View Full Version : How is R-Value determined?
88BlueGT
01-02-2008, 02:49
I was comparing a few different sleeping pads and was just wondering what exactally the R value meant. I actually have a BA Insulated Air Core *rectangual style* and was just trying to justify its weight. It has a higher r-value (4.1) than the others that I was looking at that were a little bit lighter (z rest, ridge rest, etc.) Thanks
Dan
Quagmire
01-02-2008, 03:00
from what i gather the r-value just means its ability to resist heat transfer..or cold transfer. Not sure how exactly they rate each item..lab testing of some kind. But like you have pointed out, the higher the r-value the lower the temps can get before it starts transferring the temps from one side tot he other basically.
88BlueGT
01-02-2008, 03:11
Yea I see. I just want to see how big of a difference 3.5 to 4.1 is (for example). Just want to be able to compare r-values of certain pads with some sort of knowledge. I dont want to spend an exta 30-100 dollars on another pad with 1 value higher and it only be like 5 degrees you know?
Quagmire
01-02-2008, 03:54
i gotcha. Let me see if i can find you anything better than my lame knowledge.
Quagmire
01-02-2008, 04:07
and i cant find anything useful. I am not sure there is a definitive temp number that can be assigned to a given r-value due to different variables, such as type of material, climate conditions such as moisture in the air and wind. Just a few things i have picked up while reading. For what it is worth, i have the same pad and will be trying it out in a few weeks on the trail. I shoul dbe in the smokies by mid february so hopefully the 4.1 is enough. MY friends and i did a trial run in wisconson a few weeks ago and camped on about 14 inches of snow. It got down to 20 degrees roughly and everyone was just fine as far as not being cold at the pad.
Rasputen
01-02-2008, 08:54
Try Here (http://www.hammockcamping.com/Free%20Reports/DWombleFoamPads.htm) this should help sort things out and maybe help you keep your load lighter...
Tipi Walter
01-02-2008, 09:54
The best way is to flop a pad down on snow or ice and see how you sleep. I remember thinking I could get by with a RidgeRest, so light!! but then tried it for a week on frozen ground and snow and quickly deep-sixed the useless thing.
Some people swear by using two pads, 2 blue foam or a combo of closed cell and an inflatable. To me it just doesn't make sense, why carry two pads when one thicker one will work? And I don't like the bulky things hanging off my pack. I've been a thermarest freak for years, but let's not forget Insul-Mat and the Expeds. Worth checking out.
Just Jeff
01-02-2008, 11:17
...why carry two pads when one thicker one will work?
B/c CCF is a better insulator for the weight than almost anything else out there, and it's pretty much bombproof. But it's uncomfortable. So use a CCF and an inflatable and you find a happy medium. Plus, you can use CCF for a sit pad anywhere, but an inflatable sit pad takes more care.
My current inflatable is an Exped Downmat 7. It takes a while to heat it up. But with a CCF pad on top I understand you don't have to wait for that heat. Haven't tried it myself...I usually just wait for it.
from what i gather the r-value just means its ability to resist heat transfer..or cold transfer. Not sure how exactly they rate each item..lab testing of some kind. But like you have pointed out, the higher the r-value the lower the temps can get before it starts transferring the temps from one side tot he other basically.
There is no temperature associated with R-values. All warm bodies transfer heat energy to cold. It's a continuous process. It works when the temperature difference between hot and cold is 1 degree and when it is 1,000 degrees. And it happens regardless of R value. It just happens slower at higher R values.
Neither 3.5 or 4 is very large as R values go. The walls of a typical new house in northern climes tend to be about R-20. Roof R values are in the 30s or higher.
But neither do you need a very high R value for sleeping outdoors in winter. Your body is constantly generating heat from the food you eat. All you need is enough insulation so that the heat loss doesn't exceed the heat generated. Snow itself is an insulator. If given a choice between bare ground and snow, choose snow if you have a good waterproof tent bottom to separate you from the snow your body will melt.
For winter use in snow I use a 3/8th or 1/2 inch solid foam pad from Walmart. Price $10 or less. Just create pockets in the snow for your hips and shoulders, place the pad, and sleep.
Bare ground is more of a challenge. In those conditions I use a full length foam pad and a light weight self inflating mattress.
Weary
Mara factor has an excellent discussion of this whole business here (http://friends.backcountry.net/m_factor/pad.html).
One suprising conclusion (to me, as a Thermarest "partisan") is that closed-cell pads are really more effective than Thermarest in the winter. Thermarest gives comfort... but closed-cell pads insulate better.
copythat
01-02-2008, 12:14
ok, i know i say this every time i get the chance ...
... i'm a cold sleeper and i've found i can get by with a standard 3/4-length closed-cell pad IF i also toss a cc scrap under my toesies, inside the bag. i carry a 3/8"x14x14 scrap, tapered to fit inside the footbox of my bag. when i roll at night i can readjust it with my feet and i stay toasty warm. (success to single-digits. :eek: )
11 oz for the pad, less than 2 more for the scrap, and it WORKS! and it cost less than my gaiters.
One of the mantras of UL hiking is "multiple use." Simple example: if you're using an internal-frame pack, chances are you'll empty it completely each night and pack it again each morning. So why not use a shorty pad, and put the empty pack under your feets? Whatever... it works for me.
