Sleeping mat matters

[marmottungsten]

Good explanation, Neil.


If the sleeping bag touches the foil, then heat is lost through conduction, negating the effectiveness of the foil. If there is an air gap, then conduction cannot occur, and the foil will reflect say 95% of the heat back (there will also be losses by convection within the air gap, but these will be minimal).

Nonsense...Mylar is both an insulator and a reflector...It cannot conduct heat away, it just reflects it back towards you!

[marmottungsten]

Sorry Richard I just posted another reply whilst you wrote this so I'm out of sync.


But yes, any air gap will let it work, where that gap exists. However I wanted to see how much a sleeping bag flatted out so tried pressing up against a glass door and even laid over a glass coffee table in my down bag (OK I might be a bit of nerd!) and it flattens out very well unfortunately - very few and very small air spaces that I could see. Different bags may crinkle up more perhaps?


Slivered mats that are ridged will work much better hence the Thermarest designs, and there are orientations one can think of that do produce air gaps. My own sleeping mat has vertical baffles so there are gaps between them so under that on a flat groundsheet would help a little. But people sticking a reflector under a flat pad, or lying on a space blanket thinking it will "reflect their bodyheat" are generally wasting their time. I'm not convinced about silvered flat mats. Call me cynical but just because a company sells something, doesn't mean it works 


I looked up human heat loss a while back and according to a medical paper looking at people in surgery, a naked person in still air loses most of their heat through radiation.

And space blankets reflect this radiation back to you!...They are also called emergency blankets and are used to stop people getting hypothermia.
Thermarest mats are just inflatable mats with a space blanket bonded to one of the inside surfaces...Which is why they make so much noise when you so much as twitch on them...They are doing the exact same thing I am doing...putting a space blanket under a sleeping bag...The only difference is they are combining it with the inflatable mat and I am doing it seperately.

[richardh1905]

Nonsense...Mylar is both an insulator and a reflector...It cannot conduct heat away, it just reflects it back towards you!

The mylar that you speak of is a fraction of a millimetre thick, and will conduct heat very readily. Try wrapping your space blanket around a metal mug of hot coffee and picking it up if you don't believe me.


On second thoughts, don't; I don't want you to get scalded.




Edit - thermal conductivity of Mylar is 0.15W/M/K, similar to that of wood!

[NeilC]

And space blankets reflect this radiation back to you!...They are also called emergency blankets and are used to stop people getting hypothermia.
Thermarest mats are just inflatable mats with a space blanket bonded to one of the inside surfaces...Which is why they make so much noise when you so much as twitch on them...They are doing the exact same thing I am doing...putting a space blanket under a sleeping bag...The only difference is they are combining it with the inflatable mat and I am doing it seperately.

Nonsense...Mylar is both an insulator and a reflector...It cannot conduct heat away, it just reflects it back towards you!

I don't think you realise how Infra red radiation propagates Neil...It does not need air to travel through at all...It will happily travel through a vacuum...For instance the heat from the Sun that we feel on Earth...It is infra red radiation from 93 million miles away, despite there being a deep vacuum between the two.  And Space blankets (or Mylar, to give it it's proper name) is highly reflective to infra red radiation...That is why they work as emergency blankets, despite being extremely thin.  When you wrap one around yourself it reflects the body heat you are radiating back towards you, keeping you warm...And it does exactly the same under a sleeping bag!

I'd replied to all your points in turn but I'm not sure it's really going to help much. I studied physics at university, black body radiation being the very first course we did  I think I've got a basic understanding.

If you're going to insist a reflector in direct contact with an opaque material still reflects IR then you need to be able to explain why a mirror doesn't work when covered in a thin sheet but why you think a reflective sheet will reflect IR when covered in a sleeping bag or flat foam mat, neither of which are transparent to IR. What are the differences that make on impossible but the other possible?

