Picnic mat on top or under inflated pad

Discussion in 'Sleep Mats' started by PhilHo, Feb 4, 2020.

  1. PhilHo

    PhilHo Thru Hiker

    I've read threads about improving warmth/r-value of sleeping mats. I have some of the ccf with silvered film both sides stuff.

    I'm easily able to keep it attached to the pad either on top or below with current sleeping bag that has system for attaching it to pad.

    So is it best, in terms of warmth improvement, to put it under the mat or between the mat and the sleeping bag? Yes I could try it both ways and find out for myself but I can ask you what you've found best without standing up from my office chair. :finger:
  2. WilliamC

    WilliamC Thru Hiker

    We've only tried this out with a non-reflective CCF mat with a Klymit Insulated Static V. Camping on snow, we were both feeling the cold when we put the CCF mat underneath. After half an hour or so, we switched it to the top and it immediately felt warmer.
    Caveat: We were using a quilt and the Klymit has channels between the Vs; with a different mat or with a sleeping bag, the difference might be less noticeable.
    Speculation: With a silvered mat, I would imagine it's better on the top as it will reflect heat back to you; with a silvered mat on the bottom, that reflected heat will be partly blocked by the inflatable mat's fabric and insulation.
    PhilHo likes this.
  3. Gobila

    Gobila Trekker

    I have found it warmer with the ccf mat on top although I tend to do it the other way around as I find the feeling of the ccf on top of the inflatable a bit strange.
    PhilHo likes this.
  4. Patrick

    Patrick Trail Blazer

    I've been trying to think about the physics of this, and its interesting. Fundamentally, the order of insulating substances doesn't affect their total insulation, which is additive. There are a couple of potential special factors which occur to me about the situation described, however.
    Firstly, the convection within an air mattress may lead to significant heat loss out of the sides, in which case it would be better to have additional insulation on top of the air mattress, causing the air mattress to be cooler, and heat loss from it less.
    Secondly, there's the whole issue of whether silvered surfaces work at all. In theory, they only work with an air gap above them - the heat transmitted to them is then predominantly radiant, which they are very good at reflecting back. If there's no air gap, then the heat transmitted to them is predominantly conductive, and since they are also good conductors it just runs straight through them. Given this, they probably only work well when whatever is above them is only partially in contact with them. I'm not sure whether a sleeping bag or an air mattress will have a greater proportion of its surface actually in contact with the silvered surface, but I suspect that the ridges in an air mattress may be sufficient to lift a reasonable proportion of it off the reflective surface. If this is the case, then this would argue for the reflective surface to be below the air mattress.
    That's my take on the theory, but how it balances out in practice is anybody's guess!
    Diddi, PhilHo and Gobila like this.
  5. Teepee

    Teepee Thru Hiker

    The reflective foam mats work best underneath. They contort and move about if they are above. Having them underneath is much easier.
    It's not that they don't insulate on top, I often use them to line my hammock underquilt with them when it's colder than the rating...they also get used inbetween the layers and so get full bodyweight and conduction on them. I can feel the difference when they are the reflective side facing down; they are not as warm.

    Reflective layers are proven to work as insulation. From Mountain rescue to NASA, X-Therm mats to roofing membranes; they are in widespread use.
    PhilHo likes this.
  6. Chiseller

    Chiseller Thru Hiker

    Radiant opposed to conductive is good science.. Put a frozen steak on a large sheet of silver foil and it defrosts quicker than on a rack.... Yet when my mat did one on a cold night below wetherlam... I slept on a silver sheet with less than two mm foam below.... I was using a quilt and the silver got me through the night.... Maybe because my body was directly on the silver (clothed and wind bivi) and the conducting effect isn't the same when heat is laid on it as opposed to a cold body (Eg my frozen steak).... Hmmm
  7. Rog Tallbloke

    Rog Tallbloke Thru Hiker

    "Reflective layers are proven to work as insulation"

    They don't work as insulation, they work by reflecting radiant heat. If they're backed by foam, they will insulate as well as reflecting radiant heat.

