tests for common materials.
It is prudent for a backpacker to provide a total insulation package (bag and pad) that will loose no more that 75 watts (Metric) or 240 BTUs (English) of heat in the worst case environment they will be in. Having the pad provide a margin of insulation is prudent, because any excess heat can be vented by the sleeping bag zipper.May 18, 2005 at 11:07 am #1337346
Michael KirbyBPL Member
@strider518Locale: Whatcom County
I appreciate all responders to my question regarding the Insulated Air Core Mattress. I now have one, have inflated it, examined it and wondered how such a thin layer of insulation can live up to expectations. I’d better test this mattress before I leave on a 6 day trip in the Eagle Cap Wilderness of Northeastern Oregon. I may end up modifying the bottom of my Horsethief bag to accomodate the oldstyle, full length ‘blue’ Thermorest. Now that’s a pad!May 18, 2005 at 5:30 pm #1337357
Now for your recommendation. I’m 53, 5’11 and weigh 170.
What pad or pad combo (e.g. BA Insulated Air Core + blue foam) would you recommend for routine use in conditions of 25 degrees plus (but usually 30 plus)?
I have a BA Insulated Air Core and have used it once in 50 plus conditions. I’m wondering whether to supplement it with a blue foam pad or go with the Thermarest (lighter weight version) in light of your R-value testing.
Thanks.May 18, 2005 at 10:30 pm #1337362
In my response I assumed no insulation added by the clothing that you wear to bed in 25 degree weather. That condition would also apply to garments such as silk weight long johns.
If your sleeping bag has no insulation in the bottom (like a BA) or you are using a quilt then you will need 1.13 m2K/V or an R value of 6.42 for a pad. If you are using a sleeping bag with significant compression resistance, such as Polarguard variant rated to at least 20 degrees, then you will only need .77 m2K/V or an R value of 4.37.
The BA Insulated air core provides all the cushioning you need and so I recommend using a closed cell pad augmentation as one option. For the case in which you have insulation under you, augment the BA Insulated Air Core pad with a closed cell foam pad of R value 2.6. This is achieved by using a 5/8” thick closed cell foam pad. For the case in which you have no insulation under you, augment the BA Insulated Air Core pad with a closed cell foam pad of R value 4.6. This is achieved with 1” of closed cell foam.
If you would rather just carry one pad, as an option, there are three Thermarest models ideally suited to the two scenarios. For the case where you have insulation under you consider a Thermarest Expedition pad at R value 4.4. For the case where you have no insulation under you, consider a Thermarest Standard at R value 5.8 (plus wear a fleece top to bed) or the Thermarest Luxury camp pad at R value 6.1.May 19, 2005 at 12:52 am #1337365
Many thanks for your explicit explanations. I’m guessing you have more knowledge to share. Look fwd to more if you are so inclined.
Again, many thanks,
pjMay 19, 2005 at 4:12 am #1337369
Richard, thanks for the insight. I had no idea I could be a ‘test subject’, but I certainly understand my situation and the overall dynamics of why we get cold. I have a Ridge rest pad that I use for the frame in my Gossamer Gear Mariposa, so I guess when I’m on the trail I’ll stick that under my torso. That will be the next test this weekend.
Keep typing lessons, we all need to learn this in order to enjoy our trips and be warm and safe.
Thanks, Jim EllsMay 20, 2005 at 9:10 pm #1337404
Hey anybody remember Olivia Newton John?
Let’s get technical, technical, I wanna get technical, technical…:))
Isn’t radiant heat loss a reality with darker coloured fabrics, due to the fact that the emissivity climbs towards 1.0 as the material becomes “blacker”? i.e Black materials are more transparent to infra-red than visible light is?
Based on this sketchy remembrance of thermal physics, I think that perhaps a mattress such as the BA IAC could indeed benefit from a little reflective mylar (space blanket or Reflectix), especially since it’s black.
Over to you Richard N.!May 21, 2005 at 5:43 am #1337409
Jim ColtenBPL Member
What your eye sees as darkness in a material is an indication of the material’s
albedo for visible light.
Radiant heat loss is due to infrared radiation
Thermal emissivity is a measure of how well a material emits/absorbs infrared.
A material does not have to be visually dark to have high thermal emissivity. Perhaps the best common example is snow, which is not at all dark but has thermal emissivity greater than 0.9May 21, 2005 at 11:07 am #1337424
Richard S said, “Isn’t radiant heat loss a reality with darker coloured fabrics, due to the fact that the emissivity climbs towards 1.0 as the material becomes “blacker”?
The short answer is “No”.
The long answer is that color has no affect on the long-wave radiant heat loss from our body. Color is only a factor in the absorption or reflection of visible light energy. In sunlight black will absorb most of the visible light energy and covert it to heat energy. In sunlight white will reflect most of the visible light energy and consequently be much cooler.
Our 95F skin temperature generates long-wave infrared energy and not visible light energy. The higher the source temperature, the shorter the IR waves emitted from the material will be. Reflective Mylar will efficiently reflect visible light energy, long-wave IR energy (95F source heat from your body) as well as short-wave IR energy (11,000F source heat from the sun) only if there is a sufficiently large air gap between the Reflective Mylar and the heat source. When you lay directly on reflective Mylar there isn’t a sufficient air gap to reflect the long-wave IR.
Now if Richard S. could figure out a way to levitate above the reflective Mylar without adding a lot of weight to our backpacks, that would give Olivia something to sing about.
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