Cumulative R Values
 Display Avatars Sort By: Date (Chronological) Date (Reverse Chronological)
 Bob Bankhead (wandering_bob) - MLife Locale: Oregon, USA Cumulative R Values on 09/02/2009 22:38:02 MDT 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 inchPad Two has R value of 2.6 and a thickness of 5/8 inchIf 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. Edited by wandering_bob on 09/02/2009 22:41:36 MDT.
 Dan Durston (dandydan) - M Locale: IntoCascadia.com R-values on 09/02/2009 22:50:00 MDT 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 (ashleyb) - F Cumulative R Values on 09/02/2009 22:59:22 MDT Add them. (And if you have an air pad and a foam pad, the foam pad should go on the top.) Edited by ashleyb on 09/02/2009 23:00:28 MDT.
 Dan Durston (dandydan) - M Locale: IntoCascadia.com Foam on top on 09/02/2009 23:03:47 MDT 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 (ashleyb) - F Re: Foam on top on 09/02/2009 23:18:44 MDT 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 (rcaffin) - BPL Staff - MLife Locale: Wollemi & Kosciusko NPs, Europe Re: Cumulative R Values on 09/03/2009 01:00:51 MDT 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 (Derekoak) Locale: North of England Cumulative R Values" on 09/03/2009 06:26:49 MDT 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.