    A space blanket over your body will reflect some radiant heat back to you. However, it's bigger thermal effect is that you lose less heat because it presents a barrier to convection and conduction. You'll be almost as warm under a non reflecting material of the same weight.
    Last edited: Feb 4, 2020
  8. Teepee

    Teepee Thru Hiker

    Is thermal insulation not a reduction in loss of thermal energy?


    This is a blanket statement. :) .......

    ....it doesn't take into account what clothing is worn. A naked Rog (again) curled up under a space blanket is going to feel the benefit of that reduction in loss of heat by evaporation, convection and conduction than a clothed Philho inside a sleeping bag on a mat over a reflective layer. Using a reflective layer under a mat means that it's not affecting heat transfer rate by evaporation, and convection is minimised because the layer is 'sandwiched' with a minimal/nil air gap.
  9. Rog Tallbloke

    Rog Tallbloke Thru Hiker

    I'd be interested to see some real measured temperatures for a warm 'artificial body' with a specific energy input laid over silvered and unsilvered foam mats under inflatable mattresses. I predict the difference will be minimal, but I subscribe to the old adage "In God we trust, all others bring data".
  10. PhilHo

    PhilHo Thru Hiker

    TBH if I was choosing between one and the other it might be relevant but since I only have silvered in my garage I'm going to stick with that. Previously I've put a 5mm ccf pad under my inflatable pad but I want to cut weight so have the picnic mat and just wondered if there was any advantage in putting it on top. I think I'll actually stick it under my inner to give that a bit of poke through protection and it sounds like the difference will be minimal. Santa brought me a nice new Exped SynMat HL LW Sleeping Mat but I've not had the chance to use it yet.

    By the way @Rog Tallbloke Mrs Ho was over your way on Saturday doing the Rombolds Stride. Apparently it was a tad breezy on Ilkley Moor which almost lead to her cap blowing away. We all know what a series of dangers that might have lead to.
    WilliamC likes this.
  11. Rog Tallbloke

    Rog Tallbloke Thru Hiker

    Did she spot any ducks playing football?
    PhilHo likes this.
  12. Patrick

    Patrick Trail Blazer

    That's clearly very true, and I wasn't trying to deny that they can be very effective. However, in all the examples you give above, the reflective layer is used with an air gap between it and the material it is keeping warm, which allows it to use its reflective properties well. Where I have slight misgivings is when we put it under ourselves when sleeping, when it seems to me its largely in contact with the thing its trying to keep warm. This will mean its predominantly acting as a good conductor, which we don't want, rather than a good reflector, which we do.

    Having said that, I use a piece of silvered radiator foam under my mat, and swear it makes a very noticeable difference - but I don't have any hard data!!
    Teepee and PhilHo like this.
  13. Stuart

    Stuart Ultralighter

    I don't have any relevant physics qualifications but I definitely find having a cheap thin CCF mat on top of an inflatable mat is warmer and also more comfortable.
    PhilHo likes this.
  14. Rog Tallbloke

    Rog Tallbloke Thru Hiker

    How beneficial depends to some extent on whether the inflatable pad is itself insulated. If it is, then your bag lofting into the ridges might give you more than having your ccf under the pad loses due to sidewards conduction losses. If not, and if you use a quilt, ccf on top of inflatable is better.
  15. Stuart

    Stuart Ultralighter

    Or when your bag has hardly any remaining loft on the bottom, CCF on top is better!!
  16. lakeshore

    lakeshore Trail Blazer

    Not sure what the conclusion was here, I guess it depends on the gear you have. I had been wondering about this subject- I sometimes put a thin foam mat under my Klymit sleeping pad mainly to protect it from puncture. Is there any value in putting a silver thermal sports blanket anywhere in the system. I have used one as ground cloth to keep the floor of the tent clean. I have Zpacks Poncho which I have put under the mats as a bit more insulation, but I might be waiting my time there.
  17. tom

    tom Thru Hiker

    According to the physics of reflecting heat one way and cold the other, below the airmat should be most effective (e.g. keeping frozen ground as far as possible from the body). But you may "feel" the effect more in some temp ranges when the silver layer is closer to the body. Heat loss sideways from an airmat may also change the equation .... I mostly go with uninsulated air mats with a silver sheet below and add a 2mm foam sheet on top when its colder.

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