Here is some good info on R-values and temp ratings. It compares 2 different formulas:
http://www.hammockcamping.com/Newsletters/2005/Jan2005.htm
As Quagmire posted, "R-value" is resistance to heat transfer. Single pane window glass has an R-value of 1. Double pane window glass has an R-value of 2.
In actuallity, the difference between a pad that is 3.5 and 4.0 isn't enought to worry about.
To specifically address the question about the difference between "R-4.1 and R -3.5" it seems to me to be about 7 F. The R value quoted by mats manufacturers appears to be linear and looking at those tables from 32 F to 0 F an extra .5 of R- value will give you another 7F of protection. In other words if the R -3.5 mat is comfortable for you at 32F, a R-4 should be comfortable at 25F. My conclusion is not scientific but should be close enough considering how many variants there are in the "keeping warm" equation
check this out
This is from UNC at Chapel Hill:
A thermal insulator is a material, manufactured in sheets, that resists conducting heat energy. Its thermal conductance is measured, in traditional units, in Btu (http://www.unc.edu/~rowlett/units/dictB.html#btu)'s of energy conducted times inches of thickness per hour of time per square foot of area per Fahrenheit degree of temperature difference between the two sides of the material. The R value of the insulator is defined to be 1 divided by the thermal conductance per inch. This means R is an abbreviation for the complex unit combination hr·ft2·°F/Btu. In SI (http://www.unc.edu/~rowlett/units/sipm.html) units, an R value of 1 equals 0.17611 square meter kelvins per watt (m2·K/W).
If you can noodle it, it makes some semblance of sense, in that wacky sort of sciency way. Like how the gas constant for the ideal gas law is in units of Liter*atmospheres per Kelvin Mole.
So your R value for insulation looks like a measure of how fast it will transfer heat energy from one side of it to the other.
So your R value for insulation looks like a measure of how fast it will transfer heat energy from one side of it to the other.
Correction, it would be more correct to say that it is the time it takes per unit volume/area to transfer heat energy towards equilibrium. Which for the sake of argument of staying warm is essentially moving your heat energy out of you into your surroundings.
I keep seeing scientific explanations like that, the problem is that after I read them I know a little less than I did before. In other words, it would not explain to me the difference between a Therm A Rest Prolite 3 (R-2.3) and a Prolite 4 (R-3.2)
88BlueGT
01-03-2008, 00:37
^^ LoL I have to agree. Unfortunatly, when I read things like that my head spins :)
Hmmm given your response this may be moving in the wrong direction but...
I've been doing more reading and it seems to be that the R values are given per inch of material most of the time. So if your pad has less than an inch of material, it will not insulate at a level consistent with the R value advertised.
Also, the R value measures only surface temperature of the insulation on both sides (and a difference between them), not how warm you will actually be (what's called perceived thermal comfort).
Basically I would just go with with reviews say, because even though they are very subjective, they're at least in real life situations. R values are tested in 70 degree farenheit rooms with zero air current and reflect mostly data that isn't relevent for camping sleep pads. Housing insulation has a value of something like 12, the difference between 12 of housing insulation and 2.3 of your pad is pretty big, but only in still, perfect conditions. The difference between R-2.3 and R-3.2 should be negligible and almost imperceptible to human senses. So disreguard most of the R values you see because in real life, they don't matter that much for a camp pad.
Repeat, go with reviews, not R values. They are almost worthless. Like saying gasoline has more stored energy than alcohol...well we still use alcohol don't we? That means the amount of stored energy is irrelevant because other factors outweigh it. Same deal with the R value.
The difference between R-2.3 and R-3.2 should be negligible and almost imperceptible to human senses. So disreguard most of the R values you see because in real life, they don't matter that much for a camp pad.
After your scientific explanation, I found that comment a bit strange. According to my conclusion posted above , the difference should be about 14 F , not a negligible amount in my mind.
So I had another look around and found a comparison chart at the Exped site that have their Sim Lite 1" mat rated R-2.2 with a minimum temperature rating of 32F. In the same series, the 1.5" version is rated R-3.2 as having a minimum temperature rating of 23F. The 2" version ,R-4.6 is rated at 5 F.
So my estimate was off but still a bit closer than "negligible" Note that both Pacific Outdoor and Therm A Rest call their 1" mats 3 season and the 1.5" 4 season (in the same series)
http://www.exped.com/exped/web/exped_homepage.nsf/b43HomePageE?openframeset
Click the comparison option
88BlueGT
01-03-2008, 01:58
That sounds about right. My BA IAC pad is rated at 15* on their site and it has a 4.1 R value.
BTW, that does not mean that their 1.5" mat will keep ME warm at whatever, it just means that it will keep me warm at a temperature about 9F lower than the 1" version does.
More important than the R value of a sleeping pad, is how much high energy food you eat before sleeping. A basic rule for winter camping is to never go to bed hungry, regardless of how tired and cold you may be after making camp.
Food generates the warmth you need to counteract the slow loss that occurs with any sleeping pad.
Weary
I would have said the opposite. Never mind the reviews. Go with the r-values. Still, there is more to the equation than simple r-values, because we are not in full contact with the pad and our sleeping bag or quilt is in loose contact with it where we aren't on it and so forth. It gets further complicated if you feel the need for more comfort than a simple blue foam pad provides. Then you are sacrificing weight and warmth for comfort, and if its cold enough then you need to get the warmth back somehow.