Reflecting layer, where to put it?

[Tord Hoppe]

Hi, what are your thoughts or experiences regarding where to place a reflecting layer such as those car window sheets or "mylar space blankets"?

The layer should obviously not go against your naked skin since that will conduct heat like crazy, but how much insulation should one place in between?

1. Place it on top of your hammock, laying on the reflector. Only whatever clothes you wear will insulate from conduction loss to the reflector.
2. In between the double layers if you have such a hammock. Slightly more insulation.
3. Place it under your hammock, inside of an UQ. Most likely a layer of air between the hammock and reflector.
4. Strap it under your UQ. Enough heat coming through to reflect?

Personally I lean towards 3. Since the reflector is either losely pressed against the hammock fabric or have a thin layer of air in between the risk of conduction through the material should be minimal, while there will still be heat left to actually reflect. Number 4 can work in warmer tempeatures, but as it gets colder the freezing point will creep inside of the UQ. Will this occur at even lower temperatures with a reflecting layer outside of the UQ? I think it should but I still think it´s more effecient to have the reflector inside of the UQ.

Any thermodynamicists here? (Sorry, english isn´t my first language )

[Swjoergensen]

Glad you're asking that question, been pondering a bit myself.
And while I'm no thermodynamicist-whatchamacallit, I'll have a guess anyway.
I'd say number 3 as well. As far as I understand it, condensation occurs if there's too much temperature difference between the two sides of a layer. Thus, keeping the reflector loose between the hammock and uq seems like the best place. There will be some wetness from perspiration during the night, but hopefully, that will not collect as water inside the reflector. Hopefully

I'll give it a try when I get the time, and get rid of my snotty nose. I have a PLUQ I'd like to see take me into the fall with the help of either a space blanket or a slightly thicker and insulated windscreen reflector.

So far it seems people are getting very varied results, so I'd say give a try for yourself and see how it works. Then do it again and see if you can reproduce the effect or learn something new

There are many ways to learn, but this still appears to be the best.

Looking forward to seeing your results.

[slbear]

I've been thinking about this as I've been putting together my first 3-season setup (we don't have a real winter in California - except at altitude). As such, my comments are thinking-out-loud and worth what you've paid for them. Hopefully some of the Winter-Is-Coming Northfolk can share more experience.

In any case, I've heard of people adding a space blanket inside their DIY UQ, but as far as I know I don't see any of the professional (cottage) vendors offering that. I think the mylar is doing multiple things, i.e. vapor barrier, reflector and wind barrier.

The vapor barrier seems to be counter productive because of blocking your body's perspiration escaping. Except in an emergency, where you do not have the insulation necessary to keep you warm, then put the vapor barrier as close as your skin as possible. You will be soaked in your own sweat, but your insulation should stay drier and it might get you through the night if you are caught out without an UQ/TQ/etc. That was the theory I've been taught, and I'm happy to say I never tried it.

Reflective properties are obvious for radiated heat, but I think we're talking conducted heat transfer. I'll admit I used to put a space blanket below my sleeping pads hoping it would warm up my tent, but the mylar will work in both directions, and your UQ will be more isolated from your body. That makes me think it should go on the outside.

WRT wind barrier, there is some debate on UQ protectors, but I think a slight edge in preference goes to breathable versus non-breathable options. Both will block the wind/breeze outside your cocoon, and would protect you as well from rain/splashing. I haven't seen an UQP with mylar, but that would be my choice if you thought the reflector was worth keeping the moisture in.

[MAD777]

For maximum heat reflection, the reflective surface should be as close to the heat source as possible, (that's you).
That being said, I wouldn't want to squirm around on a mylar blanket, trying to keep it under me all night. So I would do the next nearest thing and put it between the layers of a double hammock. In the case of a single layer hammock, it would go between the hammock & underquilt.

Definitely not outside the underquilt. While that would block the wind, incipient moisture from your sweat might saturate the quilt. Use a breathable UQ protector and/or good tarp deployment to combat the wind.

[FLRider]

The primary felt warmth from a space blanket (where it prevents you from losing the most heat to the outside environment) is in its use as a vapor barrier, not a reflective surface. Humans just don't radiate enough heat compared to the surrounding environment (even at 0* F, the difference between your skin temp and the air is only ~100*) compared to a campfire (which is where the reflective properties really shine; you can regain a significant portion of the lost radiative heat when the difference between the fire and the air is ~400* or more) for the reflection of that radiative heat to really be worth worrying about.

Now, heat loss through evaporated water, on the other hand is a significant issue in normal temperatures (especially as one gets colder; the relative humidity may not change much, but the absolute humidity, relative to the air next to your body, makes for a significant difference in how much vapor gets pulled out of your microclimate). So, keep your space blanket between you and your underquilt (personally, I prefer it between the hammock and underquilt, as that keeps me from feeling clammy; the hammock absorbs the small amount of water I'm losing in that direction and dries exceptionally quickly once I'm out of it).

Hope it helps!

[Swjoergensen]

Thanks FLRider
You came of sounding quite clever on that one will have to try it out as soon as possible.
How much further does adding the space blanket, allow you to extend the rating of your quilt? Do you attach the space blanket to anything, or does it just lie loose between quilt and hammock?
I'm in a PLUQ that's good to about 50, slightly into the 40's. Hoping a space blanket will make a noticeable difference.

[brianb]

Interesting. I'm in NC, USA and I've been using the car reflector sheet you mentioned but it's only been getting down to the 50's (about 13c). I have a single layer hammock and I lay the reflective in first, and then an open weave shelf liner on top of that. shelf_liner.jpg Not sure how readily available they are in Scandanavia, but you can pick them easily here in the States.

There wasn't a lot of planning in this setup, it just kind of made sense. I don't think it will hold up when it gets colder, but so far I've been able to use those two as my bottom insulation and an SOL Bivy and been fine. Soon I'll need to start bringing a liner for the bivy or a proper quilt or bag, but so far it's kept my pack light and me warm.

Curious to follow this thread to see where it goes. By the way, those open weave shelf liners are super useful for a lot of DIY stuff.

Cheers, Brian

[FLRider]

Loose, for me. I've only done it a couple of times; my best guess is somewhere between 5 and 10 degrees. It's real use, for me, is in preventing water vapor from condensing in my quit when I use a non-breathable undercover.

Hope it helps!

[MAD777]

My view of an space blanket is a last ditch effort to keep from freezing to death. That's why they call it an emergency space blanket, right?
Do I carry one in winter? Yesserie! Just in case I calculated my insulation wrongly. The space blanket is my extra measure of protection, not calculated during trip preparation. It's part of my 1st Aid kit.

[olddog]

I have used the #3 method but was using a Pellon product called insulfleece, a perforated mylar sheet with a fleece coating on both sides. Being perforated it is not a true vapor barrier and it has helped keep me warm into the low 20°s F last Jan. at the Florida Hang. It was a stacked Rube Goldberg affair with a PLUQ outer, insulfleece middle and a WL summer series UQ inner. Needed everything I had and was glad I brought it all. Was all toasty and comfy in the hammock until the 3:00 am nature call when I climbed out to realize it was COLD.

http://www.pellonprojects.com/produc...-insul-fleece/