Apr 4, 2012 at 11:16 am #1288302
I've been using the same 32*F down sleeping bag for several years, and I generally used it with just a few pieces of generic blue foam, typically about three squares of 1/4" or 3/8". That is comfortable enough for me until the temperature dips below about 30-32*F. So, the rating of the sleeping bag seemed reasonable, and the foam padding was adequate.
Lately I purchased a new down sleeping bag rated for 30*F. However, it is no official rating, only the estimate by the manufacturer, so I don't know how much to trust it. In order for that temperature rating to be fair, it requires a thermally-neutral sleeping pad, and I am thinking that my old pad arrangement may not meet that. Of course, that might make the sleeping bag seem cold.
The question is: How much R-value do I need in a foam sleeping pad?
This is for summer use, but the temperature might drop below 30*F. I have down garments that I can use to get through the cold night, but I am trying to focus just on the pad R-value. I'm thinking maybe R=2.
–B.G.–Apr 4, 2012 at 12:22 pm #1863598
There it is again, the term "thermally-neutral". What is that?
The colder it is, the more insulation you need, there's no point where you reach neutrality???
To answer your question, pads like Prolite (R = 2) or Neo-Air (R = 3) are supposed to be good above freezing or so, but much below that, or if sleeping on snow you need something more.Apr 4, 2012 at 12:39 pm #1863612
Jerry, the term implies that it is not so thin that it is sucking massive heat out of the sleeping bag (like cold ground does), but it is about the correct thickness that there is almost zero heat loss into it, and it doesn't "feel" warm from inside the bag.
Sure, the colder it is, yada yada… but that is not the premise. The premise is for summer non-snow use where I expect the ambient air temperature to get no colder than 30 or 32*F, and it absolutely shouldn't dip below 20*F.
I don't need much thickness to be comfortable, but I need enough R-value that the new sleeping bag won't seem like a poor performer. It seems like we are circling around R=2.
–B.G.–Apr 4, 2012 at 12:46 pm #1863615
@richard295Locale: San Francisco Bay Area
It means at a specific MET level and specific insulation ensemble your core will stay at 98.6F indefinitely. The other two possible options eventually result in sweating or slow-onset hypothermia. Eventually is determined by your individual heat capacity.
For sleeping, the MET level is defined as being .8.Apr 4, 2012 at 12:54 pm #1863618
"but it is about the correct thickness that there is almost zero heat loss into it"
that's not how it works
the heat loss is a constant – maybe 50 Watts per square meter – ignoring that it varies depending on how much you ate and exercised… you could have more heat loss on your head and less on your bottom or whatever…
for a given temperature, you need a specific amount of insulation to maintain a comfortable temperature
like Richard's plot of clo vs temperature:
http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/9378/index.htmlApr 4, 2012 at 12:56 pm #1863620
@ckrusorLocale: Northwest US
I would agree that R=3 is good for most men down to about freezing. My exped synmat UL7 (R=3) begins to feel cold in the upper 20s F.
Richard Nisley has described the term "thermoneutral" in at least one of his older posts, I think. That explanation might be difficult to find using the search function, though.
I think it describes a condition in which a person can maintain normal body temperature without cold-stress induced compensation (shivering, etc.). I don't know the explicit definitions of this metabolic "comfort zone" and the generalized "person" but I think these concepts originate at the Army Natick research center.
So, I think you are thermoneutral if you can effectively thermoregulate (your core temp is normal and neutral: it is neither climbing nor dropping) without relying on shivering or other metabolic tools of last resort.
Edit: I was hoping Richard would chime in, and he did just as I was posting this…Apr 4, 2012 at 12:56 pm #1863621
So, "thermally neutral" means you waited long enough for it to stabalize?Apr 4, 2012 at 1:01 pm #1863625
@ckrusorLocale: Northwest US
Jerry, thermoneutral describes you, not the pad. An adequate pad will allow you to remain thermoneutral. This happens when the pad slows the loss of heat enough that you don't gradually cool (you stay at a normal core temp).Apr 4, 2012 at 1:05 pm #1863628
My own personal heat capacity has no measured number on it. I know my maximum METS score on a treadmill, but it has no relevance to me in a horizontal position in a sleeping bag.
But the old sleeping bag seemed fairly true to its rating with me in it. If the new sleeping bag is used with a proper pad, it won't feel like there is any loss of heat into the pad, although of course there is. I'm just trying to figure out what a proper pad is to give the new sleeping bag a fair comparison.
I could use something with R=10, but that would be tremendous overkill. R=1 is probably too thin. That's why R=2 is the first estimate that came up.
–B.G.–Apr 4, 2012 at 1:13 pm #1863631
On one summer trip two years ago, I was sleeping in my old 32*F bag on some blue foam padding, inside a tarp shelter. I was around 10,500 feet elevation on a windy night. I woke up in the middle of the night, chilled and shivvering, and I was puzzled. I looked around and found that I had slid off my foam pad, and the down sleeping bag was tight up against the tarp shelter on the windward side. Those inefficiencies explain why I was cold. I donned a down inner jacket, pulled my warm hat over my ears, and went back to sleep. So, there wasn't any issue with time.
–B.G.–Apr 14, 2012 at 12:42 pm #1867223
@pjstarichLocale: N. Rocky Mountains
Interesting discussion. I've been sleeping on snow for many winters now, usually inside a snow shelter where air temp rarely falls below 32F. I've found a pad (or pads) with R3.2-R3.5 quite comfortable on snow.
Does air temp really matter? The pad serves to insulate you from the ground surface which in winter is often warmer than the air temp. Snow, by itself, is mostly air and a very good insulator. Frozen ground offers less insulation; ice almost none. So isn't ground temp and it's R-value the real factor in choosing a pad?Apr 14, 2012 at 12:47 pm #1867225
"Snow, by itself, is mostly air and a very good insulator. Frozen ground offers less insulation; ice almost none."
It isn't as simple as this. There is a huge variation in snow. Maybe some of the dry Colorado snow is mostly air, but a lot of our California snow transitions into ice pretty quickly. As a first approximation, we consider snow to be a 50-50 mixture of ice and air.
–B.G.–Apr 14, 2012 at 2:00 pm #1867232
The most I genrally use is a couple of GG torsolite pads beneath a ridgerest solite. Combined R would be pushing 4.5-5 I suppose. In the 20s I can literally feel heat radiating back at my backside, its a very nice feeling. I know I dont get that with thinner pads alone, but Im still OK with one or the other down to about freezing in a 30f bag.Apr 14, 2012 at 9:23 pm #1867356
an R-value around 2.5 or so is pretty typical of pads like the Ridgerest or the lighter self-inflating pads or the NeoAir, so that seems like a useful standard summer pad rating to me.
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