Cumulative R Values

[Bob Bankhead]

Technical question for you engineers:

Given 2 sleeping pads with different R values:
Pad One has R value of 2.2 and a thickness of 1 inch
Pad Two has R value of 2.6 and a thickness of 5/8 inch

If I put one pad on top of the other, I get a combined thickness of 1.625 inches (and will naturally sleep more comfortably) but am I really sleeping any warmer; what is the R value of the combination?

In other words, are R values additive, averaged, just equal to the larger of the two, or does it take some sort of wierd thermal balance equation to determine the combined R value?

Thanks.

[Dan @ Durston Gear]

I think you add them together (2.2 + 2.6 = 4.8) but it might be multiplication (2.2 x 2.6 = 5.72)

[Ashley Brown]

Add them. (And if you have an air pad and a foam pad, the foam pad should go on the top.)

[Dan @ Durston Gear]

Why should the foam go on top? I have a RidgeRest and a NeoAir. While I normally use the NeoAir, I would bring the RidgeRest for winter trips. I've always pictured myself putting the RidgeRest on the bottom because it seems more comfortable to sleep directly on the NeoAir. A firm pad like the Ridgerest seems like it would conform to my body less than the NeoAir, so I would be less comfortable.

[Ashley Brown]

It may be more comfortable to have the NeoAir on top, but it is warmer the other way around. I must confess to not being an expert on the physics of why this is the case, but…

I believe it is related to the fact that the air mat is more efficient at transporting heat away from your body — because the air inside the mattress is circulating (convection). So the whole air mat is acting as a heat sink/radiator. A foam mat on the other hand does not spread out your body warmth to the far corners of the mat (because it conducts poorly). It therefore has less surface area which is getting rid of your precious body heat into the air and ground.

If someone has a better explanation please pipe up!

[Roger Caffin]

Add them.

But it may not be that simple (it never is). For instance, putting one flat foam mat on top of another flat foam mat usually means a simple addition, but putting one Ridgerest on top of another one may mean addition plus a fudge factor (which may be +ve or -ve). It ll depends on what sort of air gap is trapped between them.

Cheers

[Derek Goffin]

I think Ashley's explanation of the physics is superb. You might decide on balance to defend the inflatable from thorns and sharp rocks by putting the foam underneath anyway.

[Bob Bankhead]

So R values are – for all intents and purposes – additive. Thank you.

The plan is to put two Ridgerests under the Thermarest, to protect it from puncture and to make a thicker, warmer sleeping pad. I'm a rolly-polly side sleeper for whom a good night's sleep is worth the extra weight.

It also allows me to use a shorter (20 x 47 x 1 inch) and lighter weight (11 oz) Thermarest Prolite to provide shoulder to knee comfort while the two 20 x 5/8 inch Ridgerest pads (one 48 and another 24 inches, total 14 oz) form the bottom 72 inch layer, provide both cushioning and insulation for my head, lower legs, and feet. Total thickness = 1-5/8 inches; total R = 4.8; total weight = 25 oz.

I can get the same overall system weight and thickness by placing a 20 x 72 x 1.5 inch) Thermarest Prolite Plus (24 oz) atop a 19.5 X 59 x 1/8 inch Gossamer Gear Nightlite pad (2 oz), but the total R value (about 4) and puncture protection is much less this way.

Either system provides greater flexibility than just a Thermarest alone. I get a 48" backpad for my pack, a 24 " sit pad, and a sleeping pad, all with the same stuff. Also, if I should puncture the Thermarest and not be able to find/fix it, I still have an insulated sleeping pad, just not as thick and cushy as it once was.

The same system would work with the XS (36 inch) Thermarest Prolite, or any other torso-size inflatable (like the BPL pad), with a corresponding reduction in total weight.

[Alex H]

You may have seen my post in another thread but I actually think it is the perfect combination for both warmth, padding, weight and durability. At around -5C and lower (or on snow or frozen ground) I will bring a full length Ridgerest and put the torso pad (prolite XS) on top (20 oz.), otherwise I use a cut down 36" Ridgerest and a torso pad (14 oz.). You can deflate the torso pad a bit and get the shoulder and hip hole effect and it is the most comfortable sleep I have had even on solid rock or wooden shelter floors. If the torso pad deflates for some reason I still have at least some pad underneath me.

[Roger Caffin]

Hi Bob

That's getting close to snow rating.

We used a 3/4 ProLite and some 5 mm foam just at the feet (ie not double-layer) in Switzerland this year. No worries.

Cheers

[Bob Bankhead]

Hi Roger,

Thanks for your help. I don't voluntarily sleep on snow, but I'm getting ready for the Colorado Trail next summer, and it is notorious for cold and wet.

Wanted something a little beefier than my Sierra set-up. also went through 2 big Agnes Insulated Air Core pads this summer on the JMT – a total valve failure and a tiny leak I never could find, even under water.

[Rick Cheehy]

In the cold I layer, heat sheet, gg thinlght, neoair, ground. No real science but guessing about heat refelction. It's pretty warm and cozy.

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