Does 2.8 + 2.8 = 5.6 ?? Pad question

[Swamp Buck]

Looking at upgrades to my sleep system, similar to Twinndaddy I am intrigued by the quilt idea . I have a Thermarest ridgerest which has a 2.8 R value. If I just got another one and stacked them, would I then have a 5.6 R value?

Does anyone else use multiple pad system that you can interchange depending on the comfort level wanted and type of hunt you are on in different weather conditions? Is one $200 pad better than two $100 or less pads?

 

[colonel00]

I'm a side sleeper so I "had" to have an inflatable pad with enough cushion to keep my shoulder and hips from bottoming out. However, if you are sleeping on a single ridgerest now then I assume this may not be an issue for you. That said, I'd still recommend looking at an inflatable versus two ridgerests simply for the weight and packed size factors. However, recently I have cut down a ridgerest into 4 equal sections. I will carry one or two with me to use when sitting in the cold or wet as a nice barrier and insulation. I have also put these two sections under my X-lite pad as a little bit of a buffer from the rough ground when sleeping.

Anyway, if a R2.8 pad is working for you now, I wouldn't think there would be that much difference when using a quilt but I'd also say that if you can spare the pennies, a nice inflatable pad is well worth the money.

 

[airlocksniffer]

Like Colonel00, my hips can bottom out when sleeping. I have an XLite now that is way thicker than my old Lite Foam so that's less of an issue. If I'm going to do any winter camping, I'll either bring the ridge rest or a full length Z rest in addition to the XLite for added insulation. Once nice part about packing foam is it's basically indestructible so I'll use it for something to sit on at camp. Kind of bulky but the weight penalty is negligible.

 

[COlineman78]

R = L/k where L is the thickness and k is the thermal conductivity. Now, a Ridgerest or any pad for that matter isn't a solid material so simply doubling the thickness can't necessarily be assumed to double R value. A Ridgerest is closest to a solid material as any other pad so it is probably more accurate than with other types of pads. When considering complex systems like most inflatable pads the more applicable equation is R= heat transfer / (unit area / unit time). This can only really be measured empirically, but hypothetical numbers can be produced through computer simulation. Due to the way that some pads increase R value by limiting convective flow through multiple chambers, it would be reasonable to assume that if you stacked inflatable pads they would be more than just additive. As such, combining a ridge rest with a lower r-value inflatable pad, let's say 2.8 for simplicity's sake, should result in more than 5.6 whereas 2 ridge rests should be right around 5.6.

 

[WoodBow]

R = L/k where L is the thickness and k is the thermal conductivity. Now, a Ridgerest or any pad for that matter isn't a solid material so simply doubling the thickness can't necessarily be assumed to double R value. A Ridgerest is closest to a solid material as any other pad so it is probably more accurate than with other types of pads. When considering complex systems like most inflatable pads the more applicable equation is R= heat transfer / (unit area / unit time). This can only really be measured empirically, but hypothetical numbers can be produced through computer simulation. Due to the way that some pads increase R value by limiting convective flow through multiple chambers, it would be reasonable to assume that if you stacked inflatable pads they would be more than just additive. As such, combining a ridge rest with a lower r-value inflatable pad, let's say 2.8 for simplicity's sake, should result in more than 5.6 whereas 2 ridge rests should be right around 5.6.

Listen to that guy. Sounds like he knows what he is talking about.

 

[COlineman78]

Heat transfer occurs 3 ways: convection (moving air), conduction (direct contact, like cooking) and radiation (sun and fire). For example, The Therm-a-rest XTherm uses a combination of segmented baffles to reduce air movement (convection) and thermally reflective materials (radiation and conduction) to reduce thermal conductivity. Just about every other inflatable pad uses a more traditional insulator such as synthetic fibers or down to accomplish the same thing. Here is what the cross-section of an XTherm looks like:

So, how does this relate to what I was talking about earlier? Well with a Ridgerest or any other non-inflatable pad there is no convective heat transfer (unless you count the small amount that happens on a microscopic scale in the closed-cell foam). Therefore all heat transfer is based on the thermal conductivity of the material and therefore the R=L/k is the applicable function. As the XTherm shows, adding more cells helps in convective heat transfer which is why I would make the argument that if you have multiple inflatable pads stacked the resultant R-value would be higher than a simple sum of the individual parts.

 

[Swamp Buck]

So to put it simply, yes??? but also to add that if I were to add an inexpensive inflatable on top it could be > R5.6

I am also a side sleeper and currently use two pads. First I put down the ridgerest down and add a cheap but also much heavier self inflating pad on top. I have had success with this system but did not know how this equated in the science of camping.

 

[elkguide]

Thanks for the great insight.

