Is there any science on reflective ie. metallic looking/ reflective faced fabrics.

These look interesting - https://www.etsy.com/uk/listing/1063748 ... -quilt-new

Rab are using it in sleeping bags as well:
https://rab.equipment/uk/mythic-ultra-180-sleeping-bag

Not sure how comfortable it would be next to skin, perhaps full thermals are a necessity.

Colombia have used it for a while too, they should have some good bumf to back it up, I think they call theirs omniheat

Double.post sorry

Richpips wrote: ↑Wed Oct 26, 2022 12:26 am Is there any science on reflective ie. metallic looking/ reflective faced fabrics.

These look interesting - https://www.etsy.com/uk/listing/1063748 ... -quilt-new

Yes, it's the resoning behind tinfoil.

You need to think about IR, not heat, as being reflected. Just another wavelength of light - albeit invisible to our eyes. That causes a change in temperature as it transfers it's energy.

Part of the current GCSE spec as it happens. Though we teach it to Y8 and 9 as well.

I tend to think the problem with 'reflective' is often peoples misunderstanding of what it can do - a good example is that of placing a reflective blanket beneath you. As it has limited insulation properties, it's unlikely to make much difference as little of the heat loss to the ground will be via radiation.

Berghaus also doing this in some of their jackets

The theory is sound, there must be some help in real life, particularly if you can do it with no weight penalty

Bearbonesnorm wrote: ↑Wed Oct 26, 2022 9:14 am I tend to think the problem with 'reflective' is often peoples misunderstanding of what it can do - a good example is that of placing a reflective blanket beneath you. As it has limited insulation properties, it's unlikely to make much difference as little of the heat loss to the ground will be via radiation.

Exactly the problem. People forget about conduction.

columbia fairbanks winter boots have the reflective inner. it does work in those. toasty boots.

I believe in it and thus I'm warmer

Hard to tell if the alu lining of a Therm-a-rest makes a difference. After many nights you can see, holding the mat against the light, that the reflective part disappears. At this stage the mat is leaking anyways, so it's impossible to tell a difference, once replaced with a new one. But I believe it does, which is enough for me to make a difference
My Berghaus Ramche 2.0 has reflective coated fabrics and with (albeit quite a lot of good down) only 450 something grams it's really, really warm.

As Ton says, some boots come with them and it probably doesn't harm to make them warmer.

Quite possibly a daft question. The quilt pics show the foil layer "next to skin" so I assume it's intended to reflect back into the (often poorly sealed) airspace immediately around the body. Would it be better putting the reflective layer on the inside of the outer shell so it reflects back into the insulating layer, which itself is intended to "trap" warmed air, so sort of double-bubble?

Just curious.

Quite possibly a daft question. The quilt pics show the foil layer "next to skin" so I assume it's intended to reflect back into the (often poorly sealed) airspace immediately around the body. Would it be better putting the reflective layer on the inside of the outer shell so it reflects back into the insulating layer, which itself is intended to "trap" warmed air, so sort of double-bubble?

Tempted to say not - in theory (at least) the insulation should prevent heat reaching the outer layer of the bag / quilt, so there'll be very little at the outer surface to 'reflect' back'. What did get 'reflected' back would then be prevented from reaching you by the insulation properties of the insulation ... if that makes sense.

IR is an electromagnetic radiation.
Some call it a wave, some a particle, some go by it being both (Heisenberg's uncertainty principle), for those in a sleeping bag it warms the trapped air (also down and fabrics). Add a reflective layer on the inside of the outer shell fabric and you keep more radiation inside, warming the air more efficiently than without any reflection.

The further away from a black-body, the better = the more reflection, the better.

Interesting Gian. Couple of questions / thoughts.

Even though the insulation is 'holding' or retaining heat, would it count as a heat source in the same way as a body would?

Could the insulation itself provide a barrier to any reflected heat making its way back through to the inside of the bag?

Heisenberg's uncertainty principle

This says you can either know exactly where something is or how fast its going but not both [ 100% accurately]

My guess is that the material on the inside warms the inside air faster and the loft slower [ as less energy is getting to it] You may then lose heat[energy] more slowly [ to the outside from the bag] and therefore be warmer
I could be wrong and would be interested if anyone knew for certain [ but i am enjoying the debate]

I wondered about this when reading an article by a guy doing a lot of bike riding in Alaska and how to keep your sleeping bag dry overnight so it doesn't freeze during the day. He had tried wearing an impermeable layer which kept body condensation in to stop it going into the down and then freezing, ultimately reducing the downs effectiveness day on day. So you could use a foil emergency bag as the impermeable layer to trap condensation and keep you a bit warmer.