You might also ask yourself why The Department of Energy states an air gap is required when using radiant heat barriers, if they're not needed?: https://www.energy.gov/energysaver/weatherize/insulation/radiant-barriers

I can provide a much more detailed and technical explanation of the physics involved but I suspect 99% of the board are bored stiff already - but I will if you really want 

[jimbob]

Not nonsense. I think you owe the man an apology.
Anything that directly touches another thing causes conduction to occur, with heat passing from the hottest to the less hot.The only time it doesn't happen is when both articles are at the same temperature.
The efficiency of that heat transfer is dependant upon the insulating capabilites of the item itself. If you look closely at those capabilities it is usually more efficient if the material has trapped air, i.e. down ,polystyrene, as opposed to denser less airy materials such as concrete or engineering brick.

[marmottungsten]

I'd replied to all your points in turn but I'm not sure it's really going to help much. I studied physics at university, black body radiation being the very first course we did  I think I've got a basic understanding.

Seems you didn't study hard enough!

If you're going to insist a reflector in direct contact with an opaque material still reflects IR then you need to be able to explain why a mirror doesn't work when covered in a thin sheet but why you think a reflective sheet will reflect IR when covered in a sleeping bag or flat foam mat, neither of which are transparent to IR. What are the differences that make on impossible but the other possible?

So you think a sleeping bag is not transparent to IR???  This assumption is totally wrong!...Any sleeping bag will allow IR to radiate out of it...if it didn't you would quickly overheat!  It is the quality and thickness of the insulation in the sleeping bag that slows down IR radiation loss (or "traps" it), but it can never stop it.  Even the warmest sleeping bag will still allow IR radiation through, it just traps more IR for longer than less well insulated sleeping bags can.  That is why some sleeping bags are warmer than others.
And when the down in the bottom of a sleeping bag is compressed it's ability to trap IR radiation is greatly reduced, allowing more IR radiation to exit from the bottom of the sleeping bag than the uncompressed top...With nothing between the floor of the tent and the bottom of the sleeping bag, this IR radiation would be quickly conducted away to the cold floor below, because heat is lost to cold objects, and your back would feel cold.  A space blanket is an effective reflector of IR radiation (why you find this so hard to grasp I cannot fathom?), so when placed between the floor of the tent and the bottom of the sleeping bag it reflects the IR being radiated from the bottom of the sleeping bag back upwards, preventing it being conducted away by the cold floor below, keeping your back warmer than it would otherwise be. 

You  might also ask yourself why The Department of Energy states an air gap is required when using radiant heat barriers, if they're not needed?: https://www.energy.gov/energysaver/weatherize/insulation/radiant-barriers

What do you think the compressed down in the bottom of the sleeping bag is providing between it and the space blanket...a Vacuum?  No, it provides the very air gap you keep going on about!

I can provide a much more detailed and technical explanation of the physics involved but I suspect 99% of the board are bored stiff already - but I will if you really want 

I wouldn't bother, you clearly don't have a clue about the physics involved!

[richardh1905]

Oh dear....

[NeilC]

I give up! 

[alan de enfield]

Deleted

[jimbob]

MMT you are not coming across at all well.

Hopefully you are being mischievous and not just trolling for effect as so many do on other forums.
Clearly you do not fully understand thermodynamics, so let it rest.

[Owen]

Seems you didn't study hard enough!

What's that old saying about pots and kettles?

[Mel]

Marmottungsten, do you speak to people in real life like this?:

Seems you didn't study hard enough!

I wouldn't bother, you clearly don't have a clue about the physics involved!

 
Fair enough, disagree and discuss, but don't disparage.

 
It seems I'm not the only one who thinks you are a troll.  Please prove us wrong.

 

 

[Doddy]

I use the Klymit Inertia 3/4 length at 5ft 7 does me fine. I did get a leak which was devil to fix; seemed OK till I put my full weight on it.

[April]

Here is a link to a review of the Klymit Inertia X-lite by daveswildcamping

http://daveswildcamping.com/?page_id=2101

As Dave says, it isn't good.

[alan de enfield]

Here is a link to a review of the Klymit Inertia X-lite by daveswildcamping

http://daveswildcamping.com/?page_id=2101

As Dave says, it isn't good.

I remember seeing another review with a similar result, the summary was something like :


If you are over 5' 8", are a 'side-sleeper', have shoulders wider than a match-stick, want to camp in temperatures below +10c then forget this mat - it is useless.