 

[mrbillbrown]

Low tech personal experience.... Temps in the 20's, snow on the ground, son and I were backyard "camping" testing some new gear.

I used a Big Agnes Insulated Air Core (rated to 15* per website, R-4 something). Son used a Ridgerest Sol (R-3.5) and a cheap Academy brand heavy inflatable pad.

He was toasty warm in a 28* SD backcountry bed bag and I froze my tail off with a 10* down quilt. I could feel the cold ground sucking the life out of me. Tried his Sol & inflatable pad combo and it was noticeably warmer underneath me.

Granted, way Low tech personal experiment but the 2 pads worked great. Not sure i'd want to pack the extra weight and bulk around the high mtns.

 

[moxford]

I used a TR Z-Rest and TR NeoAir in the snow last weekend.

Bonus point is that the foam helps keep the NeoAir from sliding around. =)

-mox

 

[Dos Perros]

COlineman, I'm not sure that's correct. The pad, regardless of construction, should be seen as a system with an overall thermal resistance R. It doesn't matter what's going on between the two external surfaces (conduction, convection, or radiation). The only thing that matters is the rate at which heat flows from one side to the other. R1 + R2 = R3 would be as good of an approximation as possible.

 

[COlineman78]

COlineman, I'm not sure that's correct. The pad, regardless of construction, should be seen as a system with an overall thermal resistance R. It doesn't matter what's going on between the two external surfaces (conduction, convection, or radiation). The only thing that matters is the rate at which heat flows from one side to the other. R1 + R2 = R3 would be as good of an approximation as possible.

I agree that a simple sum is the best estimation that you can do, but my supposition is that with complex systems that rely on restricting convective flows the sum is greater than it's parts. The thing you have to remember is that R-value is not thermal resistance (k), but the first derivative of a change in temperature across a system's surfaces. Take the design of the XTherm for instance: the resultant R value is certainly greater than the sum of the components. If you were to take any sheet of fabric used the R-value would be virtually nothing due to how thin the fabric is, no matter how high the thermal resistance is (remember for a solid R=L/k). Also, they are each resistant to different types of heat transfer and therefore by themselves their resistance to heat transfer might actually be quite low. For example the foil layers are very effective against radiative heat transfer they are conductive and therefore their R-value would be quite low, but in a system combined with something that has a high resistance to conduction (and in the case of air pads convection) the system is much more effective.

Another example is double pane windows. Their thermal resistance (R-value) is significantly greater than the sum of the 2 panes as can be demonstrated by their R-values both with and without a broken seal. When there is a vacuum (or low density gas) between the panes the overall system is much more thermally resistant.

As I stated in my earlier post, without either empirical evidence or computer simulation the R-value of a complex system can't be determined. The only thing that you can guarantee is that it is equal to or greater than the sum of it's parts.

 

[Dos Perros]

I agree that a simple sum is the best estimation that you can do, but my supposition is that with complex systems that rely on restricting convective flows the sum is greater than it's parts. The thing you have to remember is that R-value is not thermal resistance (k), but the first derivative of a change in temperature across a system's surfaces. Take the design of the XTherm for instance: the resultant R value is certainly greater than the sum of the components. If you were to take any sheet of fabric used the R-value would be virtually nothing due to how thin the fabric is, no matter how high the thermal resistance is (remember for a solid R=L/k). Also, they are each resistant to different types of heat transfer and therefore by themselves their resistance to heat transfer might actually be quite low. For example the foil layers are very effective against radiative heat transfer they are conductive and therefore their R-value would be quite low, but in a system combined with something that has a high resistance to conduction (and in the case of air pads convection) the system is much more effective.

Another example is double pane windows. Their thermal resistance (R-value) is significantly greater than the sum of the 2 panes as can be demonstrated by their R-values both with and without a broken seal. When there is a vacuum (or low density gas) between the panes the overall system is much more thermally resistant.

As I stated in my earlier post, without either empirical evidence or computer simulation the R-value of a complex system can't be determined. The only thing that you can guarantee is that it is equal to or greater than the sum of it's parts.

Nerding out is fun sometimes.

The R-value, at least how Thermarest does it, is a measured value.

http://www.cascadedesigns.com/therm-a-rest/blog/r-value-meaning/

They put a pad between two metal plates and put some heat to one side, measure temperature on the other, and derive R that way. So as I said, it's an R value of the pad as a system independent of material or types of heat transfer.

Edited to add: You're right that in this instance R is not related to L and k. It is however derived by the temperature gradient delta T and the flow rate of heat through the pad, where R = Qdot / (T1 - T2) .

 

[justinspicher]

I carry a thermarest luxury lite cot and an Xtherm pad and sleep quite warm and well with my EE Rev 20 quilt. Even down to zero.