Looking for someone to go into the arctic to try

Aye, vapour barrier Phil. Rab and a few others do a non-breathable inner bag for such things and socks too.

When you see runners at the end of a marathon being given an emergency blanket its always made me wonder how much heat we actually lose to radiation. I suspect that from a near hyperthermic hiker its next to nil and the main use is as a wind barrier.

Lazarus wrote: ↑Thu Oct 27, 2022 1:16 pm

Heisenberg's uncertainty principle

This says you can either know exactly where something is or how fast its going but not both [ 100% accurately]

I know the feeling... I must sort my Garmin display settings out.

There is no uncertainty with Garmin's messing up....it's inevitable

Has anybody used a vapour barrier? Do you just accept you're going to be warm but wet? It must be pretty miserable,like sleeping in a poly bag. Would sleeping with a little bag of rice or some other desiccant help?

Rob S wrote: ↑Thu Oct 27, 2022 7:32 pm Has anybody used a vapour barrier? Do you just accept you're going to be warm but wet? It must be pretty miserable,like sleeping in a poly bag. Would sleeping with a little bag of rice or some other desiccant help?

Have used VBL socks in extreme (as in -20C) cold. It does work but you need to be quite specific and precise in how you use the system. You need the thinnest layer you can get underneath the VBL so that it doesn't retain much moisture - I used Helly Hansen Lifa socks. Also the system needs to be enclosed so no venting at the collar or cuffs so that the environment next to your skin remains humid enough to trick the body into stopping sweating.

Personally I wouldn't use above about -5C as things would get much too clammy.

https://bikepacking.com/plog/fueled-by-fire/
This was the article. I like his solution to not having to use a vapour barrier!

Bearbonesnorm wrote: ↑Thu Oct 27, 2022 12:57 pm Interesting Gian. Couple of questions / thoughts.

Even though the insulation is 'holding' or retaining heat, would it count as a heat source in the same way as a body would?

Could the insulation itself provide a barrier to any reflected heat making its way back through to the inside of the bag?

Perhaps it becomes more clear to understand the insulation (trapped air in the bag/quilt) as a function of slower loss. If the air would move inside the bag and mix with the air outside, there wouldn't be any insulation. So less 'heat retaining', more of a 'more controlled heat loss' (if that makes sense). This becomes obvious when you're in a cold water pot and don't move. After a couple of minutes the unmoved water closely surrounding your skin is significantly warmer.
Reflective fabrics/lining/coating let some/most of our IR radiation bounce back. I bet this effect is significant (lab test) and likely noticeable (outdoors) when compared back to back.

RE: VBL. Just as whitestone wrote.

The idea behind VBL has nothing to do with reflecting heat, but keeping insulation dry, despite some (like Western Mountaineering) used to (?) sell reflective polycro bags as VBLs (little sense, reflective lining close to body, rather that outmost layer of insulation).
I've measured 250 g of ice inside my downbag after 5 nights in the Alps at -15 to -30 °C without VBL, guessing there's some loft loss of at least 5°C, likely more. I only used my downbag once for 10 days of winter mountaineering with out a VBL and only because it's a very warm monster of a down bag (rated -40 °C) and wearing some additional clothes I stayed warm at -36 °C. The bag was heavily compromised (same will happen with synthetic insulation or functionalised down). And that's why every semi sensible person uses a VBL on long trips in most wintry conditions (with no or hardly any option to dry).
I use pure polypropylene underwear, socks, long johns, long sleeve T inside a VBL. It only gets moist and is nowhere as bad as it sounds. I also don't change base layers between day and night, that's how dry it is.
The little bit of humidity inside the VBL can be shaken off after turning it inside out once left to freeze, which at eg -20 °C just takes seconds.
For eg skimo or fatbiking trips across 3 - 4 nights I don't bother with VBL, except for my feel during skinning/skiing or riding.

There's not much worse but needing to thaw your feet inside a solid boot before leaving your bivy. Not just unpleasant, but seriously risky for your tissue.
Tricky part is, VBL during the day, VBL during sleeping leaves the skin ... tender. One must take care of the feet inbetween to stay clear from blisters.

And that's why eg riding from Anchorage to Nome during the ITI using cabins, heated tents etc. is very different to doing the same ride completly unsupported. Sounds like not much of a difference, but it's huge.

fatbikephil wrote: ↑Fri Oct 28, 2022 3:38 pm https://bikepacking.com/plog/fueled-by-fire/
This was the article. I like his solution to not having to use a vapour barrier!

The classic way of travel sleeping in the boreal belt as it has been done for about 12000 years.

Once you leave the boreal belt (higher altitude/different latitude) it back to the inevitable, oppressive, smelly moisture of the body bag