Determining ideal R-value for pads

[Jason Brinkman]

Does anyone have a reliable method for relating single layer loft to an R-value?

I would like to carry a lighter pad, but I want to make sure that I select one that has a similar R-value to what my bag is providing on the top.

I would guess that some of the lightest pads out there have lower R-values than the bags or quilts with which they're paired.

Along the same lines, does anyone have a good list of the R-values of different lightweight pad options (like REI 3/8" standard blue closed cell foam)?

[Richard Nisley]

The average insulation value for 1" of dead air space (loft) is 4 clo or R = 3.5. Conventional high loft insulation such as Polarguard or old style insulation like wool blankets will closely match the quoted equivalence. Primaloft One or 800+ fill down will provide up to 30% better numbers.

A 3/8" blue foam pad has an R value of 1.35. The BPL Torsolight pad has an R value of 3.5. You can combine one or more closed cell foam pads with a self inflatable such as the Torsolight. If the pads are layered, the R values are additive.

MEC has an accurate list of R values for most popular pads.

[Jason Brinkman]

Many thanks Richard. Now if only I can predict the temperature of the ground at 10,000 feet in July!

[EndoftheTrail]

Hmmm…

ProLite 4 – 1.5" thick – 3.2
REI Lite Core – 1.5" thick – 3.1
ProLite 3 – 1" thick – 2.3
BPL Torsolite – 1" thick – 3.5!!!

How is it that the ultralite/ultrathin Torsolite manages such high R value?

[Jason Brinkman]

Hmmm, indeed…

Therm-a-Rest Trail Lite (no holes) – 1.5" thick – R 3.8

Torsolite sounds like it doesn't account for die cut holes.

[Richard Nisley]

I also started out saying "Hmmm, indeed" but, came away a believer.

The Torsolite uses the same construction as the POE Maxlite. I tested the R value of the '05 version of the POE Maxlite. My tests showed an R value of 3.3 for max inflation. Coring shape, thickness, and the surface type can all affect the battle between viscous forces and buoyant forces in the core. The following is my old test data.

[Michael Martin]

Great test, Richard!

How did you measure heat flow through the pad? Or did you estimate it from your metabolism, bag insulation, and perceived thermal comfort level?

Cheers,

-Mike

[Richard Nisley]

Mike,

For the heat source, I interval measured (computer based system) the temperature on my back, 18" down from my neck. For the heat sink, I interval measured the temperature on the bottom of the pad at the same spot. I then averaged (yellow lines) these captured temperature combination values to come up with the average R value.

I measured all of my sleeping pads I after spending a hypothermic night on a fully inflated BA Insulated Air Core. My take away thoughts were: 1) other than the BA Insulated Air Core rating, the mainstream sleeping pad vendors were accurately representing their products; 2) foam core pads are warmest with maximum inflation; and 3) the BA Insulated Air Core shouldn’t be inflated over ½ for maximum warmth. The summary from my '05 tests are as follows:

-BA Insulated Air Core at max inflation R=1.5 versus vendor rating N/A (rated to 15 degrees).
-BA Insulated Air Core at 1/2 inflation R=1.8 versus vendor rating N/A (rated to 15 degrees).
-Thermarest LE self inflated level R=3.6 versus vendor rating of 4.1
-Thermarest LE maximum inflation level R=4.0 versus vendor rating of R=4.1
-InsulMat max-lite 1.0 ¾ at maximum inflation level R=3.3 versus vendor rating of R=3-4.
-Old Thermarest Standard at max inflation (Used for EN13537) R=5.0 versus vendor's ratings of 5.2.

[EndoftheTrail]

Richard:

This is very enlightening! At the risk of sounding greedy, can you provide a rough translation of R value to comfortable temperature:

R=1 good for temps down to xF
R=2 good for temps down to xxF
R=3 good for temps down to xxxF

[Richard Nisley]

Ben-Reference my 1/11/07 post at http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/5625/index.html?skip_to_post=40672#40672

Multiply the clo value, at the top of my chart, by 0.8832059100018 to get the corresponding R value.

[Richard Nisley]

Ben-This should be easier to understand. A simplistic way to determine a reasonable pad to use is to use the "Sleeping Pads Used with the Speer SPE line" temperature rating. The accurate way is significantly more complicated. The details on how to do this were covered in prior posts that I made.

[EndoftheTrail]

Thanks, Richard.

[Michael Martin]

Thanks, Richard!

I think I'm still missing a piece of the puzzle, though. I see how you derived an average value for delta T. (btw, that looks like an excellent test that accurately represents real-world performance of the pad.) But what figure did you use for Q to calculate the R-value?

Cheers,

-Mike

[Richard Nisley]

Mike,

R=(DT*A)/Q

Q and A are constants because I am using my body and the same bag for all tests. The only variables are R and DT. So by measuring DT, I can calculate R.

I used the Thermarest Standard Pad as a comparative reference at R value = 5. This pad was verified as a reference standard by the EN13537 process. For the same environment I measured these values:

a=Thermarest Standard average DT(3.6F)
b=POE Maxlite average DT(5.4F)

a/b*5=R value of tested pad

I think of this thermal test as being similar in concept to your wind shirt test achieved by making a fabric cup and blowing through it. Neither of these tests would pass as International standards, yet they still get the job done. I had to pick nights in which the ground temp and air temp were the same as the reference test.

By contrast, the bottom of the BA Insulated Air Core, fully inflated, increased by 11.7F average.

[Michael Martin]

Nice Work, Richard!

>> I think of this thermal test as being similar in concept to your wind shirt test achieved by making a fabric cup and blowing through it.

Hey, I'm glad to see somebody reads my posts…. ;-)

Best Regards,

-Mike

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